WN stars
type: Concept
broader: wolf-rayet-stars
definition: A Wolf-Rayet star whose spectrum is dominated by emission lines of ionized nitrogen: N II 3995 Å, N III 4634-4661 Å, N III 5314 Å, N IV 3479-3484 Å, N IV 4058 Å, N V 4603 Å, N V 4619 Å, and N V 4933-4944 Å. This type is divided in sub-types WN2 to WN11.

Stellar photospheres
broader: stellar-atmospheres
related: Solar photosphere
type: Concept

Transit timing variation method
type: Concept
broader: timing-variation-methods

Galaxy spurs
broader: galaxy-physics
type: Concept

Quadrants
broader: galactic-coordinate-system
type: Concept

Hertzsprung Russell diagram
broader: stellar-evolutionary-models
related: Stellar physics
related: Zero-age horizontal branch stars
related: Henyey track
related: Stellar evolutionary types
definition: A display of stellar properties using a plot of effective temperature (or instead color or spectral type) along the abscissa versus luminosity (or absolute magnitude). The temperature is plotted in the inverse direction, with high temperatures on the left and low temperatures on the right. On the diagram the majority of stars are concentrated in a diagonal strip running from upper left to lower right, i.e. from high temperature-high luminosity massive stars to low temperature-low luminosity low-mass stars. This feature is known as the main sequence. This is the locus of stars burning hydrogen in their cores (proton-proton chain). The lower edge of this strip, known as the zero age main sequence (ZAMS), designates the positions where stars of different mass first begin to burn hydrogen in their cores. Well below the main sequence there is a group of stars that, despite being very hot, are so small that their luminosity is very small as a consequence. These are the class of white dwarfs. These objects represent old and very evolved stars that have shed their outer layers to reveal a very small but extremely hot inner core. They are no longer generating energy but are merely emitting light as they cool (white dwarf cooling track). Stars with high luminosities but relatively low temperatures occupy a wide region above the main sequence. The majority of them have used up all the hydrogen in their cores and have expanded and cooled as a result of internal readjustment. Called red giants, they are still burning helium in their cores (helium burning, carbon burning). There are also stars with very high luminosities, resulting from their enormous outputs of energy, because they are burning their fuel at a prodigious rate. These are the supergiants. They can be hot or cool, hence blue or red in color.
related: Hayashi track
related: Evolved stars
type: Concept
related: Zero-age main sequence stars

Visibility function
type: Concept
broader: interferometry
definition: The Fourier transform of a source's brightness distribution, weighted by the characteristics of the interferometer's antennas.

Optical observatories
broader: observatories
type: Concept

Lyman alpha forest
broader: spectroscopy
type: Concept
definition: The appearance of many differentially redshifted Lyman alpha lines in absorption in a quasar's spectrum, caused by intervening hydrogen clouds along our line of sight to the quasar.

Atomic spectroscopy
broader: spectroscopy
related: Atomic physics
type: Concept

Variable stars
broader: stellar-types
related: Time domain astronomy
type: Concept
related: Purkinje effect
related: Kappa mechanism

Quantum gravity
type: Concept
definition: A theory of gravity, yet to be developed, that would properly include quantum mechanics. Because of the tensor nature of general relativity, it is not renormalizable as a field theory in perturbation from flat space. So far various attempts to quantize general relativity have been unsuccessful.
broader: non-standard-theories-of-gravity

Galaxy pairs
type: Concept
broader: interacting-galaxies

Sungrazers
broader: near-sun-comets
type: Concept
definition: Objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit.

Minimum chi-squared
broader: regression
type: Concept

Relativistic binary stars
broader: relativistic-stars
broader: binary-stars
type: Concept
broader: high-energy-astrophysics

Gravitational microlensing [Exoplanets]
type: Concept
deprecated: :__
use instead: Gravitational microlensing

Spectroscopic binary stars
broader: binary-stars
type: Concept

Stellar coronal holes
type: Concept
broader: stellar-coronae
related: Solar coronal holes

Delta Scuti variable stars
broader: short-period-variable-stars
definition: A member of a class of pulsating stars with periods less than 0.3 days, spectral types A or F, and visual light amplitudes in the range from a few thousands of a magnitude to about 0.8 mag. On the H-R diagram, δ Scuti stars form a group which lies in an instability strip which includes the classical Cepheids at its brightest end and the pulsating white dwarfs at its faintest limit. These stars can show very complex light variations since, while some of them are pulsating in one radial mode only, others may be pulsating simultaneously in several radial and non-radial modes.
broader: multi-periodic-variable-stars
type: Concept
broader: pulsating-variable-stars

Artificial satellites
broader: space-vehicles
definition: A man-made equipment that orbits around Earth or a solar system body.
type: Concept

F dwarf stars
broader: f-stars
broader: dwarf-stars
type: Concept

Double-mode Cepheid variable stars
type: Concept
broader: double-mode-variable-stars
broader: cepheid-variable-stars

Jupiter
definition: The largest planet in the Solar System and the fifth from the Sun, lying at a mean distance of about 5.2 astronomical units from the Sun. Jupiter is a gas giant, mostly hydrogen and helium, with a mass of 1.898 x 10^27 kg, or about 0.001 solar masses, or 318 times Earth masses. It is more than twice as massive as all the other solar system planets combined. Jupiter's diameter measures 11 times that of Earth. Its rotation period, 9.93 hours (Jupiter/Earth ratio = 0.41), is the shortest of all the solar system planets. Its orbital period is 11.857 Earth years. Jupiter has an extensive family of satellites (79 known) and a faint ring system; Jupiter's ring. Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth masses. Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements is possible only at pressures about 3 million bars, as is the case in the interior of Jupiter (and Saturn). Under the extreme pressure found deep inside Jupiter, the electrons are released from the hydrogen molecules and are free to move about the interior. This causes hydrogen to behave as a metal; it becomes conducting for both heat and electricity.
type: Concept
related: Jovian satellites
broader: outer-planets

Damped Lyman-alpha systems
broader: intergalactic-clouds
type: Concept

Millisecond pulsars
broader: pulsars
type: Concept

Solar differential rotation
type: Concept
broader: solar-interior
broader: solar-rotation

Silicate grains
broader: interstellar-dust
type: Concept

Support vector machine
broader: classification
type: Concept

Upsilon Sagittarii stars
type: Concept
broader: eclipsing-binary-stars

Comet surfaces
type: Concept
broader: comet-nuclei

Astronomy data analysis
broader: computational-astronomy
type: Concept

Solar-planetary interactions
broader: planetary-science
related: Star-planet interactions
type: Concept
related: Space weather

Orbital elements
type: Concept
broader: orbits
definition: A fixed point in a body through which the resultant force of gravitational attraction acts.

Planetary system formation
broader: planetary-science
type: Concept

Flux calibration
type: Concept
broader: spectrophotometry
broader: calibration
definition: The calibration of the flux received by a detector in terms of absolute units.

White dwarf evolution
deprecated: :__
type: Concept

Water masers
type: Concept
broader: astrophysical-masers
definition: An interstellar maser phenomenon in which water (H_{2}O) molecules undergo the processes of population inversion and stimulated emission. H_{2}O masers are detected toward star formation regions and the envelopes of evolved stars. The maser emission comes from regions that are typically quite small, not larger than the solar system. The main emission frequency is 22 GHz, which shows up in strong lines. There are, however, other H_{2}O maser transitions at 380 GHz and 183 GHz, which are much weaker than the 22 GHz line. The former transitions are sporadically detected since they are strongly absorbed in the Earth's atmosphere, because of its high water vapor content.

X-ray telescopes
type: Concept
definition: A telescope designed to focus X-rays from astronomical objects. X-ray telescopes function from orbital satellites because X-rays are absorbed by the Earth's atmosphere. These telescopes require special techniques since the conventional methods used in optical and radio telescopes are not adequate.
broader: space-telescopes

Metal-containing molecules
type: Concept
broader: molecular-physics

R Coronae Borealis stars
type: Concept
deprecated: :__

Ultraviolet color
broader: ultraviolet-photometry
type: Concept

Calendars
broader: astronomical-reference-materials
definition: Any of various systems for measuring and recording the passage of time by dividing the year into days, weeks, and months. A table showing the months, weeks, and days in at least one specific year.
type: Concept

Cosmic microwave background radiation
related: CMBR detectors
broader: cosmic-noise
related: Redshifted
type: Concept
related: Non-Gaussianity
broader: cosmic-background-radiation

True anomaly
type: Concept
definition: One of the standard orbital elements, which is the angle measured at the focus nearest the periapsis of an elliptical orbit, between the periapsis and the radius vector from the focus to the orbiting body.
broader: orbital-anomalies

Algorithms
broader: astrostatistics
definition: A step-by-step problem-solving procedure, especially an established, recursive computational procedure for solving a problem in a finite number of steps.
type: Concept

Milky Way disk
related: Galaxy disks
broader: milky-way-galaxy-physics
type: Concept

Parallax
type: Concept
definition: The apparent shift of a nearby object's position in relation to more distant ones when the nearby object is observed from different viewing angles.
broader: astrometry
related: Microlensing parallax

Cirrus vertebratus
type: Concept
deprecated: :__

Molecular data
broader: molecular-physics
type: Concept

Fireballs
broader: meteors
type: Concept

Neutrino oscillation
deprecated: :__
type: Concept

Relativistic aberration
broader: special-relativity
type: Concept
broader: astrometry
definition: The aberration of light for an object moving with relativistic speed. In contrast to the classical case, the Lorentz transformation between the rest frame of the observer and that of the object must be used. Relativistic aberration is expressed by the equation: cos φ' = (cos φ - )/(1 - cos φ.v/c), where φ is the classical aberration angle, v the speed of the Earth, and c the speed of light.

Solar energetic particles
type: Concept
broader: solar-particle-emission
broader: interplanetary-physics
broader: solar-storm

Volcanoes
type: Concept
definition: An opening in the Earth's crust from which lava, ash, and hot gases flow or are ejected during an eruption.
related: Tectonics
broader: planetary-structure

Stellar nucleosynthesis
broader: nucleosynthesis
definition: The nuclear reaction process taking place inside stars, whereby chemical elements are produced from pre-existing nuclei heavier than hydrogen and helium.
type: Concept

Supergiant stars
broader: evolved-stars
definition: A star with maximum intrinsic brightness and low density. The radius of a supergiant star can be as large as 1000 times that of the Sun.
type: Concept

Late stellar evolution
type: Concept
broader: stellar-evolution

Quiet sun
related: Active sun
type: Concept
definition: The Sun when the 11-year cycle of solar activity is at a minimum.
broader: solar-cycle
related: Quiescent solar prominence

Stellar inner cores
type: Concept
broader: stellar-cores

Radiative transfer simulations
broader: astronomical-simulations
type: Concept

Solar magnetic flux emergence
type: Concept
broader: solar-magnetic-fields

Amateur astronomers
broader: astronomical-research
definition: A person who engages in astronomy as a pastime rather than as a profession.
related: Amateur astronomy
type: Concept

Intergalactic abundances
type: Concept
broader: chemical-abundances

Interplanetary shocks
broader: shocks
type: Concept
broader: interplanetary-physics

Post-asymptotic giant branch stars
broader: stellar-evolutionary-types
definition: A star in a short-lived evolutionary stage evolving from the asymptotic giant branch toward higher effective temperatures. The majority of low and intermediate mass stars (1 to 8 solar masses) are believed to pass through this stage on their way to becoming planetary nebulae.
type: Concept
related: Post-asymptotic giant branch

ZZ Ceti stars
broader: white-dwarf-stars
broader: multi-periodic-variable-stars
type: Concept
definition: A member of a class of non-radially pulsating stars that change their brightnesses with periods from 30 seconds to 25 minutes and amplitudes from 0.001 to 0.2 mag in V. Their location on the Hertzsprung-Russell diagram identifies them as white dwarf stars that have entered the instability strip as they evolve along the white dwarf track.

Giant molecular clouds
broader: molecular-clouds
type: Concept
definition: A massive complex of interstellar gas and dust, consisting mostly of molecular hydrogen, that typically stretches over 150 light-years and contains several hundred thousand solar masses. Giant molecular clouds are the principal sites of star formation.

Chemical reaction network models
type: Concept
broader: reaction-models

Multivariate analysis
type: Concept
broader: astrostatistics-techniques

Photoelectric magnitude
definition: The magnitude of an object as measured with a photoelectric photometer.
broader: apparent-magnitude
type: Concept

Classical Kuiper belt objects
type: Concept
broader: trans-neptunian-objects

Ganymede
type: Concept
definition: The seventh and largest of Jupiter's known satellites. This Galilean satellite has a diameter of 5270 km, slightly larger than Mercury, a mass about 1.48 × 10^23 kg (about 2 Earth Moons); an orbital period of 7.155 days, and an eccentricity of e = 0.0015. It was discovered by Galileo and Marius in 1610. The mean surface temperature of Ganymede is -160 °C. It is the only moon known to have a magnetosphere.
broader: galilean-satellites

Solar corona
type: Concept
related: Stellar coronae
definition: The outermost atmosphere of the Sun immediately above the chromosphere, which can be seen during a total solar eclipse. It consists of hot, extremely tenuous gas extending for millions of kilometer from the Sun's surface.
broader: solar-atmosphere

Optical aberration
type: Concept
broader: astronomical-optics

Lunar mountains
type: Concept
broader: lunar-features

cD galaxies
type: Concept
broader: d-galaxies
broader: giant-galaxies

Ultraviolet transient sources
type: Concept
broader: transient-sources
broader: ultraviolet-sources

Solar coronal waves
type: Concept
broader: solar-corona

Deprecated: Solar composition
type: Concept
deprecated: :__
use instead: Solar abundances

OGLE small amplitude red giant stars
broader: multi-periodic-variable-stars
type: Concept
broader: red-giant-stars
broader: long-period-variable-stars

H I regions
related: Astrochemistry
type: Concept
definition: A region of neutral (atomic) hydrogen in interstellar space. At least 95 percent of interstellar hydrogen is H I. It emits radio waves that are 21 cm long.
broader: nebulae

Schmidt-Cassegrain telescopes
broader: catadioptric-telescopes
type: Concept
definition: A mixture of the Cassegrain telescope with a very short focal length and of a Schmidt design (due to the presence of the corrective plate), used mainly in amateur astronomy. The main advantage of this telescope is its compact design. However, Schmidt-Cassegrain telescopes produce fainter images with less contrast than other telescope designs with similar aperture sizes. This is due to the comparatively large secondary mirror required to reflect the light back the eyepiece.

Neutron star cores
broader: neutron-stars
type: Concept

Archaeoastronomy
type: Concept
related: Early astronomical instruments
broader: cultural-astronomy
definition: The study that deals with the astronomical knowledge of prehistoric peoples (season events, calendars, observing sites, astronomical alignments) and its influence on their cultures and societies (mythologies, religions, life). Archaeoastronomy covers the intersection between astronomy and archaeology.

Helioseismic pulsations
broader: helioseismology
broader: solar-oscillations
type: Concept
broader: solar-interior

Ethnoastronomy
definition: The study of the beliefs, interpretations, and practices of specific cultures regarding celestial objects or phenomena. Ethnoastronomy uses the tools and methodologies of ethnology in the study of astronomical conceptions.
type: Concept
broader: cultural-astronomy

RGU photometry
broader: broad-band-photometry
type: Concept

Supernovae
related: Supernova dynamics
related: White dwarf stars
broader: stellar-types
type: Concept
definition: A violent stellar explosion which blows off all or most of the star's material at high velocity leaving a compact stellar remnant such as a neutron star or black hole. At maximum light, the supernova can have luminosity about 108 or 109 times the solar luminosity. The phenomenon results from the later evolution of stars when an instability sets in the core following the nucleosynthesis of iron. In massive stars, the supernova occurs when the star has used up all its available nuclear fuel and it reaches a lower energy state through gravitational collapse to form a more compact object. In white dwarfs forming binary systems, accretion of mass onto the surface of a neutron star can be sufficient to take the star over the upper mass limit for stability as a white dwarf. Consequently, the white dwarf collapses in a supernova explosion to form a neutron star. There are several supernova types.
related: Burst astrophysics
related: Ejecta
broader: stellar-remnants

Dust shells
broader: circumstellar-shells
type: Concept

Friedmann-Robertson-Walker metric
type: Concept
broader: spacetime-metric

Micrometeoroids
broader: meteoroids
type: Concept

Binary source microlensing
broader: gravitational-microlensing
type: Concept

Two-dimensional gravity
type: Concept
broader: non-standard-theories-of-gravity

RRb stars
use instead: RRab variable stars
type: Concept
deprecated: :__

Solar system
related: Solar system evolution
related: Exoplanet systems
broader: solar-system-astronomy
definition: The collective name for the Sun and all objects gravitationally bound to it. These objects are the eight planets, their 166 known moons, five dwarf planets, and billions of small bodies. The small bodies include asteroids, icy Kuiper belt objects, comets, meteoroids, and interplanetary dust. The solar system is roughly a sphere with a radius greater than 100,000 AU. Planets, satellites, and all interplanetary material together comprise only about 1/750 of the total mass. Geochemical dating methods show that the solar system chemically isolated itself from the rest of the Galaxy (4.7 ± 0.1) x 10^9 years ago.
type: Concept
related: Solar system formation
related: Cosmogony

Random Forests
type: Concept
broader: classification

Supergalactic latitude
type: Concept
broader: supergalactic-coordinate-system

Galaxy chemical evolution
broader: galaxy-evolution
type: Concept

Stellar accretion
broader: stellar-properties
related: Stellar accretion disks
type: Concept
broader: accretion

Maximal Kerr black holes
type: Concept
broader: kerr-black-holes

Stellar spectral types
related: Morgan-Keenan classification
type: Concept
broader: stellar-types

Dark matter
broader: intergalactic-medium
definition: Matter that has no radiation and therefore cannot be detected directly, but whose presence can be inferred from dynamical phenomena produced by its gravitational influence. The existence of dark matter is deduced mainly from the rotational speeds of galaxies, velocities of galaxies in clusters, gravitational lensing by galaxy clusters, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter plays also a central role in cosmic structure formation. There exists a large number of non-baryonic dark matter candidates. They include, the hypothetical stable particles WIMPs, neutralinos, axions, gravitinos, etc. Among unstable candidates are gravitinos with mild R-parity violation and sterile neutrinos.
related: Cosmology
broader: high-energy-astrophysics
type: Concept
related: Navarro-Frenk-White profile
related: Particle astrophysics

G subdwarf stars
type: Concept
broader: subdwarf-stars
broader: g-stars

RV Tauri variable stars
type: Concept
broader: type-ii-cepheid-variable-stars

Galactic coordinate system
broader: astronomical-coordinate-systems
type: Concept

Galaxy magnetic fields
broader: galaxy-properties
related: Milky Way magnetic fields
type: Concept

Measurement error model
broader: regression
type: Concept

Planetary nebulae
type: Concept
broader: emission-nebulae

Pre-main sequence stars
related: Pre-main sequence
definition: A star that evolves in the Hayashi phase and has not yet reached the zero-age main sequence.
related: Young stellar objects
broader: stellar-evolutionary-types
type: Concept

Hyperbolic orbit
definition: An orbit that is an open curve whose ends get wider apart at any rate between that of an ellipse and a straight line. Some comets' orbits become hyperbolic through the gravitational influence of a planet the comet passes near.
broader: orbital-motion
type: Concept

Semi-regular variable stars
broader: late-type-giant-stars
type: Concept
broader: long-period-variable-stars
definition: A type of giant or supergiant pulsating variable star, with intermediate or late spectra, showing noticeable periodicity in its light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light curve may be rather different and variable with each cycle. The amplitudes may be from several hundredths to several magnitudes (usually 1-2 magnitudes in the V filter). Examples are Betelgeuse, Antares, and Rasalgethi. Semiregular variables are classified in several subtypes: SRA, SRB, SRC, and SRD.

Antarctic observatories
broader: observatories
type: Concept

Stellar winds
broader: stellar-interstellar-interactions
broader: stellar-physics
type: Concept
related: Solar wind

K subdwarf stars
type: Concept
broader: k-stars

Compact objects
type: Concept
related: Compact radiation sources
definition: An astronomical object that is substantially denser or more compact than most objects of its class. More specifically, a neutron star or a stellar black hole.
broader: stellar-remnants

F subdwarf stars
type: Concept
broader: f-stars
broader: subdwarf-stars

Stellar K corona
related: Solar K corona
type: Concept
broader: stellar-coronae

Ram pressure stripped tails
type: Concept
broader: galaxy-tails

Gravitational microlensing
related: Free floating planets
broader: gravitational-lensing
type: Concept

Magnetic variable stars
type: Concept
broader: intrinsic-variable-stars
broader: magnetic-stars

Helium-weak stars
type: Concept
definition: Chemically peculiar stars with very weak helium lines. Examples include 3 Sco, HD 176582, HD 217833, HR 2949, and HD 21699. The He-weak stars do not form a homogeneous group. Some of them display intense Si, or Ti and Sr lines, and are considered a hot extension of the magnetic Ap/Bp stars. Others show overabundances of P and Ga, typically noted for HgMn stars. The star HD 139160 belongs to the non-magnetic subgroup of He-weak stars.
broader: chemically-peculiar-stars

Horizontal branch stars
type: Concept
definition: A star lying on the horizontal branch.
related: Horizontal branch
broader: stellar-evolutionary-types

Delta sunspots
type: Concept
broader: sunspots

Green valley galaxies
type: Concept
broader: galaxy-colors

Cosmic ray detectors
type: Concept
broader: astronomical-detectors

Isotopic abundances
broader: chemical-abundances
type: Concept

Soft gamma-ray repeaters
broader: irregular-variable-stars
broader: gamma-ray-sources
type: Concept
broader: magnetars

X-ray observatories
type: Concept
broader: observatories

Drift scan imaging
type: Concept
broader: astronomical-techniques

Solar radius
broader: solar-physics
related: Stellar radii
definition: A unit of length, representing the radius of the Sun, used to express the size of stars in astrophysics. It is equivalent to: 695,700 km, 0.00465047 astronomical units, 7.35355 x 10^-8 light-years, and 2.32061 light-seconds.
type: Concept

Yellow hypergiant stars
type: Concept
definition: An evolved, very massive star of spectral type F or G with a very high luminosity (~10^5 times solar) lying near the empirical upper luminosity boundary in the H-R diagram (Humphreys-Davidson limit). Yellow hypergiants have high mass loss rates (10^-5 - 10^-3 solar masses per year) and are in a short, transitional evolutionary stage. Their evolutionary state is thought to correspond to post-red supergiants rapidly evolving in blueward loops in the H-R diagram. In their post-RSG blueward evolution these stars enter a temperature range (6000-9000 K), called yellow void, with increased dynamical instability. Their link to other advanced evolutionary phases of massive stars such as Luminous Blue Variables and Wolf-Rayet stars is still an open issue in stellar evolution theory. The most famous yellow hypergiant is Rho Cassiopeiae.
broader: hypergiant-stars

Bessel photometry
broader: broad-band-photometry
type: Concept

Excitation rates
broader: molecular-physics
broader: atomic-physics
type: Concept

Galaxy bulges
definition: A spheroidal region at the center of a spiral galaxy which mostly contains old stars. Galactic bulges are generally classified into two types: classical bulges and pseudo-bulges.
type: Concept
related: Galactic bulge
broader: galaxy-structure

Baade-Wesselink method
type: Concept
definition: A method used to determine the size of certain types of pulsating stars, including Cepheids, from their magnitude variations (photometry) and the corresponding radial velocities (spectroscopy).
broader: astrometry

Adaptive interferometry
broader: interferometry
type: Concept

Zenith telescopes
definition: A telescope that is mounted on a vertical axis or moves only a small amount from the vertical. It is primarily used to determine positional measurement of stars moving near the zenith. The advantage is that there is no atmospheric refraction occurring at the zenith. If a star on one night passes through the center of eyepiece, one must observe it six month later, and see if the star has been offset by the center. A shift would mean a measure of parallax.
broader: transit-instruments
type: Concept

Main belt asteroids
related: Asteroid belt
definition: Asteroids in our solar system that are found in the area between Mars and Jupiter.
type: Concept
broader: asteroids

Interplanetary grains
broader: interplanetary-medium
type: Concept

Optical double stars
definition: A pair of stars that lie close to each other in the sky by chance, but are not physically associated, in contrast to a true binary star.
broader: optical-observation
related: Binary stars
type: Concept

Impact phenomena
related: Meteorites
related: Lunar impacts
type: Concept
broader: planetary-surfaces

Infrared photometry
type: Concept
broader: waveband-photometry
related: Medium band photometry
related: Broad band photometry

Protoplanetary nebulae
broader: nebulae
type: Concept
definition: A short-lived transition object between the asymptotic giant branch (AGB) and planetary nebula phases.

Kirkwood gaps
broader: orbital-resonances
definition: Regions in the asteroid belt within which few asteroids are found. The Kirkwood gaps are due to the perturbing effects of Jupiter through resonances with Jupiter's orbital period.
type: Concept

Weak gravitational lensing
type: Concept
definition: A gravitational bending of light by structures in the Universe that distorts the images of distant galaxies. The distortion allows the distribution of dark matter and its evolution with time to be measured, thereby probing the influence of dark energy on the growth of structures. Weak gravitational lensing is generally difficult to identify in individual images, in contrast to strong gravitational lensing.
broader: gravitational-lensing

Star lore
broader: celestial-sphere
type: Concept

Comet dynamics
type: Concept
broader: planetary-science
related: Comets

Center to limb observations
broader: optical-observation
type: Concept

Electron recombination reactions
broader: molecular-reactions
type: Concept

Hadron barrier
type: Concept
broader: big-bang-theory

Baily's beads
definition: A phenomenon that occurs during a total eclipse of the Sun. Just prior to and after totality, sunlight shines through the lunar valleys on the Moon's limb, causing the dark face of the Moon to appear to be surrounded by a shining 'necklace of pearls'.
broader: eclipses
type: Concept

N galaxies
type: Concept
broader: compact-galaxies

Azimuth
type: Concept
definition: The angular distance from the North point eastward to the intersection of the horizon with the vertical circle passing through the object. Azimuth is 0° for an object due north, 90° due east, 180° due south, and 270° due west. Altitude and azimuth constitute the horizon coordinate system.
broader: horizontal-coordinate-system
related: Alt-azimuth mounts

Hayashi track
type: Concept
related: Hertzsprung Russell diagram
definition: The path on the Hertzsprung-Russell diagram that is followed by a fully convective pre-main sequence star to reach the zero-age main sequence. Hayashi tracks for low-mass stars are near vertical. At higher masses, stars become increasingly radiative as they contract and the Hayashi tracks are almost horizontal.
broader: stellar-evolutionary-tracks

DB stars
type: Concept
broader: helium-rich-stars
broader: white-dwarf-stars
definition: A white dwarf whose spectrum shows strong He I in the absence of hydrogen or metal lines.

Isotope shifts
type: Concept
broader: atomic-physics

Warm-hot intergalactic medium
type: Concept
broader: high-energy-astrophysics
definition: The space containing a cluster of galaxies filled with a tenuous gas of temperature 105 to 107 K and density 10-6 to 10-4 cm-3. WHIM has been continuously shock-heated during the process of structure formation. It is so highly ionized that it can only absorb or emit far-ultraviolet and soft X-ray photons, primarily at spectral lines of highly ionized C, O, Ne, and Fe. WHIM is thought to be the main reservoir of missing baryons.
broader: intergalactic-medium-phases
broader: large-scale-structure-of-the-universe

BW stars
broader: helium-weak-stars
broader: b-stars
type: Concept

Pre-main sequence
related: Pre-main sequence stars
broader: hertzsprung-russell-diagram
related: Helmholtz-Kelvin contraction
type: Concept

Ptolemy stones
broader: early-astronomical-instruments
type: Concept

Thermosphere
definition: The region of the upper atmosphere in which temperature increases continuously with height, starting at roughly 100 km. The thermosphere includes the exosphere and most of the ionosphere.
type: Concept
broader: earth-atmosphere

RGB photometry
type: Concept
broader: broad-band-photometry

Sigma8
type: Concept
broader: cosmological-parameters

High-resolution microlensing event imaging
broader: gravitational-microlensing
type: Concept

DQ Herculis stars
broader: novae
related: AM Canum Venaticorum stars
related: AM Herculis stars
definition: A type of cataclysmic variable binary star system with a white dwarf and a cool main-sequence secondary star.
type: Concept

Helioseismology
related: Asteroseismology
definition: The branch of astrophysics that investigates the interior structure of the Sun by studying its surface wave oscillations. The surface of the Sun vibrates much like a bell. A piano has 88 keys or musical tones, whereas the Sun has millions of notes. These vibrations are the result of internal pressure waves that reflect off the photosphere and repeatedly cross the solar interior. They are detected through the Doppler shift of absorption lines formed in the photosphere. Because these vibrations make the solar surface move up and down, analysis of the surface patterns is used to study conditions far below the Sun's surface. The mean period of the vibrations is about five minutes, which corresponds to a frequency of about 3 mHz.
broader: solar-physics
type: Concept
related: Solar interior

Dark energy
definition: A hypothetical form of energy that fills all the space and tends to increase the rate of expansion of the Universe. Assuming the existence of dark energy is a way to explain recent observations that the Universe appears to be expanding at an increasing rate. Dark energy seems to be a kind of anti-gravity force and is supposed to be related to vacuum energy. Where gravity pulls things together at the more local level, dark energy tears them apart on the grander scale. The acceleration equation, one of Einstein's equations for the homogeneous Universe, indicates that if the Universe is accelerating, the pressure of the driving component should be strongly negative. The first indication of dark energy was provided by the observation of Type Ia supernovae. Other probes of dark energy are: baryon acoustic oscillations, weak gravitational lensing, and clusters of galaxies. In the standard model of cosmology, dark energy currently accounts for almost 74% of the total mass-energy of the Universe. Two proposed forms for dark energy are the cosmological constant and exotic component such as quintessence.
type: Concept
broader: cosmological-parameters

Carbon flashes
type: Concept
broader: stellar-evolution

Earth trojans
broader: near-earth-objects
type: Concept
broader: trojan-asteroids

Circumstellar dust
definition: Interstellar dust grains localized around various types of stars, such as asymptotic giant branch stars. Circumstellar dust occurs in the form of a spherical shell or a disk and is at the origin of an infrared excess for the central star.
type: Concept
broader: circumstellar-matter

High contrast techniques
type: Concept
broader: direct-imaging

Einstein field equations
type: Concept
broader: general-relativity

Redshift surveys
broader: surveys
related: Redshifted
type: Concept

Solar constant
definition: The amount of solar radiation in all wavelengths received per unit of time per unit of area on a theoretical surface perpendicular to the Sun's rays and at Earth's mean distance from the Sun. Its mean value is 1367.7 W m^-2 or 1.37 x 106 erg sec^-1 cm^-2. In other words, the solar constant is the mean solar irradiance on the outer atmosphere when the Sun and Earth are spaced at 1 astronomical unit.
type: Concept
broader: astrometry

Astrometric exoplanet detection
broader: exoplanet-detection-methods
type: Concept
related: Astrometry

White dwarf stars
definition: A compact star of high surface temperature, low luminosity, and high density (10^5 - 10^8 g cm^-3), with roughly the mass of the Sun (mean mass ~ 0.6 M_sun) and the radius of the Earth (R ~ 0.01 R_sun), representing the end-point of the evolution of all stars with masses less then ~ 5-9 solar masses. A white dwarf is what remains after the central star of a planetary nebula fades and becomes cool. The Chandrasekhar limit of 1.43 solar masses is the highest mass that a white dwarf can achieve before electron degeneracy pressure is unable to support it. In the Hertzsprung-Russell diagram, white dwarfs form a well-defined sequence around 8 magnitudes fainter than the main sequence. They are composed of a core of carbon and oxygen nuclei and degenerate electrons surrounded by a thin shell of helium and an outer skin of hydrogen. White dwarf's radiation is the leftover heat from the star's past when its core was an active nuclear reactor. The star slowly cools as heat escapes through the non-degenerate envelope. The first white dwarf to be discovered was Sirius B, the companion of Sirius. White dwarfs are divided into several types, according to their spectral features, which depend on the type of shell burning that dominated as it became a planetary nebula: DA white dwarf; DB white dwarf; DC white dwarf; DO white dwarf; DZ white dwarf; DQ white dwarf.
broader: compact-objects
related: Chandrasekhar limit
related: Type Ia supernovae
related: Supernovae
type: Concept
broader: stellar-evolutionary-types

Accelerating universe
type: Concept
definition: The deduction based on the observation that the most distant Type Ia supernovae are fainter than that expected from their redshifts in a matter-only dominated expanding Universe. The faintness is attributed to larger distances resulting from an accelerating Universe driven by presence of a new component with strongly negative pressure. This component that makes the Universe accelerate is named dark energy.
broader: deceleration-parameter

Comets
related: Comet dynamics
type: Concept
related: Near-Earth objects
related: Exocomets
related: Astrophysical masers
definition: A small body of gas and dust which revolves around the Sun in a usually very elliptical or even parabolic orbit. It is seen to be composed of a head and often with a spectacular gaseous tail extending a great distance from the head. The rocky-icy head is called the comet nucleus. As the comet nears the Sun, the increased temperature causes the ice in the nucleus to sublimate and form a gaseous halo around the nucleus, called the coma. Comets often possess two tails, a dust tail that lies in the orbit behind the comet generated by surface activity, and a brighter, ionized gas tail, that points away from the Sun, driven by solar wind. Cometary reservoirs are thought to represent primordial solar system material. A comet with a dust coating on its surface that inhibits gas production might be classified as an asteroid. Because of this ambiguity, objects such as Chiron, Centaur asteroid, have been reclassified as comets. Comets are primarily composed of amorphous water ice, but also contain carbon dioxide, carbon monoxide, formaldehyde, methanol, methane at a few percent level (with respect to water), and many other molecules at a lower level.
broader: small-solar-system-bodies
related: Comet origins
related: Interstellar objects
related: Interplanetary medium

R Coronae Borealis variable stars
type: Concept
broader: flare-stars
broader: irregular-variable-stars

Stellar atmospheric opacity
related: Stellar atmospheres
type: Concept
broader: astronomical-optics

Educational software
type: Concept
broader: astronomy-software
related: Astronomy education

Jovian satellites
broader: natural-satellites-solar-system
related: Jupiter
type: Concept

Nuclear reaction cross sections
type: Concept
broader: nuclear-physics

Position angle
broader: orbital-elements
definition: The convention for measuring angles on the sky in astronomy (abbreviated as PA). It is the direction of an imaginary arrow in the sky, measured from north through east: 0° = north, 90° = east, 180° = south, and so on to 359° and back to 0°. Applied to a binary system it is the direction of a secondary body or feature from a primary, measured in the system.
type: Concept
related: Proper motions

Astrophysical processes
type: Concept

Dwarf elliptical galaxies
definition: A galaxy that is much smaller than other members of the elliptical class; it is designated as dE. A subtype of dwarf ellipticals is called a dwarf spheroidal galaxy (dSph). The basic characteristics of the class are low surface brightness and smooth light distribution. They range in luminosity from that of the faintest dSph galaxies M_V ~ -9 to about -17. In the Local Group there are 19 known dEs. They are very common in galaxy clusters.
broader: dwarf-galaxies
type: Concept

Mercury-manganese stars
type: Concept
definition: Chemically peculiar stars of late B-types. The most distinctive features of Mercury-manganese stars are extreme atmospheric overabundance of Hg (up to 5 dex) and of Mn (up to 3 dex). The origin of abundance anomalies observed in late B-type stars with HgMn peculiarity is still poorly understood. More than two thirds of the HgMn stars are known to belong to spectroscopic binaries with a preference of orbital periods ranging from 3 to 20 days.
broader: magnetic-stars
broader: chemically-peculiar-stars
broader: b-stars

B dwarf stars
broader: dwarf-stars
broader: b-stars
type: Concept

Ion-storage rings
type: Concept
broader: experimental-techniques

Brightness temperature
definition: In radio astronomy, the temperature of a source calculated on the assumption that it is a blackbody emitting radiation of the observed intensity at a given wavelength.
broader: astrometry
type: Concept
broader: radio-astronomy

Red giant tip
broader: red-giant-branch
type: Concept

Martian satellites
broader: natural-satellites-solar-system
type: Concept
related: Mars

Giant elliptical galaxies
broader: giant-galaxies
broader: elliptical-galaxies
type: Concept

Solar K corona
type: Concept
related: Stellar K corona
broader: solar-corona
definition: The inner part of the solar corona which extends to about two solar radii. It is due to the Thomson scattering of light from the photosphere by the free electrons in the corona. The K corona exhibits a linearly polarized continuous spectrum. The high speeds of the scattering electrons (on the average 10,000 km s^-1 for a temperature of 2 million K) smear out the Fraunhofer lines except the H and K lines.

Interstellar objects
broader: small-solar-system-bodies
type: Concept
definition: A body other than a star or substellar object not gravitationally bound to a star. Its hyperbolic orbit would indicate an object not bound to the Sun. The first known ISO is 1I/'Oumuamua. ISOs are icy planetesimals that are expected to behave like the long-period comets of the solar system; volatile ices sublimate when the ISO approaches the Sun, developing a coma and a dust tail -- features that should make them bright and therefore easy to spot. The rocky ISOs, on the other hand, only reflect sunlight. As their albedo is expected to be extremely low they become dark (after eons of bombardment by high-energy cosmic rays), they would be extremely faint and hard to detect.
related: Comets
related: Exocomets

Abundance ratios
broader: chemical-abundances
type: Concept
definition: The relative amount of a given chemical element with respect to other elements.

Alt-azimuth mounts
definition: A telescope mounting which has its two axes of movement aligned with the horizon and the zenith.
related: Azimuth
type: Concept
broader: telescope-mounts

Great Attractor
definition: A hypothesized large concentration of mass (about 10^16 solar masses), some hundred million light-years from Earth, in the direction of the Centaurus supercluster, that seems to be affecting the motions of many nearby galaxies by virtue of its gravity.
type: Concept
broader: gravity-anomaly
broader: large-scale-structure-of-the-universe

Intermediate population stars
deprecated: :__
type: Concept

RRA stars
deprecated: :__
use instead: RRab variable stars
type: Concept

Atomic interferometry
type: Concept
broader: interferometry
related: Atomic physics

Exoplanet astronomy
type: Concept

Radio source counts
related: Radio sources
type: Concept
broader: surveys

Almanacs
definition: A book of tables, usually covering a period of one calendar year, that lists the future positions of the Moon, planets, and other prominent celestial objects, together with other useful astronomical data.
broader: astronomical-reference-materials
type: Concept

Stratopause
type: Concept
broader: earth-atmosphere

Roche lobe overflow
related: Roche lobe
type: Concept
broader: multiple-star-evolution

WC stars
broader: wolf-rayet-stars
type: Concept
definition: A Wolf-Rayet star whose spectrum is dominated by emission lines of ionized carbon: C III 5696 Å, C III / C IV 4650 Å, C IV 5801-12 Å. This type is divided in sub-types WC4 to WC9.

Parallel field
broader: field-of-view
type: Concept

James Webb Space Telescope
broader: space-observatories
type: Concept
broader: infrared-observatories

Irregular satellites
type: Concept
definition: A satellite whose orbit around its planet is eccentric, inclined with respect to the equatorial plane, and relatively far from the planet. Strong solar perturbations cause the orbit to precess.
broader: natural-satellites-solar-system
related: Irregular natural satellites (Extrasolar)

Heliocentric model
broader: cosmological-models
type: Concept

O subdwarf stars
broader: subdwarf-stars
type: Concept
broader: o-stars

Solar F corona
type: Concept
definition: The exterior part of the solar corona, illuminated by solar light scattered or reflected by dust particles. The same phenomenon also produces the zodiacal light, much farther away from the Sun. The dust particles are at most several microns in size and make up a disk stretching over almost 1 solar radius (700,000 km) from the Sun's surface. Unlike electrons, which are responsible for the K corona, the dust particles move relatively slowly. Thus, the light scattered by them has the same spectrum as the photosphere and shows the Fraunhofer lines. The F corona is the most luminous part of the corona at 1.5 solar radii from the Sun's surface.
related: Stellar F corona
broader: solar-corona

Electron impact excitation
broader: collision-processes
type: Concept

Protoplanetary disks
broader: circumstellar-disks
broader: interplanetary-medium
definition: A circumstellar disk of gas and dust surrounding a pre-main sequence star from which planetary systems form. Protoplanetary disks are remnants of accretion disks which bring forth stars. Typically, their sizes are ~100-500 AU, masses ~10^-2 solar masses, lifetimes ~10^6 - 10^7 years, and accretion rates ~10^-7 - 10^-8 solar masses per year. According to the standard theory of planet formation, called core accretion, planets come into being by the growth of dust grains which stick together and produce ever larger bodies, known as planetesimals. The agglomeration of these planetesimals of 100 to 1000 km in size into rocky Earth-mass planets is the main outcome of this theory. Beyond the snow line in the disk, if the masses of these cores of rock and ice grow higher than 10 times that of Earth in less than a few million years, gas can rapidly accrete and give rise to giant gaseous planets similar to Jupiter. If core building goes on too slowly, the disk gas dissipates before the formation of giant planets can start. Finally the left-over planetesimals that could not agglomerate into rocky planets or core of giant planets remain as a debris disk around the central object that has become a main sequence star. An alternative to core accretion theory is formation of planets in a massive protoplanetary disk by gravitational instabilities. The validity of these two theories is presently debated.
broader: protostars
broader: planetary-system-formation
type: Concept
related: Planet formation

South celestial pole
broader: celestial-poles
definition: The point in the southern hemisphere where the rotation axis of the Earth touches the celestial sphere. In contrast to the north celestial pole, no bright star is visible in that direction.
type: Concept

Stellar mass functions
type: Concept
broader: stellar-properties
related: Stellar masses

Hot Neptunes
broader: exoplanets
type: Concept

Solar chromosphere
broader: solar-atmosphere
type: Concept
related: Stellar chromospheres

Magnetic stars
type: Concept
definition: A star whose spectral lines show the Zeeman effect.
broader: chemically-peculiar-stars

Solar coronal mass ejections
broader: solar-storm
related: Stellar mass loss
type: Concept
definition: A huge eruption of material from regions of the solar corona in which the magnetic field is closed, but which suffer an extremely energetic disruption. Over the course of several hours up to 10,000 billion kg of this material is ejected into interplanetary space with a speed of as high as 3000 km/s. CMEs are most spectacularly observed by a white light coronagraph located outside Earth's atmosphere. Such observations from Skylab in the early 1970's were the first to reveal this phenomenon. CME's disrupt the flow of the solar wind and can produce intense electromagnetic disturbances that can severely damage satellites and disrupt power grids on Earth. When these ejections reach the Earth, they give rise to geomagnetic storms. The frequency varies with the solar cycle; during solar minimum they come at a rate of about one per week, and during maximum there is an average of about two or three per day.
related: Stellar coronal mass ejections
related: Ejecta

DA stars
type: Concept
definition: A white dwarf whose spectrum shows the Balmer lines of hydrogen only, with no helium or metals.
broader: white-dwarf-stars

Achondrites
broader: meteorites
definition: A class of stony meteorites that lack chondrules. They are made of rock that has crystallized from a molten state. Achondrites are relatively rare, accounting for about 8% of all meteorite falls.
type: Concept

Stratosphere
type: Concept
broader: earth-atmosphere
definition: The second major layer of Earth's atmosphere, just above the troposphere and below the mesosphere, extending from about 20 km to 90 km above the Earth. It is characterized by little vertical increase in temperature.

Ultraviolet astronomy
related: Ultraviolet sources
definition: The study of astronomical objects in the ultraviolet portions of the electromagnetic spectrum, in the waveband 3000 Å to about 10 Å. At these wavelengths, the atmosphere prevents ultraviolet radiation from reaching the Earth surface. Therefore ground-based observatories cannot observe in the ultraviolet. Only with the advent of space-based telescopes has this area of astronomy become available for research.
broader: astronomical-methods
type: Concept

Line positions
broader: atomic-spectroscopy
broader: molecular-spectroscopy
type: Concept

Solar ultraviolet emission
broader: solar-electromagnetic-emission
type: Concept

Solar x-ray emission
type: Concept
broader: solar-electromagnetic-emission

Periapsis
definition: The point in an orbit of a body where it is closest to its primary; opposite of apoapsis.
broader: apsides
type: Concept

Slow irregular variable stars
broader: long-period-variable-stars
type: Concept

Diffuse x-ray background
broader: x-ray-sources
type: Concept

Irregular cadence
type: Concept
broader: time-series-analysis

R-process
broader: nucleosynthesis
type: Concept
definition: A nucleosynthesis process in which chemical elements heavier than zinc are created through the intense bombardment of other elements by neutrons in rapid succession. The essential feature of the r-process is the release of great numbers of neutrons in a very short time (less than 100 seconds). The r-process is a 'rapid' version of the s-process, occurring in supernova core collapse and possibly when a neutron star merges with a black hole in a binary star.

Stellar streams
broader: galaxy-structure
type: Concept

Stellar rings
type: Concept
broader: galaxy-stellar-halos

Phase switching
broader: interferometry
type: Concept

Normal distribution
definition: A theoretical frequency distribution for a set of variable data, usually represented by a bell-shaped curve with a mean at the center of the curve and tail widths proportional to the standard deviation of the data about the mean.
broader: astrostatistics-distributions
type: Concept

Software available on request
type: Concept
broader: software-licensing

Photometric systems
related: Magnitude
type: Concept
broader: photometry

Deprecated: Henry Draper Catalogue
type: Concept
deprecated: :__
use instead: Henry Draper catalog

Radio sources
related: Radio pulsars
broader: astronomical-radiation-sources
related: Quasars
definition: A point or small portion of the sky giving stronger radio emission than the sky in its vicinity.
related: Radio astronomy
related: Active galactic nuclei
related: Radio source counts
related: Radio galaxies
related: Radio cores
type: Concept

Intermediate-mass black holes
type: Concept
broader: astrophysical-black-holes

Extreme ultraviolet astronomy
type: Concept
broader: ultraviolet-astronomy

Mass spectrometers
broader: spectrometers
related: Mass spectrometry
type: Concept

Relativistic fluid dynamics
broader: relativistic-mechanics
type: Concept

Exoplanet evolution
broader: exoplanet-astronomy
type: Concept

L subdwarfs
broader: brown-dwarfs
type: Concept

M giant stars
type: Concept
broader: m-stars
broader: late-type-giant-stars

Voids
broader: large-scale-structure-of-the-universe
type: Concept
broader: intergalactic-medium
broader: cosmic-web
definition: A large region of cosmic space without galaxies. The first of these voids to be discovered lies in the direction of Bootes and is some 300 million light-years across. It is estimated that voids take up about 98% of the volume of the Universe, with clusters of galaxies concentrated in the thin walls that surround them.
broader: galaxy-physics

Proper motions
related: Position angle
related: Time domain astronomy
definition: The apparent motion of a star across the sky (not including a star's parallax), arising from the star's velocity through space with respect to the Sun. Proper motion is usually tabulated in star catalogs as changes in right ascension and declination per year or century.
broader: stellar-motion
type: Concept

Short period variable stars
type: Concept
broader: periodic-variable-stars
definition: A variable star that has a relatively short period with respect to stars of similar types.

Exoplanet structure
broader: exoplanet-astronomy
type: Concept
related: Planetary structure

Remote telescope astrophotography
type: Concept
broader: astrophotography

Comet dust tails
broader: comet-tails
type: Concept

Extrasolar rocky planets
broader: exoplanets
type: Concept
related: Solar system terrestrial planets

Dark matter distribution
broader: large-scale-structure-of-the-universe
type: Concept

Single-dish antennas
type: Concept
broader: radio-telescopes

Cosmological evolution
type: Concept
broader: cosmology

Photometric parallax
broader: parallax
type: Concept
definition: A method of deriving the distance of a star using its apparent magnitude and the absolute magnitude inferred from its spectral type.

OH/IR stars
related: Infrared sources
broader: giant-stars
definition: An evolved Mira-type star which is associated with strong OH maser and strong infrared (IR) emission from the surrounding shell of warm gas and dust.
related: Hydroxyl sources
type: Concept
related: Cocoon stars

Common envelope binary stars
related: Common envelope evolution
broader: interacting-binary-stars
type: Concept

Cirrus castellanus
deprecated: :__
type: Concept

Gravitational instability
broader: newtonian-gravitation
definition: The process by which fluctuations in an infinite medium of size greater than a certain length scale (the Jeans length) grow by self-gravitation.
type: Concept

Irregular galaxies
definition: A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape and tend to smaller than others. Two types of irregular galaxies are defined, Irr I galaxy and Irr II galaxy.
related: Hubble classification scheme
type: Concept
related: De Vaucouleurs Sandage classification
broader: galaxies

Origin of the universe
type: Concept
broader: cosmology

Deprecated: FU Orionis Stars
deprecated: :__
use instead: FU Orionis stars
type: Concept

Opposition
definition: The position of a solar system body having its orbit outside that of the Earth when the Earth is in a line between the Sun and the body. At opposition the body has a solar elongation of 180°, and is closest to the Earth. It will, in principle, be visible throughout the night. It will rise in the east as the Sun sets in the west and it will set as the Sun rises. This is because, at opposition, the body and the Sun are 12 hours apart. The inner planets can never be in opposition. The opposite of opposition is conjunction.
type: Concept
broader: astronomical-location

Main sequence stars
broader: stellar-evolutionary-types
related: Beta Cephei variable stars
related: Main sequence
related: Dwarf stars
type: Concept

O dwarf stars
type: Concept
broader: o-stars
broader: dwarf-stars

Observational cosmology
broader: cosmology
broader: astronomical-methods
type: Concept
definition: The application of observational data to the study of the Universe as a whole.

Ursa Minor dwarf spheroidal galaxy
type: Concept
broader: dwarf-spheroidal-galaxies

Early-type stars
type: Concept
broader: stellar-types
definition: Hot, luminous stars of spectral type O, B, A, and F0 to F5. They were originally thought, wrongly, to be at an earlier stage of evolution than late-type stars.

Stellar surfaces
broader: stellar-structures
type: Concept
related: Solar surface

Star clusters
broader: stellar-astronomy
type: Concept
related: Field stars
definition: A group of stars held together by the mutual gravitational attraction of its members, which are physically related through common origin. They are of two types: open clusters and globular clusters.

Telescope focal plane photography
type: Concept
broader: astrophotography

Ultraviolet spectroscopy
type: Concept
broader: spectroscopy

LINER galaxies
broader: active-galaxies
broader: emission-line-galaxies
type: Concept

Astronomical techniques
type: Concept
broader: observational-astronomy

Conjunction
definition: A position of two bodies in the solar system when they have the same celestial longitude, seen from the Earth. The bodies can be a planet and the Sun, two planets or the Moon and a planet. The superior planets are in conjunction with the Sun, when, seen from the Earth, they are right behind the Sun. The inferior planets, such as Mercury and Venus, have two conjunctions with the Sun: inferior conjunction, when they are between the Earth and the Sun, and superior conjunction, when they are on the other side of the Sun.
broader: astronomical-location
type: Concept

Lightning
broader: planetary-atmospheres
definition: A flash of light produced by an electric discharge in response to the buildup of an electric potential between clouds and a planet's surface, or between different portions of the same cloud.
type: Concept

Gas-to-dust ratio
type: Concept
definition: The mass ratio of gas to dust. It amounts to approximately 100 in the interstellar medium, but may vary in molecular clouds and circumstellar disks due to dust grain evaporation, dust settling, condensation of gas, etc. The gas-to-dust ratio depends on the metallicity. It is larger in galaxies with lower metallicity.
broader: astronomical-models

Apparent superluminal motion
broader: celestial-mechanics
type: Concept

Galaxy circumnuclear disk
broader: galaxy-nuclei
type: Concept

Pickup ions
type: Concept
broader: interplanetary-physics

Radar astronomy
type: Concept
broader: astronomical-methods

Active solar chromosphere
type: Concept
broader: solar-chromosphere

Topocentric microlensing parallax
type: Concept
broader: microlensing-parallax

RRab variable stars
broader: rr-lyrae-variable-stars
type: Concept

Troposphere
type: Concept
broader: earth-atmosphere
definition: The lower part of the Earth's atmosphere in which temperature decreases with height, except for local areas of temperature inversion.

Galactic bar
broader: milky-way-galaxy-physics
related: Galaxy bars
type: Concept

Visual binary stars
broader: binary-stars
type: Concept

Galaxy distances
type: Concept
broader: galaxy-physics

RRc variable stars
type: Concept
broader: rr-lyrae-variable-stars

Plasma astrophysics
type: Concept
broader: cosmology
broader: high-energy-astrophysics
related: Plasma physics

Solar system evolution
type: Concept
broader: planetary-system-evolution
related: Solar system

Infrared spectroscopy
broader: spectroscopy
type: Concept

Mass-to-light ratio
type: Concept
broader: astronomical-models

Lunar evolution
broader: lunar-science
type: Concept

Quasars
type: Concept
definition: An compact, extragalactic object which is highly luminous and looks like a star. Their redshifts can be large and their brightness varies. Quasars have an intrinsic luminosity which can reach some 100 times that of bright galaxies. They are thought to be active galactic nuclei with a size a little larger than the solar system. The first quasar to be identified as such, in 1963, was the radio source 3C 273 at a redshift of 0.158. With its 13th magnitude, it is the optically brightest quasar as observed from Earth. Some quasars are strong radio sources.
related: Radio sources
broader: supermassive-black-holes
broader: active-galactic-nuclei

Lunar mineralogy
type: Concept
broader: lunar-surface

Transit photometry
broader: exoplanet-detection-methods
type: Concept
broader: photometry

Kron-Cousins photometry
broader: broad-band-photometry
type: Concept

Charge exchange ionization
broader: ionization
type: Concept

Haro galaxies
type: Concept
definition: A type of galaxies characterized by strong emission in the blue and violet regions of the spectrum. They are often elliptical or lenticular.
broader: active-galaxies

Interplanetary medium
related: Asteroids
definition: The material contained in the solar system in the space through which the planets travel. It consists of the smaller objects such as asteroids, comets, meteorites and also a general pervading interplanetary dust. Moreover a plasma of charged particles streaming outward from the Sun in the form of the solar wind contributes to the interplanetary medium.
broader: solar-system
type: Concept
related: Comets

Circumstellar disks
type: Concept
broader: circumstellar-matter
definition: Any concentration of material in the form of a disk orbiting around a star.

Cooling flows
type: Concept
broader: intracluster-medium
definition: A phenomenon observed in a cluster of galaxies, whereby the cluster core loses energy via X-ray radiation because of the collisions between the gas particles. The radiation rate is proportional to the square of the density, and the cooling time, which remains in the outer parts too large, becomes smaller than the Hubble time in the core. As a result, the central regions of clusters of galaxies cool down; and since in the center of a cluster gas pressure and gravitational attraction are in equilibrium, the gas density has to rise to maintain the pressure necessary for supporting the outer layers of gas. To cause its density to rise, the cooled gas has to flow inward. As the densest gas, which cools quickest, is already concentrated in the center of the cluster, the inward flow will start at the center, soon followed by the outer layers. This flow of gas is called the cooling flow. Cooling flows are moderated through feedback due to the supermassive black hole in the nucleus of the central galaxy. The gas inflow to the center fuels the active galactic nucleus (AGN). The latter then heats again the gas through its radio jets.
broader: cool-intergalactic-medium

Lithosphere
broader: planetary-structure
type: Concept
definition: The solid portion of the Earth, as compared to the atmosphere and the hydrosphere. The lithosphere consists of semi-rigid plates that move relative to each other on the underlying asthenosphere. The process is known as plate tectonics and helps explain continental drift.

Cloud monitors
broader: all-sky-cameras
type: Concept

Gaussian mixture model
broader: mixture-model
type: Concept

Photometric standard stars
broader: photometry
type: Concept
broader: standard-stars

Zeeman-Doppler imaging
type: Concept
broader: doppler-imaging

Cavity-enhanced Fourier-transform
type: Concept
broader: microwave-spectroscopy
broader: millimeter-wave-spectroscopy

Cosmic ray sources
type: Concept
broader: cosmic-rays

Oort cloud
definition: A huge theoretical cloud of comets surrounding the Sun between about 50,000 and 100,000 astronomical units and thought to be the reservoir of long-period comets entering the inner solar system.
type: Concept
broader: solar-system
broader: long-period-comets

Theoretical models
type: Concept
broader: laboratory-astrophysics

Field galaxies
broader: galaxies
related: Field of view
definition: A galaxy that lies in the direction of a cluster of galaxies, but is not a member of the cluster. Field galaxies are rare, less than about 5% of all galaxies.
type: Concept

Hilda group
type: Concept
broader: asteroids
definition: The asteroids found on the outer edge of the main asteroid belt in a 2:3 orbital resonance with Jupiter. The group is not an asteroid family since the members are not physically related. The group consists of asteroids with semi-major axes between 3.70 AU and 4.20 AU, eccentricities less than 0.30, and inclinations less than 20°. It is dominated by D- and P-type asteroids.

Space vehicle instruments
type: Concept
broader: space-vehicles

Gamma-rays
broader: particle-astrophysics
definition: An electromagnetic wave with a typical wavelength less than 10^-2 Å (10^-12 m), corresponding to frequencies above 10^19 Hz and photon energies above 100 keV.
type: Concept

Cosmic ray astronomy
related: Cosmic rays
broader: astronomical-methods
type: Concept

English mounts
deprecated: :__
type: Concept

Beta Lyrae stars
broader: ellipsoidal-variable-stars
type: Concept
broader: eclipsing-binary-stars

Chlorofluorocarbons
broader: greenhouse-gases
type: Concept

Galaxy masses
type: Concept
related: Milky Way mass
broader: galaxy-properties

Haumea
deprecated: :__
type: Concept

Radar telescopes
type: Concept
broader: radio-telescopes

Flare stars
broader: eruptive-variable-stars
type: Concept
definition: A member of a class of dwarf stars that undergoes sudden, intense outbursts of light (mean amplitude about 0.5-0.6 mag).

Zodiacal dust bands
type: Concept
broader: zodiacal-cloud

Spokes
broader: planetary-rings
definition: In Saturn's rings, changing structures in the radial direction. It is thought that gravitational forces alone cannot account for the spoke structure, and it has been proposed that electrostatic repulsion between ring particles may play a role.
type: Concept

Hierarchical models
broader: bayesian-statistics
type: Concept

Stellar magnetic fields
broader: asteroseismology
broader: stellar-properties
related: Solar magnetic fields
type: Concept

Plasmapause
definition: The sharp outer boundary of the plasmasphere, at which the plasma density decreases by a factor of 100 or more.
broader: planetary-magnetosphere
type: Concept

Planetary science
related: Remote sensing
broader: solar-system-astronomy
definition: The branch of astronomy that deals with the science of planets, or planetary systems, and the solar system.
type: Concept

Stellar granulation
broader: stellar-physics
type: Concept
related: Solar granulation

O subgiant stars
type: Concept
broader: o-stars
broader: subgiant-stars

Double quasars
broader: quasars
type: Concept

Big Bang theory
broader: origin-of-the-universe
definition: A theory which states that the Universe came into existence in an 'instantaneous' event some 14 billion years ago. Matter was created in that initial event and as time has gone by the Universe has expanded and the contents evolved into the galaxies and stars and of today. The Big Bang is sometimes described as an 'explosion.' However, matter and energy did not erupt into a pre-existing space, since they came into being simultaneously with space and time.
broader: cosmology
type: Concept

Lemaitre universe
type: Concept
broader: cosmological-models

Bautz-Morgan type
broader: galaxy-clusters
type: Concept

Visual observation
deprecated: :__
type: Concept

Ultraviolet sources
related: Ultraviolet astronomy
type: Concept
broader: astronomical-radiation-sources

Exozodiacal dust
type: Concept
broader: debris-disks
related: Zodiacal cloud

Hour angle
definition: A telescope based coordinate specifying the angle, in the equatorial plane, from the meridian to a plane containing the celestial object and the north and south celestial poles.
broader: equatorial-coordinate-system
type: Concept

Galaxy formation
broader: galaxy-physics
type: Concept
definition: The study dealing with the processes that gave rise to galaxies in a remarkably early Universe.
related: Milky Way formation

Astroinformatics
broader: interdisciplinary-astronomy
type: Concept

Bp stars
broader: magnetic-stars
type: Concept
broader: b-stars
broader: chemically-peculiar-stars

Bouwers telescope
type: Concept
broader: catadioptric-telescopes

Ceres
definition: Once qualified as the largest known asteroid, Ceres is now classified as a dwarf planet (2006 IAU General Assembly). It is approximately 950 km across, and resides with tens of thousands of asteroids in the main asteroid belt; it is the largest body of the belt. Its mass is 9.4 x 10^20 kg, its rotation period 9.074 hours, its orbital period 4.60 years, and its semi-major axis 2.767 AU. NASA's Dawn spacecraft, which was placed in orbit around Ceres in 2015, has mapped its surface in great detail. Dawn discovered very bright spots, which reflect far more light than their much darker surroundings. The most prominent of these spots lie inside the crater Occator and suggest that Ceres may be a much more active world than most of its asteroid neighbors.
broader: dwarf-planets
broader: main-belt-asteroids
type: Concept

Gamma-ray transient sources
type: Concept
broader: transient-sources
related: Gamma-ray bursts
broader: gamma-ray-sources

Small Magellanic Cloud
type: Concept
definition: An irregular galaxy, the smaller of the two Magellanic Clouds that are satellites of our own Galaxy, lying in the southern constellation Tucana about 20 degrees from the south celestial pole. The SMC covers an area roughly 3 by 5 degrees in dimension and has an overall visual magnitude about +2.7. The SMC is about 10,000 light-years in diameter and some 210,000 light-years (61 kpc) away. It has a visible mass of about 1/50-th that of our Galaxy and 1/10-th of that of the Large Magellanic Cloud (LMC). Its heavy element content is about a factor 5 smaller than that of the Galaxy. The SMC is the third-nearest external galaxy after the Sagittarius Dwarf Spheroidal Galaxy and the LMC.
broader: magellanic-clouds
broader: magellanic-irregular-galaxies

Polar jets
type: Concept
broader: radio-jets

Optical observation
type: Concept
broader: astronomical-techniques

Heliosheath
type: Concept
definition: The region located between the termination shock and the heliopause where the turbulent and hot solar wind is compressed as it passes outward against the interstellar wind.
broader: heliosphere

Solar mass
definition: The amount of mass in our Sun, about 330,000 times the Earth's mass. The solar mass is also the unit in which the masses of other stars, galaxies, and other large celestial bodies are expressed.
related: Stellar masses
broader: solar-physics
type: Concept

Dust physics
type: Concept
broader: solid-matter-physics

Potential energy surfaces
type: Concept
broader: quantum-chemical-calculations

Exobiology
use instead: Astrobiology
type: Concept
deprecated: :__

Stellar chemical composition
deprecated: :__
type: Concept
use instead: Stellar abundances

Stellar parallax
type: Concept
definition: The apparent difference in the position of a celestial object as seen by an observer from two widely separated locations. The parallax of an object can be used to derive its distance. The relationship between the parallax angle p (measured in seconds of arc) and the distance d (measured in astronomical units) is given by d = 206,264 / p. For a parallax angle p = 1'', the distance to the star would correspond to 206,264 AU. By convention, the distance unit parsec is defined to be equivalent to 206,264 AU. Therefore, the parallax relation takes the much simpler form: d (in pc) = 1/p (in seconds of arc). The first star whose parallax was measured was 61 Cygni (Bessel, 1838).
broader: parallax
related: Stellar distance

Stellar composition
use instead: Stellar abundances
deprecated: :__
type: Concept

Subgiant stars
definition: A star with a lower absolute magnitude than a normal giant star of its spectral type. It is a star that has exhausted the hydrogen at its center and is evolving into a red giant. Subgiants are luminosity class IV.
broader: evolved-stars
type: Concept

Extragalactic magnetic fields
type: Concept
broader: extragalactic-astronomy

UU Herculis stars
type: Concept
broader: srd-variable-stars
broader: f-supergiant-stars
broader: double-mode-variable-stars

Nucleosynthesis
definition: The process by which nuclear reactions at very high temperatures and pressures produce the various chemical elements of the periodic table, either in the Big Bang or in stellar interiors.
broader: nuclear-astrophysics
type: Concept

Hubble radius
type: Concept
broader: galaxy-distances
definition: The size of the observable Universe as derived from the ratio c/H_0, where H_0 is the Hubble-Lemaitre constant and c the speed of light.

General relativity
broader: relativity
broader: gravitation
definition: The theory of gravitation developed by Albert Einstein (1916) that describes the gravitation as the space-time curvature caused by the presence of matter or energy. Mass creates a gravitational field which distorts the space and changes the flow of time. In other words, mass causes a deviation of the metric of space-time continuum from that of the 'flat' space-time structure described by the Euclidean geometry and treated in special relativity. General relativity developed from the principle of equivalence between gravitational and inertial forces. According to general relativity, photons follow a curved path in a gravitational field. This prediction was confirmed by the measurements of star positions near the solar limb during the total eclipse of 1919. The same effect is seen in the delay of radio signals coming from distant space probes when grazing the Sun's surface. Moreover, the space curvature caused by the Sun makes the perihelion of Mercury's orbit advance by 43'' per century more than that predicted by Newton's theory of gravitation. The perihelion advance can reach several degrees per year for binary pulsar orbits. Another effect predicted by general relativity is the gravitational reddening. This effect is verified in the redshift of spectral lines in the solar spectrum and, even more obviously, in white dwarfs. Other predictions of the theory include gravitational lensing, gravitational waves, and the invariance of Newton's gravitational constant.
type: Concept

Lunar surface
related: Natural satellite surfaces
broader: selenology
type: Concept

Software tutorials
broader: software-documentation
type: Concept

Sundivers
broader: near-sun-comets
type: Concept
definition: Comets with orbits that intersect the solar photosphere.

Hirayama families
type: Concept
broader: asteroids

Long period comets
definition: A comet with orbital period of more than 200 years.
type: Concept
broader: comets

S Vulpeculae stars
deprecated: :__
type: Concept

Dimensionality reduction
broader: multivariate-analysis
type: Concept

X-ray bursters
type: Concept
broader: x-ray-binary-stars
broader: x-ray-transient-sources
related: X-ray bursts

Novae
broader: cataclysmic-variable-stars
type: Concept
related: Optical bursts
broader: stellar-types
definition: A star that experiences a sudden increase in luminosity, by a s much as 10^6. The outburst ejects a shell of matter but does not disrupt the star.
broader: stellar-remnants

Superbubbles
type: Concept
broader: interstellar-medium
definition: A cavity hundreds of light-years across filled with a hot gas blown into the interstellar medium by multiple supernovae and stellar winds. Examples are the Local Bubble in the Orion Arm of the Milky Way and the N44 Superbubble in the Large Magellanic Cloud.

L galaxies
type: Concept
broader: low-surface-brightness-galaxies
definition: In Morgan's classification, an elongated galaxy of low surface brightness.

Planetary structure
type: Concept
broader: planetary-science
related: Exoplanet structure

X-ray astronomy
related: X-ray sources
broader: astronomical-methods
type: Concept
definition: The study of celestial bodies using their X-ray emission. X-ray astronomy deals mainly with Galactic and extragalactic phenomena involving very high-energy photon emissions, covering a band of energies between 0.1 keV and 500 keV. The research field includes: X-ray binaries, cataclysmic variables, pulsars, black holes, dark matter, active galaxies, galactic clusters X-ray transients. The Earth's atmosphere absorbs most X-rays coming from outer space. X-ray astronomy therefore requires observations to be done above atmosphere. The first rocket flight which successfully detected a cosmic source of X-ray emission was launched in 1962 by an American research group. A very bright source was detected that they named Scorpius X-1. Since then several dedicated X-ray astronomy satellites have been launched, among which: Uhuru, INTEGRAL, ROSAT, Rossi X-ray Timing Explorer (RXTE), Chandra X-ray Observatory, and XMM-Newton, which have contributed to important advances in astronomy.

Schwarzschild metric
type: Concept
definition: In general relativity, the metric that describes the space-time outside a static mass with spherically symmetric distribution.
broader: spacetime-metric

The Moon
related: Lunar science
related: Earth (planet)
broader: natural-satellites-solar-system
definition: Natural satellite of the Earth. Mass 7.35 x 10^25 g = 1/81 or 0.0123 Earth's. Mean radius 1740 km = ~ 1/4 the Earth's; this relatively small size ratio makes the Earth-Moon system unique in the solar system. Mean density 3.34 g cm-3. Mean distance from Earth 384,400 km. Escape velocity 2.38 km s-1. Surface gravity 162.2 cm s-2 = 0.165 Earth's. Sidereal period 27d 7h 43m 11s. Eccentricity 0.0549. Inclination of orbital plane to ecliptic 5° 8' 43''. Obliquity 6° 41'. Synodic period 29d 12h 44m 2s.9. Orbital velocity 1.02 km s-1. The Moon's average visual albedo is 0.12, a factor of three smaller than that of Earth. The Moon's center of mass is displaced about 2 km in the direction of Earth. The average temperature on the surface of the Moon during the day is 107 °C. During the night, the average temperature drops to -153 °C. Studies of lunar rock have shown that melting and separation must have begun at least 4.5 x 109 years ago, so the crust of the Moon was beginning to form a very short time after the solar system itself. Thickness of crust ~ 60 km; of mantle ~ 1000 km. Temperature of core ~ 1500 K. It would have taken only 107 years to slow the Moon's rotation into its present lock with its orbital period. Because of this synchronous rotation, the Moon revolves once on its axis each time it orbits the Earth, thus always presenting the same face, the nearside, toward Earth. The Moon may have formed during a collision between the early Earth and a Mars-sized rocky planet about 4.6 billion years ago; Theia.
type: Concept
broader: solar-system

Gravitational wave sources
type: Concept
broader: gravitational-waves

Astronomers
broader: astronomical-research
definition: A person who practices astronomy as a profession rather than as a hobby, in contrast to an amateur astronomer.
type: Concept

Schwarzschild black holes
broader: nonrotating-black-holes
type: Concept
definition: A black hole with zero angular momentum (non-rotating) and zero electric charge derived from Karl Schwarzschild 1916 exact solution to Einstein's vacuum field equations.
broader: uncharged-black-holes

Particle astrophysics
type: Concept
broader: high-energy-astrophysics
related: Dark matter
related: Particle physics

Lunar regolith
related: Regolith
broader: lunar-mineralogy
type: Concept

Near ultraviolet astronomy
type: Concept
broader: ultraviolet-astronomy

Galaxy chemical composition
use instead: Galaxy abundances
type: Concept
deprecated: :__

Triangulum Galaxy
broader: spiral-galaxies
type: Concept
definition: One of the prominent members of the Local Group situated in the constellation Triangulum. Also known as NGC 598. M33 is a type Sc spiral galaxy seen nearly face-on. It lies 2.8 million light-years away and its diameter is 52,000 light-years. M33 is thought to be a satellite of the Andromeda Galaxy.
broader: local-group

Mantle
definition: A major subdivision of a planet's internal structure, located beneath the planet's crust and above the central core.
broader: planetary-structure
type: Concept

Close binary stars
type: Concept
broader: binary-stars
definition: A binary system in which the separation of the component stars is comparable to their diameters, so that they influence each other's evolution most commonly by the tidal forces.

Pyrolysis
type: Concept
broader: production-techniques

Reddened stars
definition: A star whose light has undergone reddening.
type: Concept
broader: interstellar-extinction

Kuiper belt
type: Concept
broader: solar-system
definition: A region of the Solar System extending roughly from the orbit of Neptune, or 30 astronomical units (AU), to 50 AU from the Sun that contains many small icy bodies. The Kuiper belt is now considered to be the source of the short-period comets.

Sky brightness
type: Concept
definition: Atmospheric (airglow, auroral emission, artificial light) or extraterrestrial (scattered sunlight from Moon, scattered starlight, interplanetary dust) foreground light that interferes with observations.
broader: atmospheric-effects

Deprecated: Gamma ray telescopes
use instead: Gamma-ray telescopes
deprecated: :__
type: Concept

Pulsation frequency method
type: Concept
broader: timing-variation-methods

Comet nuclei
broader: comets
definition: The solid, centrally located part of a comet. The nucleus is a mass of dust and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma, which is later swept into an elongated tail. Reliable measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km. The nucleus of Comet Hale-Bopp is one of the largest (perhaps 40 km). The composition of the nucleus is determined by measuring the composition of the coma (except for 67P/Churyumov-Gerasimenko). The dominant volatile is water, followed by carbon dioxide, carbon monoxide, formaldehyde, methanol, methane at a few percent level (with respect to water) and many other molecules at a lower level.
type: Concept

Stellar rotation
definition: The spinning of a star about its axis, due to its angular momentum. Stars do not necessarily rotate as solid bodies, and their angular momentum may be distributed non-uniformly, depending on radius or latitude. Thus the equator of the star can rotate at a different angular velocity than the higher latitudes. These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field.
related: Solar rotation
type: Concept
related: von Zeipel theorem
broader: stellar-properties

Milky Way rotation
broader: milky-way-galaxy-physics
type: Concept
related: Galaxy rotation

Computational astronomy
broader: interdisciplinary-astronomy
broader: astronomical-methods
type: Concept
related: Analytical mathematics

Red giant stars
type: Concept
related: Red giant branch
definition: A certain star of spectral type K or later that occupies the upper right portion of the H-R diagram. Red giants are evolved stars that have exhausted their hydrogen fuel in the core. They may have a luminosity up to 1000 times greater than main sequence stars of the same spectral type. Red giants belong to the luminosity class III or II (bright giants). They are luminous because of their great size, but have a relatively low surface temperature. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolution. When a main sequence star has converted approximately 10% of its hydrogen to helium, nuclear reactions in the core stop (Schönberg-Chandrasekhar limit). The hydrostatic equilibrium is no longer maintained, and the core contracts while the outer layers expand and cool. This process produces the low surface temperature and large size (from 10 to 100 times that of the Sun) that characterize the red giant. In the core the temperature continues to rise. When it approaches 100,000,000 K helium will begin to fuse into carbon. Prominent bright red giants in the night sky include Aldebaran and Arcturus.
broader: giant-stars
related: Cocoon stars

Reflection nebulae
definition: A type of nebula that is visible from its reflection of starlight. Bright stars near reflection nebulae emit light into the region that is reflected by the large amount of dust there. The size of the dust grains causes blue light to be reflected more efficiently than red light, so these reflection nebulae frequently appear blue in color.
related: R associations
broader: nebulae
type: Concept

Intracluster medium
definition: A diffuse, hot, magnetized plasma that exists between galaxies in a galaxy cluster and is composed mainly of H, He, and heavy elements. The Intracluster medium (ICM) strongly emits X-rays, making it the most luminous extended X-ray source in Universe. While some of the gas has been stripped out of galaxies, it is also likely that some is also primordial in nature, and has been accreted into the clusters. The origin of the ICM is subject of intense investigation. Broadly, two possibilities have been envisaged. The first one considers the intracluster gas to be once contained in galaxies and later driven in the ICM. This would explain several observations: the presence of high metallicity gas, and H I deficiency of galaxies residing in the cores of rich clusters (which suggests that gas stripping has occurred). Alternatively, the ICM is primordial, originating at the time of cluster formation. Actually the ICM may result from a combination of both scenarios.
related: Intergalactic medium
broader: high-energy-astrophysics
type: Concept
broader: galaxy-clusters

M dwarf stars
broader: late-type-dwarf-stars
type: Concept
broader: m-stars
definition: A star whose spectrum is dominated by the absorption bands of titanium oxide (TiO) and vanadium oxide (VO) and has many neutral metal lines. The effective temperature of M dwarfs ranges from about 3850 to 2600 K. They are low mass stars with masses ranging from 0.6 times that of the Sun at spectral type M0 to less than 0.1 solar masses. M dwarfs are very abundant, they account for about 70-80% of stars in the Galactic disk. The nearest star to the Sun, Proxima Centauri, is an M dwarf.
broader: dwarf-stars

Gödel universe
type: Concept
broader: cosmological-models

Galaxy jets
type: Concept
broader: galaxy-properties
broader: jets

Lunar transient phenomena
definition: A short-lived change in the brightness of patches on the face of the Moon. The TLPs last from a few seconds to a few hours and can grow from less than a few to a hundred kilometers in size. They have been reported by many observers since the invention of the telescope. However, the physical mechanism responsible for creating a TLP is not well understood. Several theories have been proposed, among which lunar outgassing, that is, gas being released from the surface of the Moon.
broader: lunar-features
type: Concept

Light year
definition: The distance that light travels in one year at about 300,000 km per second, i.e. 9.5 x 10^12 km. It is equal to about 63,000 astronomical units.
broader: distance-measure
type: Concept

Stellar classification
related: G stars
related: M stars
related: A stars
type: Concept
related: Wolf-Rayet stars
related: O stars
related: B stars
related: F stars
definition: The systematic grouping of stars and stellar objects into categories on the basis of physical, morphological, or evolutionary characteristics.
related: OB stars
related: K stars
broader: classification-systems

De-excitation rates
broader: molecular-physics
broader: atomic-physics
type: Concept

Uncertainty bounds
type: Concept
broader: astrostatistics-techniques

Equatorial zone
broader: equatorial-coordinate-system
type: Concept

N stars
type: Concept
broader: carbon-stars
broader: late-type-stars

Single x-ray stars
type: Concept
broader: x-ray-stars

Xallarap effect
broader: binary-source-microlensing
type: Concept

Phase angle
broader: astrometry
definition: For an object in the solar system, the angle "Sun-object-Earth" that is, the angle between the Sun and the observer as seen from the given object. It is 0° when the object is fully illuminated, 90° when the object is half-illuminated (like the Moon at first quarter and last quarter), and 180° when the object is between Earth and the Sun.
type: Concept

Solar dynamo
broader: solar-cycle
definition: A model for explaining the generation of the solar magnetic field and the related observational features (mainly solar cycle, Sporer's law, Hale's law, Joy's law, polarity reversal). The global frame of this model is the interaction between a turbulent plasma in the convective zone (reciprocal generation of magnetic and electric fields) and the solar differential rotation (mutual transformation of meridional magnetic field into azimuthal magnetic field).
broader: solar-magnetic-fields
type: Concept

Atmospheric effects
type: Concept
broader: atmospheric-science

Open star clusters
type: Concept
definition: A loose grouping of dozens or hundreds of young stars distributed in a region a few light-years across. Open clusters are relatively young, typically containing many hot, highly luminous stars. They are located within the disk of the Galaxy, whence their older name Galactic clusters.
broader: star-clusters

Spiral pitch angle
broader: galaxy-structure
type: Concept

Cepheid variable stars
definition: A class of luminous, yellow supergiants that are pulsating variables and whose period of variation is a function of their luminosity. These stars expand and contract at extremely regular periods, in the range 1-50 days. Their highest brightness and surface temperature occur when their expansion velocity is greatest. Similarly, their minima in brightness and temperature occur when they are in the contraction phase. The longer the period, the more luminous the star. The process that drives the pulsation of Cepheid variables is the kappa mechanism. In fact, Cepheids provide one of the most powerful tools for measuring distances to other galaxies (period-luminosity relation). However, this method is limited to the distance of the Virgo cluster of galaxies (15-20 Mpc) even with the HST or the largest ground-based telescopes. One particularly special Cepheid is the North Star, Polaris.
type: Concept
broader: pulsating-variable-stars

Hubble-Lemaitre law
broader: origin-of-the-universe
type: Concept
definition: The speed with which a galaxy cluster recedes from us is directly proportional to its distance. It can be stated as v = H_{0}d, where v is the recessional velocity, H_0 the Hubble-Lemaitre constant, and d the distance.

Relativistic mechanics
broader: relativity
type: Concept

Chromatic aberration
broader: optical-aberration
type: Concept

Orbital anomalies
type: Concept
broader: orbital-elements

Dust composition
broader: dust-physics
type: Concept

CN stars
broader: giant-stars
type: Concept

Polytropes
broader: stellar-structures
type: Concept

Intergalactic dust clouds
broader: intergalactic-clouds
type: Concept

Gravitational collapse
broader: general-relativity
type: Concept
definition: Collapse of a mass of material as a result of the mutual gravitational attraction of all its constituents.

Meyer group
type: Concept
broader: sunskirters

Radio spectroscopy
related: Radio astronomy
type: Concept
broader: spectroscopy

Protostars
broader: stellar-types
type: Concept
broader: interstellar-medium
definition: A stage in the process of star formation, after the gravitational collapse of the dense pre-stellar core and before the initiation of nuclear fusion in the central object which will eventually become a star. Protostars are classified into four groups: Class 0, Class I, Class II, and Class III.

A dwarf stars
broader: dwarf-stars
broader: a-stars
type: Concept

Pogson scale
type: Concept
broader: apparent-magnitude

Meteorites
type: Concept
related: Impact phenomena
related: Meteors
definition: An object of extraterrestrial origin that survives entry through the atmosphere to reach the Earth's surface. Meteors become meteorites if they reach the ground.
broader: meteoroids

Solar interior
related: Stellar interiors
related: Helioseismology
type: Concept
broader: solar-physics

Interstellar cyclotron emission
broader: interstellar-emissions
type: Concept

Star formation
broader: stellar-physics
definition: The process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy, star formation includes the study of the interstellar medium and molecular clouds as precursors to the star formation process as well as the study of young stellar objects.
type: Concept

G dwarf stars
broader: dwarf-stars
broader: g-stars
type: Concept

Deprecated: Long period variable stars
type: Concept
use instead: Long period variable stars
deprecated: :__

Infrared Astronomical Satellite
broader: space-observatories
type: Concept
broader: infrared-observatories

Asymptotic giant branch stars
related: Asymptotic giant branch
type: Concept
broader: stellar-evolutionary-types

X-ray sources
related: X-ray binary stars
related: Solar x-ray flares
type: Concept
broader: astronomical-radiation-sources
definition: An astronomical object whose dominant mechanism of radiation is through X-ray emission. X-ray sources contain an extremely hot gas at temperatures from 10^6 to 10^8 K. They are generated by various physical processes involving high energies, such as accretion on to a compact object, shock waves from supernovae, stellar winds, hot gas in stellar coronae, or hot spaces between galaxies in a cluster. The first celestial X-ray source, after the Sun, to be detected was Scorpius X-1 by means of rocket flight.
related: X-ray astronomy

X-ray surveys
type: Concept
broader: surveys

Circumstellar clouds
broader: circumstellar-matter
type: Concept

Beta Cephei variable stars
broader: multi-periodic-variable-stars
broader: pulsating-variable-stars
broader: short-period-variable-stars
related: Main sequence stars
type: Concept
broader: early-type-variable-stars

Stellar cores
broader: stellar-interiors
related: Solar core
type: Concept

Short period comets
broader: comets
definition: A comet with a period less than 200 years.
type: Concept

Quiescent solar prominence
broader: solar-filaments
related: Quiet sun
type: Concept
broader: solar-prominences

Stellar aberration
broader: astrometry
type: Concept

Symbiotic novae
type: Concept
broader: symbiotic-binary-stars
broader: novae

DC stars
broader: white-dwarf-stars
definition: A white dwarf showing a continuous spectrum with no readily apparent lines.
type: Concept

Telescopes
definition: An instrument used to collect and amplify light or other energy. Refracting telescopes gather light by means of a lens, reflecting telescopes by means of a mirror. Radio telescopes gather radio energy by using an antenna. Telescopes have also been built that can gather X rays, gamma rays, and other forms of energy.
type: Concept
broader: astronomical-instrumentation

Astrostatistics distributions
type: Concept
broader: astrostatistics

Red dwarf flare stars
type: Concept
broader: flare-stars

B subgiant stars
broader: subgiant-stars
broader: b-stars
type: Concept

Barium stars
type: Concept
broader: heavy-metal-stars
definition: A type of star, usually G or K giants, whose spectra show unusually strong absorption lines of barium, strontium, and other s-process elements.

Helium-rich stars
broader: chemically-peculiar-stars
type: Concept

X-ray point sources
type: Concept
broader: x-ray-sources

Cepheid distance
broader: stellar-distance
type: Concept

Gnomons
broader: sundials
type: Concept

Lunar features
type: Concept
broader: selenology

Solar meridional circulation
type: Concept
broader: solar-interior

H-alpha stars
type: Concept
broader: emission-line-stars

Relativistic cosmology
type: Concept
broader: cosmology

Spiral galaxies
related: De Vaucouleurs Sandage classification
broader: disk-galaxies
related: Hubble classification scheme
type: Concept
definition: A galaxy with a prominent nuclear bulge and luminous spiral arms of gas, dust, and young stars that wind out from the nucleus. Masses span the range from 10^10 to 10^12 solar masses.

Strong gravitational lensing
broader: gravitational-lensing
type: Concept
definition: A gravitational lensing phenomenon in which the image distortion is strong enough to be readily recognized, such as in the case of the Einstein cross or when giant luminous arcs show up in galaxy clusters (e.g. Abell 2218). Opposite to weak gravitational lensing.

Orbit determination
type: Concept
broader: orbits

Chi-squared statistic
type: Concept
broader: astrostatistics-tools

Binary pulsars
type: Concept
broader: pulsars
broader: compact-binary-stars

Saturnian satellites
related: Saturn
type: Concept
broader: natural-satellites-solar-system

Penrose limit
broader: geodesics
type: Concept

Galaxy structure
type: Concept
broader: galaxy-physics

Kreutz group
broader: sungrazers
type: Concept

Adaptive optics
type: Concept
broader: astronomical-optics

Hertzsprung gap
broader: hertzsprung-russell-diagram
definition: A region of the Hertzsprung-Russell diagram, between the main sequence and the giant branch, occupied by very few stars. It corresponds to a very short period in stellar evolution.
type: Concept

DDO classification
type: Concept
broader: galaxy-classification-systems

Triple lens microlensing
broader: gravitational-microlensing
type: Concept

Optical constants (Dust)
broader: dust-physics
type: Concept

Horizontal coordinate system
type: Concept
definition: The coordinate system based on the position of the observer. The horizontal plane is the fundamental plane and the coordinates are altitude and azimuth.
broader: astronomical-coordinate-systems

Inclination
broader: orbital-elements
type: Concept
definition: An orbital element that defines the angle between the orbital plane of a solar system body (planet, comet, asteroid) and the plane of the ecliptic.

K supergiant stars
broader: late-type-supergiant-stars
broader: k-stars
type: Concept
related: Red supergiant stars

Lunar impacts
type: Concept
broader: lunar-surface
related: Impact phenomena

Solar x-ray flares
related: X-ray sources
broader: solar-flares
type: Concept

Mira variable stars
type: Concept
broader: pulsating-variable-stars
broader: emission-line-stars
broader: late-type-giant-stars
broader: long-period-variable-stars
related: Me stars

X-ray bursts
type: Concept
broader: x-ray-sources
related: X-ray bursters
related: X-ray transient sources
definition: A rapid and intense surge of X-ray emission from some sources. They often last less than one second followed by an exponential decrease of typically a few seconds to a minute. Most X-ray bursts are believed to arise in X-ray binary systems due to nuclear fusion of material accreted onto a compact companion.
broader: burst-astrophysics

M subgiant stars
type: Concept
broader: m-stars
broader: subgiant-stars

Radio astronomy
related: Radio spectroscopy
broader: astronomical-methods
type: Concept
definition: The branch of astronomy that deals with the study of the Universe by means of radio waves.
related: Radio sources

X-ray identification
type: Concept
broader: x-ray-astronomy

Young stellar objects
related: Stellar evolution
related: Pre-main sequence stars
definition: Any star that has evolved past the protostar stage, but has not yet arrived on the main sequence. There is a variety of YSOs depending on their age, mass, and environment, including Herbig stars, T Tauri stars, and, in general, compact infrared sources embedded in molecular clouds.
broader: protostars
type: Concept

Hardness ratio
broader: x-ray-photometry
type: Concept

Thermal properties (Ice)
type: Concept
broader: ice-physics

Relaxation time
broader: astrometry
type: Concept
definition: The characteristic length of time that is required for a system undergoing relaxation to move to its equilibrium state. If the system follows an exponential law G = G_0 exp(-t / τ), the relaxation time is the time required for G to obtain the fraction 1/e of its initial value G_0.

Kinematic parallax
broader: parallax
type: Concept

GPU computing
broader: computational-methods
type: Concept

Multiple stars
broader: stellar-types
type: Concept

Interplanetary magnetic fields
related: Interplanetary physics
broader: planetary-science
definition: The magnetic field that is carried along with the solar wind and fills the solar system space. It is wound into a spiral structure by the rotation of the Sun. At the Earth's distance from the Sun, it has a strength of about 5 x 10^-5 gauss.
type: Concept

Marsden group
type: Concept
broader: sunskirters

Submillimeter astronomy
type: Concept
definition: The study of astronomical objects with submillimeter waves. As with millimeter-wave astronomy, this part of the spectrum is rich in lines emitted by interstellar molecules and dust.
broader: radio-astronomy

Annular eclipses
definition: A solar eclipse in which the Moon is close the apogee and is, therefore, too small to cover the whole disk of the Sun, leaving a visible edge or ring of sunlight. An annular eclipse can last for 12m 30s at the most.
broader: eclipses
type: Concept

Satellite microlensing parallax
broader: microlensing-parallax
type: Concept

Oe stars
broader: o-stars
broader: early-type-emission-stars
type: Concept

Galaxy stellar halos
related: Milky Way stellar halo
type: Concept
broader: galaxy-structure

Elliptical galaxies
definition: A galaxy whose structure is smooth without spiral arms and ellipsoidal in shape. Ellipticals are redder than spirals of similar mass. Giant ellipticals contain over 1012 solar masses, whereas dwarf ellipticals have masses as low as 107 solar masses.
related: Hubble classification scheme
related: De Vaucouleurs Sandage classification
broader: galaxies
related: Galaxy classification systems
type: Concept

Stellar motion
type: Concept
related: Solar motion
broader: stellar-kinematics

Magnetogram
type: Concept
broader: astronomical-techniques

Surface variability
type: Concept
related: Exoplanet surface variability
broader: planetary-surfaces

Moldavites
broader: tektites
type: Concept

Atmospheric tides
broader: planetary-atmospheres
broader: tides
type: Concept
broader: earth-atmosphere

Compact radiation sources
broader: astronomical-radiation-sources
related: Compact objects
type: Concept

Planetary system evolution
type: Concept
broader: planetary-science

Lunar domes
type: Concept
definition: Rounded volcanic structure on the Moon produced by eruption or intrusion of viscous high-silica magma. An uplift that is round or elliptical in map view of the Moon with beds dipping away in all directions from a central point.
broader: lunar-features

Solar faculae
related: Stellar faculae
broader: solar-physics
type: Concept
definition: A bright area of the photosphere of the Sun visible in white light and best seen near the solar limb, although they occur all across the Sun. Faculae raise several hundred kilometers above the photosphere and are associated with sunspots. They often appear immediately before the formation of a sunspot group and remain visible for several days or weeks after the disappearance of the spots. Faculae are formed when a strong magnetic field heats a region of the photosphere to higher temperatures than the surrounding area.

Syzygy
definition: The alignment of three or more celestial bodies in the same gravitational system along a nearly straight line. Conjunction or opposition of a heavenly body with the Sun.
type: Concept
broader: astrometry

Zenith tubes
broader: early-astronomical-instruments
broader: transit-instruments
type: Concept

Cosmic electrodynamics
broader: plasma-astrophysics
type: Concept

Intermediate-type stars
broader: stellar-types
type: Concept

Gould Belt
broader: milky-way-galaxy-physics
type: Concept

Apastron
definition: The point at which a binary star is furthest from its companion.
type: Concept
broader: apsides

Experimental Models
deprecated: :__
type: Concept

Binary stars
broader: multiple-stars
related: Optical bursts
type: Concept
related: Optical double stars
definition: Two stars gravitationally bound to each other, so that they revolve around their common center of gravity.

Landers
type: Concept
broader: space-probes
definition: A space probe designed to land on a planet or other solid celestial body.

Extrasolar gaseous giant planets
broader: extrasolar-gaseous-planets
related: Solar system gas giant planets
type: Concept

Galactic poles
type: Concept
broader: galactic-coordinate-system

Special relativity
type: Concept
broader: relativity
definition: The theory formulated by A. Einstein in 1905, which is based on the following two postulates: 1) Principle of relativity: The laws of physical phenomena are the same when studied in terms of two reference systems moving at a constant velocity relative to each other. 2) Principle of constancy: The velocity of light in free space is the same for all observers and is independent of the relative velocity of the source of light and the observer. The term 'special theory of relativity' refers to the restriction in the first postulate to reference systems moving at a constant velocity relative to each other (inertial reference frame).

Spiral arms
type: Concept
related: De Vaucouleurs Sandage classification
definition: The region in a spiral galaxy that contains concentrations of gas, dust, and massive stars. Spiral arms are created by density waves.
broader: galaxy-structure

Diffuse nebulae
definition: An irregularly shaped and low density interstellar cloud visible in the optical wavelengths.
type: Concept
broader: nebulae

Indochinites
broader: tektites
type: Concept

Stellar coronal loops
type: Concept
broader: stellar-coronae
related: Solar coronal loops

Diffuse interstellar bands
type: Concept
broader: interstellar-absorption
definition: Absorption features in the spectrum of stars identified in the ultraviolet, visible, and infrared regions. They have an interstellar origin, but despite extensive efforts, their carrier(s) have not yet been clearly identified.
broader: astrochemistry

Asymptotic giant branch
broader: hertzsprung-russell-diagram
related: Asymptotic giant branch stars
type: Concept
definition: A region of the Hertzsprung-Russell diagram populated by evolving low-mass to intermediate-mass stars. These stars have an electron degenerate core of carbon and oxygen surrounded by two burning shells of helium and hydrogen. The H and He-burning shells are activated alternately in the deep layers of the star. An extended and tenuous convection envelope, having a radius of 10^4 - 10^5 times the size of the core, lies above these shells. The loosely bound envelope is gradually eroded by the strong stellar wind, which forms a dusty circumstellar envelope out to several hundreds of stellar radii. The convective envelope, stellar atmosphere, and circumstellar envelope have a rich and changing chemical composition provided by nucleosynthesis processes in the burning shells in the deep interior.

Microlensing parallax
related: Parallax
broader: gravitational-microlensing
type: Concept

Solar magnetic bright points
broader: solar-activity
broader: solar-magnetic-fields
type: Concept

Cosmological constant experiments
type: Concept
broader: cosmological-constant

Compact binary systems
type: Concept
deprecated: :__

Galaxy groups
type: Concept
broader: galactic-and-extragalactic-astronomy

Leptogenesis
type: Concept
broader: early-universe

Late-type dwarf stars
type: Concept
broader: late-type-stars

Lunar magnetic fields
type: Concept
broader: selenology

N-body problem
definition: The mathematical problem of studying the behavior (e.g., velocities, positions) of any number of objects moving under their mutual gravitational attraction for any time in the past or future.
related: N-body simulations
broader: celestial-mechanics
type: Concept

Spectral line identification
broader: atomic-physics
type: Concept
broader: molecular-physics

Van Allen radiation belts
type: Concept
broader: planetary-magnetosphere
definition: The ring-shaped regions of charged particles surrounding the Earth from 1 to 6 Earth radii into space. The charged particles are trapped in by the Earth's magnetic field. The inner belt is between 1.2 and 4.5 Earth radii and contains high-energy electrons and protons which originate mainly from interactions between cosmic rays and the upper atmosphere. The outer belt, located between 4.5 and 6.0 Earth radii, contains lower-energy charged particles mainly coming from the solar wind.

Oef stars
type: Concept
broader: early-type-emission-stars
broader: o-stars

Broad band photometry
broader: photometric-systems
related: Infrared photometry
type: Concept

Horizontal branch
type: Concept
related: Horizontal branch stars
definition: A set of roughly horizontal points in the Hertzsprung-Russell diagram of a typical globular cluster. It displays a stage of stellar evolution which immediately follows the red giant branch (RGB) in stars with an initial mass < 1.2 M_sun. When the star's ascent of the RGB is terminated by the helium flash, it moves down to the Horizontal Branch (HB). The star's effective temperature on the HB is higher than it was on the RGB, but the luminosity is considerably less than at the helium flash. Usually HB stars have two energy sources: in addition to the helium burning in their cores, they experience hydrogen fusion in a surrounding shell. The thickness of the shell determines the color of the HB stars. A thin shell, involving low opacity, makes the star look blue. The HB domain encompasses a very large effective temperature range with several members: extreme HB, blue HB, RR Lyrae, red HB, and red clump stars. The locations depend on many parameters, including stellar mass, metallicity, age, helium abundance, and rotation.
related: Extreme horizontal branch stars
broader: hertzsprung-russell-diagram
related: Zero-age horizontal branch stars

Pointing accuracy
type: Concept
broader: astronomical-techniques

Solar coronal transients
broader: solar-corona
type: Concept

Solar magnetism
deprecated: :__
use instead: Solar magnetic fields
type: Concept

Astrobiology
type: Concept
definition: The study of life throughout the Universe, also known as exobiology.
related: Pre-biotic astrochemistry
broader: interdisciplinary-astronomy

Compact dwarf galaxies
type: Concept
broader: dwarf-galaxies

Interstellar synchrotron emission
broader: interstellar-emissions
type: Concept

Lunar origin
type: Concept
broader: lunar-science

Subdwarf stars
type: Concept
definition: A metal-poor main-sequence star with spectral type later than M7 and luminosity class VI. They are population II dwarfs which appear less luminous than their solar metallicity counterparts due to the dearth of metals in their atmospheres. Subdwarfs are halo members with high proper motions and large heliocentric velocities. They are important tracers of the chemical enrichment history of the Galaxy and belong to the first generation of stars formed in the Galaxy (at least 10 billion years old).
broader: stellar-evolutionary-types

Atmospheric evolution
related: Exoplanet atmospheric evolution
broader: planetary-atmospheres
type: Concept

Polar caps
type: Concept
definition: Either of the regions around the poles of a planet.
broader: planetary-structure

Reddening law
broader: extinction
type: Concept

Type II Cepheid variable stars
type: Concept
broader: cepheid-variable-stars

Solar granulation
related: Stellar granulation
definition: The mottled appearance of the solar photosphere, caused by convective cells, resembling granules, which rises from the interior of the Sun. Each granule has a mean size of about 1,000 km and an upward velocity of about 0.5 km/sec. Granules are separated by intergranular walls about 400 K colder. They emerge from the fragments of the preceding granules and their lifetimes are about 20 minutes.
type: Concept
broader: solar-physics
related: Solar granules

Ritchey-Chrétien telescopes
broader: cassegrain-telescopes
type: Concept
definition: A type of Cassegrain telescope in which the primary mirror is a hyperboloid. It is designed to eliminate coma and spherical aberration, thus providing a relatively large field of view as compared to a more conventional configuration.

Lb variable stars
broader: giant-stars
type: Concept
broader: slow-irregular-variable-stars

Schwarzschild radius
definition: The critical radius at which a massive body becomes a black hole, i.e., at which light is unable to escape to infinity: R_s = 2GM / c^2, where G is the gravitational constant, M is the mass, and c the speed of light. The formula can be approximated to R_s ≅ 3 x (M/M_sun), in km. Therefore, the Schwarzschild radius for Sun is about 3 km and for Earth about 1 cm.
type: Concept
broader: schwarzschild-metric

Minimum spanning tree
broader: spatial-point-processes
type: Concept
related: Single-linkage hierarchical clustering

Interstellar clouds
type: Concept
broader: interstellar-medium
definition: An aggregation of gas and dust in the interstellar medium containing large quantities of atoms, molecules, and dust. There are several types of interstellar clouds, such as diffuse interstellar clouds, dark clouds, molecular clouds.

Galaxy rotation curves
type: Concept
broader: galaxy-rotation

Binary lens microlensing
broader: gravitational-microlensing
type: Concept

Einstein rings
type: Concept
definition: The apparent shape of a background source undergoing the effect of gravitational lensing as seen from Earth, provided that the source, the intervening lens, and the observer are in perfect alignment through Einstein radius.
broader: gravitational-lensing

Astrographic catalogs
type: Concept
broader: catalogs

Quadrupole mass spectrometry
type: Concept
broader: mass-spectrometry

Carbon stars
related: Late-type giant stars
broader: chemically-peculiar-stars
type: Concept
definition: A class of red giant stars whose spectra show strong molecular bands of carbon compounds.

Zodiacal cloud
type: Concept
related: Exozodiacal dust
broader: interplanetary-dust

E+A galaxies
broader: post-starburst-galaxies
type: Concept
definition: E+A galaxies have strong Balmer absorption lines typical of young stars, but lack optical emission features (e.g. O III 5007) typical of star-forming regions and have metallic absorption lines (e.g. Ca H and K) indicative of an older population of dwarf stars. The presence of two distinct stellar populations and morphological observations suggest that E+A galaxies are the product of a gas-rich galaxy merger.
broader: early-type-galaxies

Stellar photometry
broader: photometry
definition: The precise measurement of a star's brightness, usually through several specific wavelength bands.
type: Concept

Chemical kinetics
broader: quantum-chemical-calculations
type: Concept

Shepherd satellites
definition: A natural satellite in orbit near the edge of a planetary ring, whose gravitational force on the ring particles strongly controls the distribution of material within the ring, creating ringlets and density waves within the ring and sharp edges at ring boundaries. Examples include Saturn's Prometheus and Pandora, which shepherd the narrow outer F ring and the Uranus satellites Cordelia and Ophelia and the epsilon ring. The faster-moving inside satellite accelerates the inner ring particles as it passes them, causing them to spiral out to larger orbits. At the same time the slower-moving outer satellite decelerates the outer ring particles as they pass by, causing them to spiral inward. The result is a narrow, well-defined ring.
type: Concept
broader: planetary-rings

Solar system gas giant planets
type: Concept
definition: A planet that revolves around the Sun beyond the asteroid belt, namely Jupiter, Saturn, Uranus, and Neptune.
broader: solar-system-planets
related: Extrasolar gaseous giant planets

Solar spectral irradiance
type: Concept
broader: solar-physics

Interstellar magnetic fields
type: Concept
broader: interstellar-medium
definition: A large-scale, weak magnetic field, with an estimated strength of about 1 to 5 microgauss, that pervades the disk of the Milky Way Galaxy and controls the alignment of interstellar dust grains.

Am stars
definition: A chemically peculiar A- or early F-type star showing an overabundance of heavy elements and an underabundance of calcium and scandium. An Am star cannot receive a unique spectral type, as different methods (using the metallic lines, the hydrogen Balmer lines, and the calcium Ca II K-lines) yield three different spectral types. Contrarily to Ap stars, Am stars do not have significant external magnetic fields. Their rotational velocities are about 100-120 km s-1 smaller than those of Ap stars.
broader: magnetic-stars
broader: chemically-peculiar-stars
broader: magnetic-variable-stars
broader: a-stars
type: Concept

Galilean satellites
broader: jovian-satellites
type: Concept
definition: The four largest and brightest satellites of Jupiter, that is: Io, Europa, Ganymede, and Callisto.

Atomic physics
broader: laboratory-astrophysics
related: Atomic interferometry
related: Atomic spectroscopy
type: Concept

Nuclear decay
type: Concept
broader: nuclear-physics

G stars
broader: stellar-spectral-types
related: Stellar classification
type: Concept
broader: intermediate-type-stars
definition: A member of a class of stars to which the Sun belongs. The G-type stars on the main sequence have surface temperatures of 5,300-6,000 K and therefore appear yellow in color. G type giant stars (such as Capella) are almost 100-500 K colder than the corresponding main sequence stars. G type supergiants have temperatures of 4,500-5,500 K. The spectrum of early type G stars, such as the Sun (G2), is dominated by ionized lines of calcium (H and K lines, mainly) and neutral metals. In later type G stars the molecular bands of CH molecules and CN molecules become visible. The main sequence and giant stars have masses of ~ 1 solar mass, while the supergiants are of ~ 10 solar masses. The luminosities of G-type giants are almost 30-60 times greater than that of the Sun, whereas the supergiants are 10,000-30,000 times more luminous.

Gaussian Processes regression
broader: smoothing
type: Concept

Radiative transfer equation
broader: analytical-mathematics
definition: The equation that describes the radiative transfer. It states that the specific intensity of radiation during its propagation in a medium is subject to losses due to extinction and to gains due to emission.
type: Concept
related: Radiative transfer

Radioheliographs
broader: solar-telescopes
type: Concept
broader: radio-telescopes

A stars
type: Concept
related: Stellar classification
definition: A star whose spectrum shows strong hydrogen Balmer lines accompanied by many faint to moderately strong metallic lines. In contrast to B and O stars, the lines of He I and He II are absent. Their surface temperature ranges from 7,500 to 10,000 K. The main metallic lines seen in A-type stars are those of Fe I and Fe II, Cr I and II, Ti I and II, and account for about two-thirds of all lines. Some famous examples of A star are Sirius, Deneb, Altair, and Vega. Also known as A-type star.
broader: stellar-spectral-types
broader: early-type-stars

Compact galaxies
definition: A galaxy with no disk or nebulous background and a high surface brightness that appears only barely larger than a star-like point on a sky survey photograph.
broader: galaxies
type: Concept

Interplanetary gas
broader: interplanetary-medium
type: Concept
definition: Electrically charged particles of the solar wind and gas liberated from comets within the solar system.

Low mass stars
type: Concept
broader: stellar-types
definition: A star whose mass is around that of the Sun.

Integrated magnitude
broader: magnitude
type: Concept
definition: The magnitude an extended object (nebula, galaxy, etc.) would have if all of its light were concentrated into a point source.

K giant stars
broader: late-type-giant-stars
broader: k-stars
type: Concept

A giant stars
type: Concept
broader: a-stars
broader: giant-stars

Orbital motion
type: Concept
broader: orbits

Annual equation
type: Concept
broader: lunar-theory
definition: An irregularity in the Moon's orbit, which can amount to 11 degrees in a period of one year. It results from the Sun's disturbing effect on the motion of the Moon due to varying distance between them.

Dwarf novae
broader: novae
definition: A class of novae and cataclysmic variables that have multiple observed eruptions. Their prototype is U Geminorum star. Optically, dwarf nova eruptions have amplitudes of 2-6 mag in V, a duration of a few to 20 days and a recurrence time-scale of weeks to years. Dwarf novae are thought to be semidetached binary stars consisting of a white dwarf primary accreting via Roche lobe overflow from a companion which is usually a late-type, generally main-sequence star. DN outbursts are usually attributed to the release of gravitational energy resulting from an instability in the accretion disk or by sudden mass transfers through the disk.
type: Concept

Astronomy databases
broader: astronomical-reference-materials
broader: computational-methods
broader: astronomy-data-analysis
definition: A combined and coordinated set of data that supplies astronomy information for a specific purpose in a variety of forms.
type: Concept

Hubble classification scheme
related: Spiral galaxies
type: Concept
related: Lenticular galaxies
related: Elliptical galaxies
definition: The classification of galaxies according to their visual appearance into four basic types suggested by E. Hubble: ellipticals (E), spirals (S), barred spirals (SB), and irregulars (Ir). Later on a separate class of lenticulars (S0) was appended as an intermediate type between ellipticals and spirals. The sequence starts with round elliptical galaxies (E0). Flatter galaxies are arranged following a number which is calculated from the ratio (a - b)/a, where a and b are the major and minor axes as measured on the sky. Ellipticals are divided into eight categories (E0, E1, ..., E7). Beyond E7 a clear disk is apparent in the lenticular (S0) galaxies. The sequence then splits into two parallel branches of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral and barred types are subdivided into Sa, Sb, Sc, and SBa, SBb, SBc, respectively. Along the sequence from Sa to Sc, the central bulge becomes smaller, while the spiral arms become more and more paramount. The original, erroneous idea that such arrangement of the galaxies might represent an evolutionary sequence led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies.
related: Irregular galaxies
broader: galaxy-classification-systems

Radiative processes
broader: molecular-physics
broader: atomic-physics
type: Concept

Mascon
broader: gravity-anomaly
type: Concept
definition: A region on the surface of the Moon where the gravitational attraction is slightly higher than normal due to the presence of dense rock.

Scaling relations
type: Concept
broader: astronomical-techniques
broader: galaxy-properties

Lunar petrology
broader: lunar-surface
type: Concept

Limb brightening
type: Concept
broader: solar-atmosphere
definition: An observed increase in the intensity of radio, extreme ultraviolet, or X-radiation from the Sun from its center to its limb.

Stellar elemental abundances
deprecated: :__
use instead: Stellar abundances
type: Concept

Stellar coronal plumes
related: Solar coronal plumes
type: Concept
broader: stellar-coronae

Galaxy accretion
type: Concept
related: Galaxy accretion disks
broader: accretion
broader: galaxy-processes

Solar optical telescopes
type: Concept
broader: optical-telescopes
broader: solar-telescopes

Ataxites
broader: meteorites
type: Concept

Stellar properties
broader: stellar-physics
type: Concept

Exoplanet surfaces
type: Concept
related: Planetary surfaces
broader: exoplanet-structure

Glaciers
broader: planetary-structure
related: Glaciation
type: Concept

Galaxy collisions
broader: interacting-galaxies
type: Concept

Pointing error
type: Concept
broader: pointing-accuracy

Tailed radio galaxies
broader: radio-galaxies
type: Concept
definition: A member of the class of radio galaxies that have a strong radio emission coming from a bright 'head' and a more diffuse emission from a 'tail.' They are often found in clusters.

Late-type supergiant stars
type: Concept
broader: late-type-stars
broader: supergiant-stars

Orbital theory
type: Concept
broader: orbits

Solar photosphere
type: Concept
related: Stellar photospheres
definition: The visible surface of the Sun (temperature 5700 K), just below the chromosphere and just above the convective zone. The solar photosphere is a thin layer of roughly 300 km wide. Its temperature decreases uniformly with height, from about 6,600 K (pressure 0.868 millibars) at its bottom, to about 4,400 K (pressure 125 mb), where it merges with the chromosphere. The photosphere has a 'rice-grain' appearance, called granulation, caused by rising (hot) and falling (cool) material in the convective cells just below the photosphere. Other main features of the photosphere are sunspots, faculae, and supergranulation.
broader: solar-atmosphere

Plasma jets
broader: stellar-mass-loss
type: Concept
broader: jets

Spectral line lists
type: Concept
related: Laboratory astrophysics
broader: astrochemistry

Michelson interferometry
related: Michelson interferometers
broader: optical-phase-shifting-interferometry
type: Concept

Earth ionosphere
related: Ionospheric scintillation
type: Concept
broader: earth-atmosphere
definition: The region of the Earth's upper atmosphere containing a small percentage of free electrons and ions produced by photoionization of the constituents of the atmosphere by solar ultraviolet radiation.

Ecliptic poles
definition: Either of the two points on the celestial sphere that are 90° above and below the plane of ecliptic. The north ecliptic pole lies in Draco, and the south ecliptic pole in Dorado. Due to precession, the celestial pole moves in a circle around the ecliptic poles once every 25,800 years.
broader: ecliptic-coordinate-system
type: Concept

Stellar phenomena
type: Concept
broader: stellar-physics

Optical telescopes
type: Concept
broader: telescopes

Magnetohydrodynamical simulations
broader: astronomical-simulations
related: Magnetohydrodynamics
type: Concept

Supernova evolution
type: Concept
deprecated: :__

Standard stars
definition: Stars for which accurate color indices and/or magnitudes exist, defining a standard system.
type: Concept
broader: astrometry

Astronomical instrumentation
type: Concept
broader: observational-astronomy
definition: The design and use of scientific instruments for detection, measurement, control, computation, and so on to observe and study astronomical objects.

Algol variable stars
type: Concept
broader: eclipsing-binary-stars
broader: semi-detached-binary-stars

Termination shock
definition: A shock wave inside the heliopause where the supersonic solar wind abruptly slows from an average speed of 500 km s^-1 to subsonic and becomes denser and hotter.
type: Concept
broader: heliosphere

Red straggler stars
type: Concept
broader: red-giant-stars

Orbital nodes
broader: orbits
type: Concept
definition: One of the two points of intersection of the orbit of a secondary body with the plane of reference through the primary.

AM Canum Venaticorum stars
type: Concept
definition: A binary system that has very short orbital period (less than one hour) and helium dominated spectrum. The prototype, AM Canum Venaticorum, with a period of 17 minutes, was discovered in 1967. AM CVn stars are semidetached binary systems in which accretion is going on. The donor star is hydrogen deficient and the accretor is usually a white dwarf. To fit within their Roche lobes, the donor stars must be dense, suggesting they may be degenerate too. It is at present thought that AM CVn stars represent three possible evolutionary phases in 1) double white dwarf systems, 2) white dwarf and helium star binaries, and 3) cataclysmic variables with evolved donors.
broader: cataclysmic-variable-stars
broader: hydrogen-deficient-stars
related: DQ Herculis stars

AB photometry
type: Concept
definition: A photometric system defined by reference to monochromatic magnitudes in such a way that AB is equal to the V magnitude for a source with a flat spectral energy distribution. The zero point is defined by the flux of the star Vega at 5546 Å. In this system, an object with constant flux per unit frequency interval has zero color.
broader: broad-band-photometry

Dark matter density
type: Concept
broader: density-parameters

Collisional processes
broader: planetary-science
type: Concept

Gravitational microlensing exoplanet detection
broader: gravitational-microlensing
broader: exoplanet-detection-methods
type: Concept
related: Free floating planets

High mass x-ray binary stars
type: Concept
broader: massive-stars
broader: x-ray-binary-stars
definition: A member of one of the two main classes of X-ray binary systems where one of the components is a neutron star or a black hole and the other one a massive star. HMXBs emit relatively hard X-rays and usually show regular pulsations, no X-ray bursts, and often X-ray eclipses. Their X-ray luminosity is much larger than their optical luminosity. In our Galaxy HMXBs are found predominantly in the spiral arms and within the Galactic disk in young stellar populations less than 10^7 years old. One of the most famous HMXB is Cygnus X-1 which was the first stellar-mass black hole discovered.

Sunspot flow
broader: sunspots
type: Concept

Eris
type: Concept
deprecated: :__

True position
broader: astronomical-location
type: Concept
definition: The coordinates of an object for a given date, with respect to the true equator and the true equinoxes for the instant of time in question.

Solar radiative zone
broader: solar-interior
type: Concept

Solar E corona
broader: solar-corona
type: Concept
definition: A part of the solar corona defined by the emission lines of hot gases. These emission lines include the so-called forbidden lines of the strongly ionized atoms of iron, calcium, and other elements. The E corona is thinner than the K corona and the F corona.
related: Stellar E corona

Luminosity function
definition: Number distribution of stars or galaxies (galaxy) with respect to their absolute magnitudes. The luminosity function shows the number of stars of a given intrinsic luminosity (or the number of galaxies per integrated magnitude band) in a given volume of space.
type: Concept
broader: analytical-mathematics

U Geminorum stars
type: Concept
broader: dwarf-novae

Equator position
broader: astrometry
type: Concept

Optical position
broader: astronomical-location
type: Concept

Varuna
type: Concept
deprecated: :__

Disk flaring
type: Concept
broader: stellar-flares

Cross-validation
broader: astrostatistics-techniques
related: Bootstrap
type: Concept

Gamma-ray astronomy
type: Concept
definition: The study of gamma rays from extraterrestrial sources, especially gamma-ray bursts.
related: Gamma-ray sources
broader: astronomical-methods

Cardinal directions
type: Concept
broader: horizontal-coordinate-system

Early-type supergiant stars
broader: supergiant-stars
broader: early-type-stars
type: Concept

Tektites
broader: meteorites
type: Concept
definition: Small glassy bodies whose chemical composition is unrelated to the geological formation in which they are found. They are found mostly in Australia, Java, Philippines and Indochina. Tektites are now thought to have been produced by the impact of meteorites on the earth's surface.

Circumstellar grains
type: Concept
broader: circumstellar-matter

Accreting Binary Stars
deprecated: :__
type: Concept

Galaxy physics
related: Galaxy classification systems
type: Concept
broader: galactic-and-extragalactic-astronomy

Substellar companion stars
type: Concept
broader: companion-stars

Supergalactic coordinate system
broader: astronomical-coordinate-systems
definition: A spherical coordinate system in which the equator is the supergalactic plane. Supergalactic longitude, SGL, is measured counterclockwise from direction l = 137.37 deg, b = 0 deg (between 0 and 360 deg). The zero point for supergalactic longitude is defined by the intersection of this plane with the Galactic plane. In the equatorial coordinate system (J2000) this is approximately 2.82 h, +59.5 deg. Supergalactic latitude, SGB, is measured from the supergalactic plane, positive northward and negative southward. The North Supergalactic Pole (SGB=90 deg) lies at galactic coordinates l = 47.37 deg, b = +6.32 degrees, corresponding to the equatorial coordinate system (J2000) 18.9 h, +15.7 deg.
type: Concept

CH stars
broader: giant-stars
type: Concept

Galaxy equators
type: Concept
broader: galaxy-kinematics

Compact H II region
broader: h-ii-regions
type: Concept
definition: A Galactic H II region with an electron density ≥ 10^3 cm^-3 and of a linear dimension ≤ 1 pc.

Solar active region magnetic fields
type: Concept
broader: solar-active-regions

Atmospheric circulation
definition: The large-scale movements of air around areas of high and low pressure whereby heat is distributed on the surface of the Earth. Atmospheric motion is driven by uneven heating of the planet. The atmosphere (and ocean) transfers the excess heat from tropics to poles. The flow is determined by balance between pressure gradients and the Coriolis effect.
type: Concept
broader: earth-atmosphere

Exoplanet atmospheric evolution
type: Concept
related: Atmospheric evolution
broader: exoplanet-atmospheres

Resonant Kuiper belt objects
type: Concept
broader: trans-neptunian-objects

Saturn
related: Saturnian satellites
definition: The sixth planet from the Sun and the second largest with an equatorial diameter of 120,536 km orbiting at an average distance of 1,429,400,000 km (9.54 astronomical units) from Sun. With an eccentricity of 0.05555, its distance from the Sun ranges from 1.35 billion km (9.024 AU) at its perihelion to 1.509 billion km (10.086 AU) at its aphelion. Its average orbital speed being 9.69 km/s, it takes Saturn 29.457 Earth years (or 10,759 Earth days) to complete a single revolution around the Sun. However, Saturn also takes just over 10 and a half hours (10 hours 33 minutes) to rotate once on its axis. This means that a single year on Saturn lasts about 24,491 Saturnian solar days. Saturn has a mass of 5.6836 x 1026 kg (95.159 Earth masses) and a mean density of 0.687 g cm^(-3). Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, and ammonia, similar to the composition of the primordial Solar Nebula from which the solar system was formed. The temperature on Saturn is ~ -185 °C. Like Jupiter, Saturn has a solid core of iron-nickel and rock (silicon and oxygen compounds). The core has an estimated mass of 9-22 Earth Masses and a diameter of about 25,000 km (about 2 Earth diameter). The core is enveloped by a liquid metallic hydrogen layer and a molecular hydrogen layer. Saturn's interior is hot (12,000 K at the core). The planet radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism as in Jupiter. Saturn has 62 known satellites. Saturn's ring. On 1 July 2004 NASA/ESA's Cassini-Huygens became the first to orbit Saturn, beginning a 13 year mission that revealed many secrets and surprises about Saturn and its system of rings and moons.
broader: outer-planets
type: Concept

Star-planet interactions
broader: exoplanet-dynamics
type: Concept
related: Solar-planetary interactions

Core-collapse supernovae
type: Concept
definition: A supernova arising from the core collapse of a massive star.
broader: supernovae

Few-body systems
broader: n-body-problem
type: Concept

Gamma-ray observatories
broader: space-observatories
broader: observatories
type: Concept

Interdisciplinary astronomy
type: Concept

Tectonics
broader: planetary-dynamics
type: Concept
related: Volcanoes

Antapex
definition: The direction in the sky (in the constellation Columba) away from which the Sun seems to be moving (at a speed of 19.4 km/s) relative to general field stars in the Galaxy.
type: Concept
broader: solar-motion

Cosmic inflation
type: Concept
broader: cosmology
definition: A brief exponential expansion of the Universe postulated to have occurred 10-35 seconds after the Big Bang, in response to the separation of the strong interaction from the electroweak interaction. This idea aims at explaining the flatness problem, the horizon problem, and the magnetic monopole problem.

Stellar chromospheres
related: Solar chromosphere
type: Concept
broader: stellar-atmospheres

Blazars
definition: A term specifying BL Lac objects or quasars when the continuum radiation emitted from the active nucleus is highly polarized and very variable.
broader: active-galaxies
broader: active-galactic-nuclei
type: Concept

Parabolic orbit
type: Concept
definition: An orbit whose overall shape is like a parabola; it is the limiting case between an elliptical orbit (eccentricity less than 1) and a hyperbolic orbit (eccentricity larger than 1). The speed necessary to form a parabolic orbit is known as the escape velocity.
broader: orbital-motion

Cosmic noise
broader: radio-astronomy
type: Concept

Milky Way Galaxy fountains
broader: milky-way-galaxy-physics
type: Concept
related: Galaxy fountains

Solid matter physics
related: Interstellar dust processes
type: Concept
broader: laboratory-astrophysics
related: Interstellar dust
related: Astrophysical dust processes

Cometary globules
broader: dark-interstellar-clouds
type: Concept
definition: A relatively small cloud of dust and gas in the interstellar medium shaped like a comet with a bright-rimmed head. Cometary globules are situated near young massive stars with a strong stellar wind. The wind ionizes gases on the side facing the O stars and sweep away the low-density gas toward the tail. Cometary globules are believed to be molecular cloud condensations, which are so dense that they are not disrupted when an H II region expands into the molecular cloud surrounding it. The Rosette nebula is a good example of an H II region which shows an abundance of cometary globules.

Cosmological parameters
type: Concept
broader: cosmology

Irregular variable stars
broader: late-type-giant-stars
definition: A type of variable star in which variations in brightness show no regular periodicity. There are two main types, irregular eruptive variables and irregular pulsating variables.
type: Concept
broader: pulsating-variable-stars

Uncharged black holes
type: Concept
broader: classical-black-holes

Galaxy encounters
broader: galaxy-physics
type: Concept

Hydroxyl masers
definition: A maser phenomenon created by OH molecules with characteristic OH lines. OH masers are detected toward a variety of astronomical environments, including massive star formation regions and evolved late-type stars.
type: Concept
broader: astrophysical-masers
related: Hydroxyl sources

Dwarf stars
definition: A star that burns its hydrogen content to produce its energy and therefore belongs to the main-sequence luminosity class.
related: Main sequence
type: Concept
related: Main sequence stars
broader: stellar-evolutionary-types

Magnetars
type: Concept
definition: A highly magnetized neutron star with fields a thousand times stronger than those of radio pulsars. There are two sub-classes of magnetars, anomalous X-Ray pulsar (AXP)s and soft gamma repeater (SGR)s, that were thought for many years to be separate and unrelated objects. In fact SGRs and AXPs are both neutron stars possessing magnetic fields of unprecedented strength of 10^14 - 10^16 G, and that show both steady X-ray pulsations as well as soft gamma-ray bursts. Their inferred steady X-ray luminosities are about one hundred times higher than their spin-down luminosities, requiring a source of power well beyond the magnetic dipole spin-down that powers rotation-powered pulsar (RPP)s. New high-energy components discovered in the spectra of a number of AXPs and SGRs require non-thermal particle acceleration and look very similar to high-energy spectral components of young rotation-powered pulsars.
broader: magnetic-stars
broader: neutron-stars

Zero-age horizontal branch stars
broader: horizontal-branch-stars
type: Concept
related: Horizontal branch
related: Hertzsprung Russell diagram
definition: A star that has arrived on the horizontal branch after leaving the red giant branch. It begins helium burning in its core.

Color equation
type: Concept
broader: photometric-systems

Explosive nucleosynthesis
related: Supernova dynamics
definition: The explosive processes that are believed to occur in supernovae. Explosive carbon burning occurs at a temperature of about 2 x 10^9 degrees and produces the nuclei from neon to silicon. Explosive oxygen burning occurs near 4 x 10^9 degrees and produces nuclei between silicon and calcium in atomic weight. At higher temperatures, still heavier nuclei are produced.
broader: nucleosynthesis
type: Concept

Ground telescopes
type: Concept
broader: telescopes

Tropopause
type: Concept
definition: The boundary between the troposphere and stratosphere. Its height, varying with latitude and seasons, is from 8 km at poles, to 18 km at equator; it is higher during winter time. The atmospheric temperature decreases from ground upward until the tropopause. Then it increases in stratosphere because of the absorption of solar ultraviolet radiation.
broader: earth-atmosphere

Low-luminosity active galactic nuclei
broader: active-galactic-nuclei
type: Concept

Keplerian telescopes
type: Concept
broader: refracting-telescopes

Solar neutrons
type: Concept
broader: solar-particle-emission

Red supergiant stars
related: M supergiant stars
broader: supergiant-stars
related: Cocoon stars
related: K supergiant stars
definition: A supergiant star with spectral type K or M. Red supergiants are the largest stars in the Universe, but not necessarily the most massive. Betelgeuse and Antares are the best known examples of a red supergiant.
broader: late-type-supergiant-stars
type: Concept

Lambda Eridani stars
broader: pulsating-variable-stars
broader: be-stars
type: Concept

Early universe
definition: A qualitative term used to describe a phase in the history of the Universe, from the Big Bang event to the apparition of the first structures (seeds of future galaxies), at a redshift around 30.
broader: origin-of-the-universe
type: Concept

Bondi accretion
broader: accretion
definition: The accretion of mass by a star (assumed as point particle) moving at a steady speed through an infinite, uniform gas cloud. It is directly proportional to the star mass (M) and the medium density (ρ) and inversely proportional to the relative star/gas velocity (v).
type: Concept

SX Phoenicis variable stars
broader: multi-periodic-variable-stars
type: Concept

Laboratory astrophysics
type: Concept
related: Spectral line lists
broader: interdisciplinary-astronomy

Interstellar scintillation
broader: radio-astronomy
type: Concept
broader: astronomical-optics
definition: An apparent twinkling of the signals from distant point-like radio sources (pulsars). It is due to turbulence, i.e. fluctuations in the electron density of the interstellar ionized gas, through which the signals have passed on their way to Earth.

Infrared sources
related: OH/IR stars
related: FU Orionis stars
related: Herbig-Haro objects
type: Concept
related: Cocoon stars
related: Infrared galaxies
related: Extreme carbon stars
related: Infrared astronomy
broader: astronomical-radiation-sources

Gravitational interaction
broader: newtonian-gravitation
type: Concept
definition: Mutual attraction between any two bodies that have mass.

Deconvolution
type: Concept
definition: A mathematical operation that allows to restore the original input signal, such as an astronomical image or spectrum, to its state before being affected by the atmospheric turbulence and the transfer function of the instrument.
broader: astrostatistics-techniques

Pre-biotic astrochemistry
related: Astrobiology
broader: astrochemistry
type: Concept

Galaxy quenching
type: Concept
broader: galaxy-evolution
related: Quenched galaxies

Purkinje effect
broader: amateur-astronomy
definition: The increasing sensibility of the retina for light of shorter wavelength as the brightness decreases. In those conditions red objects are perceived to fade faster than blue objects of the same brightness.
related: Variable stars
type: Concept

Astronomical object identification
broader: astronomical-techniques
type: Concept

Orreries
type: Concept
definition: A mechanical device that illustrates the relative positions and motions of the planets and moons in the solar system in heliocentric model.
broader: early-astronomical-instruments

Gravitational lensing
broader: general-relativity
definition: A concentration of matter, such as a galaxy or a cluster of galaxies, that bends light rays from a background object, resulting in production of multiple images. If the two objects and the Earth are perfectly aligned, the light from the distant object appears as a ring from Earth. This is called an Einstein Ring, since its existence was predicted by Einstein in his theory of general relativity.
type: Concept

Late-type giant stars
broader: giant-stars
related: Carbon stars
type: Concept
broader: late-type-stars

Space vehicles
broader: astronomical-instrumentation
type: Concept

Galaxy infall
type: Concept
broader: galaxy-accretion

A subgiant stars
broader: subgiant-stars
broader: a-stars
type: Concept

Pre-solar nebulae
broader: nebulae
type: Concept

Longitude of the descending node
type: Concept
broader: orbital-elements

Intergalactic medium
definition: The matter or environment between the galaxies of a cluster.
type: Concept
related: Astrophysical dust processes
related: Intracluster medium
broader: extragalactic-astronomy

Galaxy accretion disks
type: Concept
related: Galaxy accretion
broader: galaxy-physics

Radio interferometry
broader: interferometry
type: Concept

Bailey type stars
deprecated: :__
type: Concept

Vesta
type: Concept
broader: main-belt-asteroids

Stellar radii
broader: stellar-properties
related: Solar radius
type: Concept

Aperture
broader: telescope-properties
type: Concept

Exoplanet plate tectonics
broader: exoplanet-structure
related: Plate tectonics
type: Concept

Ultraviolet telescopes
type: Concept
broader: space-telescopes

Oort constants
related: Galactic center
broader: galaxy-rotation
definition: Two parameters, denoted A and B, that describe the major features of our Galaxy's differential rotation in the Sun's neighbourhood. A is one-half of the shear and equal to +14.4 ± 1.2 km s^-1 kpc^-1, and B, one-half of the vorticity, equal to -12.0 ± 2.8 km s^-1 pc^-1.
type: Concept
related: Milky Way Galaxy
broader: astronomical-models

Libration
type: Concept
definition: Small oscillations of a celestial body about its mean position. The term is used mainly to mean the Moon's libration caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon's surface can be seen over a period of time. This results from three kinds of libration working in combination: libration in longitude, libration in latitude, and diurnal libration.
broader: orbital-motion

Radio observatories
type: Concept
broader: observatories

Infrared astronomy
broader: astronomical-methods
type: Concept
related: Infrared sources
definition: The study of infrared properties of astronomical objects.

Continuum radio emission
deprecated: :__
type: Concept

Redshifted
related: Cosmic microwave background radiation
broader: cosmology
related: Redshift surveys
definition: A spectral line whose wavelength does not coincide with its theoretical value and is shifted toward longer wavelengths.
type: Concept
broader: doppler-shift

Laser ablation
type: Concept
broader: production-techniques

Black holes
broader: compact-objects
related: Black hole physics
broader: high-energy-astrophysics
definition: A fantastically compact object, predicted by the theory of general relativity, whose gravity is so powerful that not even light can escape from it. A black hole forms when matter collapses to infinite density, producing a singularity of infinite curvature in the fabric of space-time. Each black hole is surrounded by an event horizon, at which the escape velocity is the speed of light. The Schwarzschild radius for the Sun is about 3 km and for the Earth about 1 cm. There is observational evidence for black holes on a remarkable range of scales in the Universe: stellar black hole, intermediate-mass black hole, primordial black hole, mini black hole, supermassive black hole, Schwarzschild black hole, Kerr black hole.
type: Concept
related: Gravitational singularities

Dielectronic recombination
type: Concept
broader: recombination

Asteroid belt
broader: solar-system
related: Main belt asteroids
related: Asteroids
definition: The region of the solar system located between Mars and Jupiter where over a million objects bigger than 1 km across orbit the Sun.
type: Concept

Deprecated: Cometary atmospheres
type: Concept
use instead: Comae
deprecated: :__

M-sigma relation
broader: black-hole-physics
type: Concept

Radio pulsars
type: Concept
related: Radio sources
definition: A pulsar that emits pulses in radio waves. The bulk of discovered pulsars are radio pulsars. There is a small number of pulsars that emit at optical wavelengths, X-ray wavelengths, and gamma-ray wavelengths.
broader: pulsars

Seyfert galaxies
broader: emission-line-galaxies
type: Concept
broader: high-energy-astrophysics
definition: A member of an important class of active galaxies which are characterized by the presence of an intensely bright nucleus in the optical wavelengths (10^9-10^12 L_sun) displaying emission spectral lines. The presence of these emission features, which are not seen in the spectra of normal galaxies, indicates a very high degree of ionization. Moreover, the nucleus radiates non-thermal continuum emission extending over the whole electromagnetic spectrum. It is thought that a massive black hole in the nucleus of a galaxy accretes gas (accretion) from its surrounding environment to power Seyfert galaxies. These galaxies are divided into two types according to the width of their spectral lines. Type 1 Seyfert (Sy 1) galaxies have very broad emission lines (10^3 - 10^4 km s^-1), while Type 2 Seyferts (Sy 2) show relatively narrow lines (several hundred km s^-1). These spectral differences may be the result of viewing the nucleus from different angles. A Type 2 Seyfert galaxy may be a mostly edge-on view of matter spiraling in toward the supermassive black hole, whereas a Type 1 Seyfert provides a more pole-on view, allowing us to see the more turbulent region around the black hole.

Mars
definition: Fourth planet from Sun and the seventh largest. Mass 6.42 x 10^26 g (0.11 Earth's); radius 3397 km. Mean distance from Sun 1.52 AU. Sidereal period 687 days; synodic period 779.9 days. Surface temperature 248 K., rotational period 24h 37m 22.66s. Obliquity 23°59'. Atmosphere more than 90% CO_2, traces of O_2, CO, H_{2}O. Two tiny satellites (Phobos and Deimos), both of which are locked in synchronous rotation with Mars.
broader: inner-planets
related: Martian satellites
type: Concept

Solar electromagnetic emission
broader: solar-physics
type: Concept
related: Spectropolarimetry

Companion galaxies
type: Concept
broader: galaxies

Habitable zone
definition: A zone around a star where the temperature would be in the range 0-100 °C to sustain liquid water on the surface of rocky planets (or sufficiently large moons). Water is thought to be a necessary component to the formation and evolution of Earth-type life. This zone depends on the parent star's luminosity and distance; it will be farther from hotter stars. A more accurate definition of a Habitable Zone needs to include other factors, such as orbital eccentricity, heat sources other than stellar irradiation, and atmospheric properties.
type: Concept
broader: planet-hosting-stars

Stellar spectral lines
type: Concept
broader: stellar-properties

Stellar physics
definition: The field of astrophysics concerned with the study of the physical characteristics of stars, more specifically their internal structure, physical processes taking place in their interiors, atmospheres, stellar winds, mass loss, interaction with the interstellar medium, as well as the physical laws governing star formation.
related: Gravitation
related: Hertzsprung Russell diagram
related: Solar physics
broader: stellar-astronomy
type: Concept

Software licensing
type: Concept
broader: astronomy-software

Optical identification
type: Concept
broader: visible-astronomy

Waveband photometry
type: Concept
broader: photometry

A supergiant stars
type: Concept
broader: a-stars
related: Alpha Cygni variable stars
broader: early-type-supergiant-stars

Vibrational spectroscopy
broader: spectroscopy
type: Concept

Heliosphere
related: Astrosphere interstellar medium interactions
type: Concept
definition: The vast, three-dimensional region of space around the Sun filled with the solar wind and the remnant of the solar magnetic field carried in it. It is bounded by the heliopause, which is estimated to be 100 astronomical units or more from the Sun. The radius of the heliosphere is expected to vary with the solar cycle. The heliosphere may be very elongated owing to the presence of an interstellar wind of neutral hydrogen flowing from the direction of the Galactic center.
broader: solar-system
broader: stellar-wind-bubbles

Blue loop
broader: hertzsprung-russell-diagram
definition: An evolutionary behavior of certain stars, particularly massive stars, which return to the blue stage after becoming a red supergiant. The phenomenon appears as a blueward loop on the theoretical evolutionary tracks.
type: Concept

Trojan asteroids
type: Concept
broader: asteroids
definition: A member of the family of asteroids that share Jupiter's orbit and lie in elongated, curved regions around the two Lagrangian points 60° ahead and behind of Jupiter. The Lagrangian points L4 and L5 host several thousands of them. Originally, the term Trojan applied only to asteroids sharing Jupiter's orbit; however, planetoidal bodies have been discovered at the Lagrangian points of Mars and Neptune as well, and are also referred to as Mars Trojans and Neptune Trojans respectively.

Atira group
type: Concept
broader: near-earth-objects

Disk galaxies
definition: A galaxy consisting of a thin disk of stars and interstellar matter which may include spiral arms and bar.
broader: galaxies
type: Concept

Mass spectrometry
type: Concept
broader: astronomical-techniques
related: Mass spectrometers
definition: An analytical technique for identification of chemical structures, determination of mixtures, and quantitative elemental analysis, in which ions are separated according to the mass/charge ratio and detected by a suitable detector.
broader: experimental-techniques

Photovisual magnitude
broader: apparent-magnitude
type: Concept
definition: Magnitude defined for the combination of a photographic plate and a yellow filter, approximating the spectral sensitivity of the eye.

Plages
definition: A bright cloud-like feature that appears in the vicinity of a sunspot. Plages represent regions of higher temperature and density within the chromosphere. They are particularly visible when photographed through filters passing the spectral light of hydrogen or calcium.
type: Concept
broader: solar-chromosphere

Interplanetary particle acceleration
type: Concept
broader: interplanetary-physics

Small molecules
broader: molecular-physics
type: Concept

Young massive clusters
type: Concept
broader: star-clusters

Astronomical research
type: Concept
broader: observational-astronomy

Solar active region filaments
broader: solar-filaments
type: Concept
broader: solar-active-regions

Xenobiology
broader: astrobiology
definition: The study of synthesizing and manipulating biological devices and systems.
type: Concept

Astronomy education
related: Educational software
type: Concept
definition: The act or process of educating in astronomy. The knowledge or abilities gained through being educated in astronomy.
broader: interdisciplinary-astronomy

de Sitter universe
broader: cosmological-models
definition: A solution to Einstein's field equations of general relativity which contains no ordinary matter (Ω_M = 0) or radiation (Ω_R = 0), is Euclidean (k = 0), but has a cosmological constant (Ω_Λ > 0). The Universe expands exponentially forever. This solution was the first model expanding of expanding Universe.
type: Concept

Poisson distribution
broader: astrostatistics-distributions
type: Concept
definition: A probability function that characterizes discrete random events occurring independently of one another within some definite time or space. It may be regarded as an approximation of the binomial distribution when the number of events becomes large and the probability of success becomes small. The Poisson distribution is expressed by: f(x) = (λxe-λ)/x!, where λ is the mean number of successes in the interval, e is the base of the natural logarithm, and x is the number of successes we are interested in.

Geological processes
broader: planetary-geology
type: Concept

Strömgren spheres
broader: stellar-structures
definition: A theoretical sphere of ionized hydrogen created by energetic ultraviolet photons of a hot, massive star embedded in a uniform interstellar molecular cloud and lying at the center of the sphere.
type: Concept

Solar oscillations
related: Stellar oscillations
broader: solar-physics
type: Concept

Supermassive black holes
definition: A black hole of tremendous mass, equivalent to those of millions or even billions of stars, which is believed to exist and occupy the centers of many galaxies. The supermassive black hole residing in the center of our Milky Way Galaxy is the object Sgr A* with a mass of 4 x 10^6 solar masses within a radius of 100 astronomical units.
type: Concept
broader: astrophysical-black-holes

Astronomical methods
type: Concept
broader: observational-astronomy

Solar analogs
related: The Sun
definition: A member of a class of unevolved or slightly evolved Population I disk stars with an effective temperature, degree of evolution, metallicity, and kinematic property not very different from those of the Sun.
broader: stellar-types
type: Concept

Twilight
type: Concept
broader: sky-brightness
definition: The diffused light from the sky when the Sun is below the horizon, either from daybreak to sunrise or, more commonly, from sunset to nightfall. There are three types of twilight: astronomical twilight, civil twilight, and nautical twilight. They are divided on the basis of the solar depression angle.

Exoplanet surface variability
related: Surface variability
type: Concept
broader: exoplanet-surface-characteristics

Star atlases
broader: astronomical-reference-materials
type: Concept

Software documentation
broader: astronomy-software
type: Concept

Domes
type: Concept
broader: observatories
definition: A vault, having a circular plan and usually in the form of a portion of a sphere, that shelters an optical telescope.

Type Ia supernovae
related: White dwarf stars
definition: A Type I supernova that presents a singly-ionized silicon (Si II) absorption feature at 6150 Å near peak brightness. Type Ia SNe are believed to result from mass accretion to a carbon-oxygen white dwarf in a close binary system. When the white dwarf mass exceeds the Chandrasekhar limit, the degenerate electron pressure can no longer support the accumulated mass and the star collapses in a thermonuclear explosion producing a supernova. The peak luminosity of SNe Ia is set by the radioactive decay chain, and the observed photometric correlation between the peak luminosity and the time-scale over which the light curve decays from its maximum is understood physically as having both the luminosity and opacity being set by the mass of Nickel-56 synthesized in the explosion. Type Ia supernovae occur in all types of galaxies. Type Ia SNe are used as standard candles in determining cosmological distances, after normalizing their light curves with the Phillips relation.
broader: supernovae
type: Concept

Chandrasekhar limit
related: Type II supernovae
related: White dwarf stars
definition: A limiting mass of about 1.44 Solar masses that the theory predicts a non-rotating white dwarf can attain without collapsing to become a neutron star or a black hole. Over this critical mass, the degeneracy pressure will be unable to bear the load of the bulk mass.
broader: gravitational-collapse
type: Concept

Compact nebulae
type: Concept
broader: nebulae

Variable radiation sources
related: UV Ceti stars
type: Concept
broader: astronomical-radiation-sources

Stellar bow shocks
broader: shocks
broader: heliosphere
type: Concept

Humidity
definition: Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.
type: Concept
broader: earth-atmosphere

Stone circles
broader: early-astronomical-instruments
type: Concept
related: Megalithic astronomy

Line of sight
definition: The imaginary straight line connecting the object and the objective lens of the viewing device.
broader: astronomical-location
type: Concept

Kappa mechanism
type: Concept
broader: luminosity-classification
related: Variable stars
definition: A process based on the effects of opacity (κ) that drives the pulsations of many types of variable stars. Consider a layer of material within a star and suppose that it undergoes inward contraction. This inward motion tends to compress the layer and increase the density. Therefore the layer becomes more opaque. If a certain amount of flux comes from the deeper layers it gets stuck in the high κ region. The energy accumulates and heat builds up beneath it. The pressure rises below the layer, pushing it outward. The layer expands as it moves outward, cools and becomes more transparent to radiation. Energy can now escape from below the layer, and pressure beneath the layer diminishes. The layer falls inward and the cycle repeats. The κ mechanism is believed to account for the pulsations of several star families, including Delta Scuti stars, Beta Cephei variables, Cepheids, and RR Lyrae stars.

Wilson effect
broader: sunspots
definition: A phenomenon in which the shape of sunspots flattens as they approach the Sun's limb due to the solar rotation. More specifically, when a sunspot approaches the solar limbs the width of the penumbra, relative to the umbra, on the side facing the center of the Sun seems to become narrower than on the side facing the limb. This phenomenon arises from a projection effect, and is due to a geometrical depression (the Wilson depression) in the layers of constant optical depth in sunspots.
type: Concept

Kapteyn Selected Areas
type: Concept
broader: surveys

Optical phase shifting interferometry
type: Concept
broader: optical-interferometry

X-ray novae
broader: novae
type: Concept

Planetary nebulae nuclei
broader: planetary-nebulae
type: Concept

Ephemerides
broader: astronomical-reference-materials
type: Concept
definition: A table of computed positions occupied by a celestial body over successive intervals of time such as daily; plural ephemerides.

Stellar coronal mass ejections
broader: stellar-phenomena
related: Ejecta
related: Stellar mass loss
related: Solar coronal mass ejections
type: Concept

Gibbs Sampler
broader: markov-chain-monte-carlo
type: Concept

Cataclysmic variable stars
broader: semi-detached-binary-stars
type: Concept
broader: intrinsic-variable-stars
definition: A variable star that shows a sudden and dramatic change in brightness, including flare stars, novae, and some types of symbiotic stars. They are believed to be very close binary systems consisting of an accreting white dwarf primary and an evolved late-type secondary star that has filled its Roche lobe. For systems with an accretion disk, it is believed that a thermal instability is the cause of repetitive outbursts observed in cataclysmic variables called dwarf novae.

Halo stars
broader: high-velocity-stars
type: Concept

Dobsonian telescopes
type: Concept
broader: newtonian-telescopes

Perseus Cluster
broader: galaxy-clusters
type: Concept
definition: A galaxy cluster of about 12,000 members about 250 million light-years (redshift z = 0.0176) away, covering 4° of sky in the constellation Perseus. It is dominated by elliptical galaxies. At its center lies the radio source Perseus A. Also known as Abell 426 (Abell catalog).

Dredge-up
definition: A process by which the nucleosynthesis products inside a star are drawn to upper layers of the star.
type: Concept
broader: late-stellar-evolution

Galaxy counts
type: Concept
broader: surveys

Ultracompact dwarf galaxies
type: Concept
broader: compact-dwarf-galaxies
definition: A type of very bright compact stellar system (-14 ≤ MV ≥ -12) that is intermediate between globular clusters (GCs) and compact elliptical galaxies (cEs). With masses of M > 2 x 10^6 M_sun and radii > 10 parsecs (pc), UCDs are among the densest stellar systems in the Universe. Nevertheless, the nature and origin of these objects is still widely debated. Early interpretations suggested that UCDs could be the most massive GCs or possibly the tidally stripped remnants of dwarf galaxies. However, there is evidence that both formation mechanisms could contribute to the UCD population. Supermassive black holes (SMBHs) have been confirmed in most UCDs with masses M > 10^7 M_sun. The most massive UCD discovered to date, M59-UCD3 (M_* ~2 x 10^8 M_sun, radius ~ 25 pc), hosts a SMBH.

Major axis
type: Concept
broader: elliptical-orbits
definition: The greatest diameter of an ellipse; it passes through the two foci.

Solar coronal streamers
definition: A bright, narrow stream of particles traveling through the Sun's corona, visible in images taken with a coronagraph or during a total solar eclipse. Coronal streamers represent the most outwardly extended structures in the solar corona and result from the interaction between the solar slow wind and the large-scale magnetic field.
broader: solar-corona
type: Concept

BY Draconis stars
type: Concept
broader: eruptive-variable-stars

De Vaucouleurs Sandage classification
type: Concept
related: Spiral arms
related: Lenticular galaxies
related: Elliptical galaxies
related: Irregular galaxies
broader: galaxy-classification-systems
related: Spiral galaxies

Double-mode variable stars
broader: multi-periodic-variable-stars
type: Concept
broader: pulsating-variable-stars

Oscillating universe
definition: A cosmological model in which the Universe is closed and undergoes a series of oscillations, each beginning with a big bang and ending with a big crunch.
type: Concept
broader: cosmological-models

Overabundances
type: Concept
broader: chemical-abundances

Carbonaceous chondrites
broader: chondrites
definition: A rare type of stony meteorite having a higher carbon content than other classes of meteorite. They represent only ~5% of the known meteorites. Their bulk composition is mainly silicates, oxides and sulfides, whilst the minerals olivine and serpentine are characteristic. The six classes of carbonaceous chondrites are: CI chondrites, CM chondrites, CV chondrites, CO chondrites, CK chondrites, CR chondrites, CH chondrites, and CB chondrites.
type: Concept

Yellow straggler stars
broader: stellar-evolutionary-types
type: Concept
related: Blue straggler stars

Warm ionized medium
type: Concept
broader: interstellar-phases

Single-linkage hierarchical clustering
broader: clustering
related: Minimum spanning tree
type: Concept

Stellar coronal lines
broader: stellar-coronae
related: Solar coronal lines
type: Concept

Galaxy dynamics
broader: galaxy-processes
broader: galaxy-physics
related: Milky Way dynamics
type: Concept

Multiple star evolution
type: Concept
broader: stellar-evolution

Interstellar phases
type: Concept
broader: interstellar-medium

Collisional broadening
type: Concept
broader: atomic-spectroscopy
broader: molecular-spectroscopy

The Sun
broader: solar-system
related: Solar physics
type: Concept
definition: The star that governs the solar system. It is a yellow main-sequence star of spectral type G2, shines with apparent magnitude -26.74, and has an absolute magnitude of +4.83. The Sun is 4.6 billion years old and lies 27,000 light-years from the Galactic center.
related: Solar analogs

Polycyclic aromatic hydrocarbons
broader: molecular-physics
type: Concept
broader: astrochemistry
broader: interstellar-dust

Argument of perihelion
definition: The angular distance between the ascending node of an object orbiting the Sun and its perihelion. Argument of perihelion is measured in the orbital plane with respect to the Sun and in the direction of motion. It is one of the orbital elements and usually shown with the symbol ω.
type: Concept
broader: orbital-elements

H II regions
definition: A type of emission nebulae composed of very hot gas (about 10^4 K), mainly ionized hydrogen, created by the ultraviolet radiation of massive stars. H II regions originate when O or early-type stars, born in giant molecular clouds, start heating up the cold gas, causing it to become ionized and 'glow'. The effective temperatures of the exciting stars are in the range 3 x 10^4 to 5 x 10^4 K, and throughout the nebula hydrogen is ionized. Helium is singly ionized, and other elements are mostly singly or doubly ionized. Typical densities in the H II region are of the order 10 to 10^2 cm^-3, ranging as high as 10^4 cm^-3. Internal motions occur in the gas with velocities of order 10 km s^-1. The spectra of H II regions are mainly composed of strong H I recombination lines and forbidden lines such as [O III], [O II], [N II].
broader: emission-nebulae
type: Concept

Aten group
broader: near-earth-objects
type: Concept
definition: A member of a class of near-Earth asteroids with perihelion distances between 0.983 and 1.0 astronomical units. It is estimated that 6% of the total number of NEAs are Atens.

Photoelectron spectroscopy
type: Concept
broader: molecular-spectroscopy

Gravity anomaly
broader: newtonian-gravitation
type: Concept

Astronomy data modeling
type: Concept
broader: astronomy-data-analysis

Early astronomical instruments
broader: astronomical-instrumentation
related: Archaeoastronomy
type: Concept

Star counts
definition: The number of stars that appear in a given region of sky, usually counted on a photographic plate or CCD image.
broader: surveys
broader: astronomical-models
type: Concept

Astronomy data visualization
type: Concept
broader: astronomy-data-analysis

Stellar types
broader: stellar-astronomy
type: Concept

Blue straggler stars
type: Concept
broader: stellar-evolutionary-types
definition: Any of stars, often found in globular clusters and old open clusters, that lie on the blueward extension of the main sequence beyond the turnoff point. Blue stragglers have an anomalously blue color and high luminosity in comparison with other cluster members. The most probable ways in which they could form are: mass transfer or coalescence in close binary systems, encounters or collisions in overcrowded cores of globular clusters.
related: Yellow straggler stars

Fundamental parameters of stars
type: Concept
broader: astrometry

Recombination (cosmology)
type: Concept
broader: big-bang-theory
broader: early-universe

Stellar coronal dimming
type: Concept
broader: stellar-coronae

Kerr black holes
related: Wormholes
broader: uncharged-black-holes
related: Ring singularities
type: Concept
broader: rotating-black-holes
definition: A black hole that possesses only mass (not electric charge) and rotates about a central axis. It has an ergosphere and a stationary limit.

Night vision
broader: amateur-astronomy
type: Concept

History of astronomy
broader: interdisciplinary-astronomy
type: Concept

AGN host galaxies
type: Concept
related: Active galactic nuclei
broader: active-galaxies

Herbig Ae/Be stars
broader: be-stars
broader: ae-stars
broader: young-stellar-objects
type: Concept

Callisto
type: Concept
broader: galilean-satellites

Shadow bands
definition: Faint wavy lines of alternating light and dark that sometimes can be seen on flat, light-colored surfaces just before and just after a total solar eclipse. The phenomenon results from sunlight distortion by irregularities in the Earth's atmosphere.
type: Concept
broader: solar-eclipses

Baryonic dark matter
type: Concept
broader: dark-matter
definition: Dark matter made up of baryons that are not luminous enough to produce any detectable radiation. It is generally believed that most dark matter is non-baryonic. The baryonic dark matter could reside in a number of forms, including cold gas and compact objects.

Solar convective zone
type: Concept
broader: solar-interior
related: Stellar convective zones

Stellar activity
related: Solar activity
broader: stellar-properties
type: Concept

SRS variable stars
broader: semi-regular-variable-stars
type: Concept

Proton-proton reaction
type: Concept
definition: A thermonuclear reaction in which two protons collide at very high velocities and combine to form a deuterium.
broader: hydrogen-burning

Unbarred spiral galaxies
broader: spiral-galaxies
type: Concept

Active galaxies
broader: galaxies
type: Concept
definition: A galaxy that produces huge amounts of energy at its center, which cannot be attributed to normal processes from stars, interstellar medium, and their interactions. There are several types of active galaxies: Seyfert galaxies, quasars, and blazars. All of these objects show brightness variations, some as short as 3 hours. These fluctuations indicate a relatively very small size for the central object, because an object cannot vary in brightness faster than light can travel across it. For example, an object that is one light-year in diameter cannot vary significantly in brightness over a period of less than one year.

Intergalactic filaments
type: Concept
broader: intergalactic-medium

Very long baseline interferometry
definition: A technique in radio interferometry in which the individual telescopes are not directly connected together, but instead make their observations separately with very accurate timings. The data are later sent to a central correlator to be combined. With this technique the individual telescopes can be arbitrarily far apart, and so the technique provides the highest resolution images in astronomy, typically down to a few milliarcseconds.
related: Very long baseline interferometers
broader: radio-interferometry
type: Concept

Protoclusters
type: Concept
broader: star-clusters

Perturbation methods
type: Concept
broader: analytical-mathematics
related: Cosmological perturbation theory
broader: celestial-mechanics

Ergosphere
broader: black-hole-physics
type: Concept
definition: The region between the event horizon and the stationary limit of a rotating Kerr black hole. It is possible for a particle falling inside the ergosphere to break into two parts, one of which will fall into the black hole and the other will come out.

Solar M coronal region
use instead: Solar coronal holes
deprecated: :__
type: Concept

Light curve classification
broader: time-series-analysis
type: Concept
related: Light curves

I galaxies
type: Concept
use instead: Irregular galaxies
deprecated: :__

Nested sampling
broader: markov-chain-monte-carlo
type: Concept

Beryllium abundance
deprecated: :__
type: Concept

Hot ionized medium
type: Concept
broader: interstellar-phases

Solar coronal lines
type: Concept
related: Stellar coronal lines
definition: An emission line in the spectrum of the solar corona caused by highly ionized metal ions; especially those of iron, such as the red and green lines at 6375 Å and 5303 Å [Fe X] and [Fe XIV], respectively. From their discovery in 1870 until 1939, it was believed that these forbidden lines would be due to an unknown element, called coronium. Ultraviolet and X-ray coronal lines are also detectable in stars.
broader: solar-corona

Comae
type: Concept
definition: The nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun.
broader: comets

Atmospheric dynamics
type: Concept
broader: planetary-atmospheres
related: Exoplanet atmospheric dynamics

Galaxy spectroscopy
type: Concept
broader: spectroscopy
related: Galaxies

Interplanetary turbulence
broader: interplanetary-physics
type: Concept

Small Solar System bodies
definition: A term introduced by the International Astronomical Union (August 2006) to name the solar system bodies other than planets and dwarf planets. Small solar system bodies include asteroids, comets, and meteoroids.
type: Concept
broader: solar-system

Solar-terrestrial interactions
broader: solar-planetary-interactions
type: Concept

Exoplanet surface characteristics
type: Concept
broader: exoplanet-surfaces

Supernova neutrinos
type: Concept
broader: particle-astrophysics

Solar atmosphere
broader: solar-physics
type: Concept
related: Stellar atmospheres

Bipolar nebulae
type: Concept
broader: nebulae
definition: An interstellar cloud of ionized gas with two main lobes which lie symmetrically on either side of a central star. The bipolar shape is generally due to the ejection of material by the central star in opposing directions.

Land-atmosphere interactions
broader: earth-atmosphere
type: Concept

Baryon acoustic oscillations
definition: In cosmology, one of a series of peaks and troughs that are present in the power spectrum of matter fluctuations after the recombination era, and on large scales. At the time of the Big Bang, and for about 380,000 years afterwards, Universe was ionized and photons and baryons were tightly coupled. Acoustic oscillations arose from perturbations in the primordial plasma due to the competition between gravitational attraction and gas+photons pressure. After the epoch of recombination, these oscillations froze and imprinted their signatures in both the CMB and matter distribution. In the case of the photons, the acoustic mode history is manifested as the high-contrast Doppler peaks in the temperature anisotropies. As for baryons, they were in a similar state, and when mixed with the non-oscillating cold dark matter perturbations, they left a small residual imprint in the clustering of matter on very large scales, ~100 h^-1 Mpc (h being the Hubble constant in units of 100 km s^-1 Mpc^-1). The phenomenon of BAOs, recently discovered using the Sloan Digital Sky Survey data, is a confirmation of the current model of cosmology. Like Type Ia supernovae, BAOs provide a standard candle for determining cosmic distances. The measurement of BAOs is therefore a powerful new technique for probing how dark energy has affected the expansion of the Universe.
broader: early-universe
type: Concept

Unified Astronomy Thesaurus (IVOA rendering)
license: legalcode
description: This is a re-publication of the Unified Astronomy Thesaurus (see https://astrothesaurus.org), in particular using human-readable URIs and with availability in Desise.
vocflavour: SKOS
created: 2023-02-28
type: Ontology
creator: The UAT Steering Committee

Neutrino astronomy
broader: astronomical-methods
type: Concept
broader: high-energy-astrophysics

Astrometry
type: Concept
broader: astronomical-techniques
related: Astrometric exoplanet detection
definition: The precise measurement of the positions and motions of astronomical objects.

Horseshoe mounts
type: Concept
deprecated: :__

Ring resonance
type: Concept
broader: planetary-rings

Semi-detached binary stars
broader: interacting-binary-stars
definition: A binary system whose secondary member fills its Roche lobe but whose primary member does not.
type: Concept

Carbonaceous grains
broader: interstellar-dust
type: Concept

Apparent position
definition: A position given by the coordinates calculated for a star, if it were seen from the Earth's center, relative to the real equator and the real equinox, at a certain date. It includes the displacements from one heliocentric direction, given in a stellar catalogue, due to precession, nutation, aberration, proper motions, annual parallax, and light gravitational deviation.
type: Concept
broader: astronomical-location

Non-radial pulsations
broader: stellar-pulsations
type: Concept

Period determination
type: Concept
broader: apparent-magnitude

BV photometry
broader: broad-band-photometry
type: Concept

Frozen in elements
type: Concept
broader: chemical-abundances

Remote sensing
type: Concept
broader: astronomical-techniques
definition: Technique that utilizes electromagnetic waves to detect, measure, and obtain information about an object that is not in contact with the sensing apparatus.
related: Planetary science

Solar system formation
broader: planetary-system-formation
type: Concept
related: Solar system
related: Solar nebulae

B(e) stars
definition: A Be star with forbidden lines in emission in its spectrum. B[e] stars show large infrared excess due to circumstellar dust emission.
type: Concept
broader: be-stars

Effective temperature
definition: A measure of the surface temperature of a star derived from the total emitted energy, assuming that the star is a blackbody emitter.
broader: astrometry
type: Concept

CO line emission
type: Concept
broader: interstellar-line-emission

Active sun
definition: The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum.
related: Quiet sun
type: Concept
broader: solar-cycle

Galaxy magnitudes
type: Concept
broader: magnitude
broader: galaxy-properties

Particle physics
definition: The branch of physics that deals with the smallest known structures of matter and energy in order to understand the fundamental particles and forces of nature.
type: Concept
related: Particle astrophysics
broader: laboratory-astrophysics

Astrostatistics
type: Concept
definition: A branch of applied mathematics that deals with the collection and interpretation of quantitative data and the use of probability theory to estimate population parameters, applied to astronomy.
broader: interdisciplinary-astronomy

Geocentric orbit
broader: orbital-motion
type: Concept

Black hole physics
type: Concept
related: Black holes
broader: high-energy-astrophysics

Astrophysical black holes
type: Concept
broader: black-holes

Far infrared interferometry
broader: far-infrared-astronomy
broader: direct-imaging
type: Concept

Red giant branch
related: Red giant stars
definition: The evolutionary path of a star that has exhausted its available hydrogen content in the core, between the main-sequence turnoff and the helium flash.
type: Concept
broader: giant-branch

Relativistic disks
type: Concept
broader: active-galactic-nuclei

Low-mass x-ray binary stars
broader: x-ray-binary-stars
definition: A member of one of the two main classes of X-ray binary systems where one of the components is a neutron star or black hole and the other component a low-mass star with a spectral type A or later. LMXBs mainly emit soft X-rays. The ratio of their optical to X-ray luminosities is less than 0.1. They belong to old stellar populations with ages 5-15 x 10^9 years and are found in globular clusters and in the bulge of our Milky Way galaxy; some are also found in the disk. Hercules X-1 is an example of LMXBs.
type: Concept

Trojan planets
broader: exoplanets
type: Concept

Giant galaxies
broader: galaxies
type: Concept

Collision physics
broader: molecular-physics
definition: The physical interactions of atoms and molecules when they are brought into close contact with each other and with electrons, protons, neutrons or ions. This includes energy-conserving elastic scattering and inelastic scattering. Such collisions are an important probe of the structure and properties of matter.
broader: atomic-physics
type: Concept

Infrared observatories
type: Concept
broader: observatories

Post-starburst galaxies
broader: galaxies
type: Concept
related: Starburst galaxies

Charge exchange recombination
broader: recombination
type: Concept

Phase dispersion minimization
broader: period-search
type: Concept

Wolf-Rayet stars
definition: A type of very luminous, very hot (as high as 50,000 K) stars whose spectrum is characterized by broad emission lines (mainly He I and He II), which are presumed to originate from material ejected from the star at very high (~ 2000 km s^-1) velocities. The most massive O stars (M > 25 solar masses for solar metallicity) become W-R stars around 2 and 3 million years after their birth, spending only some few hundreds of thousands of years (≤ 10^6 years) in this phase until they explode as type Ib and type Ic supernovae. The minimum stellar mass that an O star needs to reach the W-R phase and its duration is dependent on metallicity.
type: Concept
related: Stellar classification
broader: early-type-emission-stars

Solstice
definition: Either of the two points on the ecliptic at which the apparent longitude of the Sun is 90° or 270°. Also the time at which the Sun is at either point. Solstices occur when the Earth's axis is oriented directly toward or away from the Sun, causing the Sun to reach its northernmost and southernmost extremes.
broader: equatorial-coordinate-system
type: Concept

Solar filament eruptions
broader: solar-filaments
type: Concept

Heliostats
type: Concept
definition: An instrument consisting of a mirror moved by clockwork for tracking the movement of the Sun and reflecting the sunlight into a stationary solar telescope. A heliostat is similar to a coelostat.
broader: stellar-tracking-devices

Posterior distribution
broader: bayesian-statistics
type: Concept

Cirrus fibratus
type: Concept
deprecated: :__

Kruskal diagram
definition: A diagram used to plot trajectories in space-time near a black hole. The vertical and horizontal axes are two complicated functions of time and distance from the black hole. Lines of constant time radiate from the origin of the diagram, with steeper slopes corresponding to later times. Lines of constant distance are hyperbolas, lines of constant time pass through the origin; photons always travel along diagonal lines at ±45° to the vertical. The trajectory of an object falling into the black hole is shown as a curving line moving upward on the diagram at less than 45° to the vertical.
type: Concept
broader: black-hole-physics

Magnetohydrodynamics
broader: astrophysical-fluid-dynamics
related: Magnetohydrodynamical simulations
broader: plasma-astrophysics
definition: The dynamics of an ionized plasma in the non-relativistic, collisional case. In this description, charge oscillations and high frequency electromagnetic waves are neglected. It is an important field of astrophysics since plasma is one of the commonest forms of matter in the Universe, occurring in stars, planetary magnetospheres, and interplanetary and interstellar space.
related: Alfven waves
type: Concept

Medium band photometry
broader: photometric-systems
related: Infrared photometry
type: Concept

Apocenter
broader: apsides
type: Concept
definition: The point on an elliptic orbit at the greatest distance from the principal focus or center of attraction.

BL Herculis variable stars
type: Concept
broader: type-ii-cepheid-variable-stars

Milky Way Galaxy
related: Oort constants
broader: spiral-galaxies
definition: A spiral galaxy, of which the solar system is a small part. It is the second largest in our Local Group of galaxies. The Milky Way is a disk-shaped system, with a diameter of between 80,000 and 100,000 light-years and a thickness of about 2,000 light-years, containing more than 10^11 stars. The stars are divided into two main categories, Population II stars and Population I stars. The core, or nucleus, of the Galaxy is surrounded by an ellipsoidal central bulge that measures some 15,000 light-years in diameter and about 6,000 light-years in the direction perpendicular to the plane of the disk. Surrounding the bulge and extending in a near spherical distribution above and below the Galactic plane is the Galactic halo. The halo contains about 200 globular clusters and an extremely thinly scattered population of individual stars. The Sun is located just over half way out from the center to the edge of the disk at a distance of about 25,000 light-years. In common with other stars, the Sun revolves around the Galactic Center. Its orbital velocity is about 220 km s^-1 and its orbital period is about 225 million years. Overall, the Galaxy exhibits differential rotation, that is stars and gas clouds closer to the center have shorter orbital periods than those that are located further out. The spiral arms of the Milky Way lie within its disk, where bright young stars, H II regions, and molecular clouds of gas and dust are concentrated into curved 'arms' that appear to radiate from the central bulge in a spiral pattern. The Galaxy's spiral pattern consists of several major arms and a number of shorter segments, one of which, the Orion arm, contains the Sun and the Orion star-forming region. Near-infrared observations have shown that the stars in the central bulge are arranged in an elongated galactic bar, about twice as long as it is wide, that is seen nearly end on from the present location of the solar system. The exact center, or nucleus, of the Galaxy coincides with a str…
related: Milky Way Galaxy physics
type: Concept
related: Magellanic Clouds
broader: local-group

Stellar effective temperatures
broader: stellar-properties
type: Concept

Stellar feedback
broader: galaxy-formation
type: Concept
definition: The process whereby large quantities of energy and momentum are released into the gas surrounding star formation regions in galaxies. More specifically, massive stars inject energy, mass, and metals back to the interstellar medium through stellar winds and supernova explosions. Feedback inhibits further star formation either by removing gas from the galaxy, or by heating it to temperatures that are too high to form new stars. Observations reveal feedback in the form of galactic-scale outflows of gas in galaxies with high star formation rates, especially in the early Universe. Feedback in faint, low-mass galaxies probably facilitated the escape of ionizing radiation from galaxies when the Universe was about 500 million years old, so that the hydrogen between galaxies changed from neutral to ionized, a process called reionization.

Galaxy triplets
type: Concept
broader: interacting-galaxies

Coelostats
definition: A flat mirror with a clock-drive mounted in such a way that it moves from east to west to compensate for the apparent rotation of the Earth in order that the image of a particular area of sky remains fixed in the focal plane.
broader: stellar-tracking-devices
type: Concept

Stellar tracking devices
broader: astronomical-instrumentation
type: Concept

Calibration
broader: astronomical-techniques
type: Concept

Microwave spectroscopy
broader: rotational-spectroscopy
type: Concept

Astronomy data acquisition
broader: astronomy-data-analysis
type: Concept

Heavy metal stars
broader: chemically-peculiar-stars
type: Concept
broader: late-type-giant-stars

Atmospheric science
type: Concept
related: Planetary atmospheres
broader: planetary-science

Closed universe
definition: A cosmological model, first formulated by Friedmann and Lemaître, in which the Universe has a finite size and lifetime and space has a positive curvature, e.g. a Universe with a density greater than the critical density.
type: Concept
broader: cosmological-models

Corotating streams
type: Concept
broader: interplanetary-physics

Asterisms
broader: celestial-sphere
type: Concept

Newtonian telescopes
type: Concept
broader: reflecting-telescopes

Sagittarius dwarf spheroidal galaxy
definition: A satellite galaxy of the Milky Way discovered only in 1994 since most of it is obscured by the Galactic disc. At only 50,000 light years distant from our Galaxy's core, it is travelling in a polar orbit around the Galaxy. Our Galaxy is slowly devouring it, as evidenced by a filament which stretches around the Milky Way's core like a gossamer loop. It is only about 10,000 light-years in diameter, in comparison to the Milky Way's diameter of 100,000 light years. It is populated by old yellowish stars has four known globular clusters: M54, Arp 2, Terzan 7, and Terzan 8. It should not be confused with the Sagittarius Dwarf Irregular Galaxy.
broader: dwarf-spheroidal-galaxies
type: Concept

PV Telescopii variable stars
type: Concept
broader: pulsating-variable-stars

Natural satellite surfaces
related: Lunar surface
related: Natural satellites (Solar system)
type: Concept
broader: planetary-science

Radio source catalogs
type: Concept
broader: catalogs

Disk stars
deprecated: :__
type: Concept

Sunskirters
definition: Objects that pass within 33 solar radii of the Sun's center, equal to half of Mercury's perihelion distance.
type: Concept
broader: near-sun-comets

Stellar interiors
broader: stellar-structures
related: Solar interior
type: Concept

Zenith
definition: The point on the celestial sphere directly above the observer's head, opposite to the direction in which gravity acts. Opposite of nadir.
type: Concept
broader: horizontal-coordinate-system

Galaxy clusters
type: Concept
definition: An aggregation of galaxies, made up of a few to a few thousand members, which may or may not be held together by its own gravity.
broader: galactic-and-extragalactic-astronomy

Ice phases
type: Concept
broader: ice-physics

Andromeda Galaxy
broader: local-group
broader: spiral-galaxies
type: Concept
definition: The nearest spiral galaxy to our own and a major member of the Local Group. It lies in the constellation Andromeda and is the most remote object normally visible to the naked eye. The earliest known reference to this galaxy is by the Iranian astronomer Sufi who called it 'the little cloud' in his Book of Fixed Stars (A.D. 964).

Extreme horizontal branch stars
type: Concept
related: Horizontal branch
broader: horizontal-branch-stars

O associations
type: Concept
broader: stellar-associations

Uranian satellites
type: Concept
related: Uranus
broader: natural-satellites-solar-system

Galaxy rotation
broader: galaxy-kinematics
related: Milky Way rotation
type: Concept

O stars
broader: stellar-spectral-types
related: Stellar classification
broader: early-type-stars
type: Concept
definition: A luminous, hot, blue star whose spectrum is dominated by the lines of hydrogen, atomic helium, and ionized helium; also known as O-type star. This is the earliest spectral type and the only main sequence star in which ionized helium is present. The effective temperatures of these stars range from about 30,000 K to 50,000 K, their luminosities from 50,000 to 1,000,000 times that of solar luminosity, and their masses from about 20 to 100 solar masses. The hottest O-type stars display high ionization emission features such as N III and He II, Of star. They are divided into subtypes O2, the hottest, to O9.7, the coldest. O-type stars are relatively rare, for each star of 100 solar masses there are 10^6 stars of solar mass. They are relatively short-lived since they spend only a few million years on the main sequence. The brightest O-type star in the sky visible with naked eye is Alnitak.

F test
broader: parametric-hypothesis-tests
type: Concept

Atomic reactions
broader: atomic-physics
type: Concept

Stellar-interstellar interactions
broader: interstellar-medium
type: Concept

G giant stars
broader: g-stars
type: Concept
broader: giant-stars

Galactic anticenter
related: Galaxy anticenters
broader: milky-way-galaxy-physics
definition: The point in the Galactic plane that lies directly opposite the Galactic center. It lies in the constellation Auriga at approximately R.A. 05h 46m, Dec. +28° 56'.
type: Concept

Friedmann universe
type: Concept
broader: cosmological-models

Apollo group
broader: near-earth-objects
definition: A member of a class of near-Earth asteroids (NEAs) that have orbital semi-major axes greater than that of the Earth (> 1 astronomical unit) but perihelion distances less than the Earth's aphelion distance (less than 1.017 AU); thus, they cross the Earth's orbit when near the perihelia of their orbits. They are named for the prototype Apollo.
type: Concept

Planetary geology
broader: planetary-structure
type: Concept

Gamma-ray bursters
broader: gamma-ray-transient-sources
definition: The object or phenomenon at the origin of a gamma-ray burst.
type: Concept
related: Gamma-ray bursts

Iron meteorites
broader: meteorites
definition: A meteorite which is composed mainly of iron mixed with smaller amounts of nickel. Iron meteorites make up about 4.4% of all meteorites.
type: Concept

AM Herculis stars
broader: stellar-remnants
related: DQ Herculis stars
type: Concept
broader: cataclysmic-variable-stars

Solar wind termination
type: Concept
broader: interplanetary-physics

M stars
broader: stellar-spectral-types
definition: A cool, red star of spectral type M with a surface temperatures of less than 3600 K. The spectra of M stars are dominated by molecular bands, especially those of TiO. Naked-eye examples are Betelgeuse and Antares.
type: Concept
related: Stellar classification
broader: late-type-stars

Smoothing
broader: spatial-point-processes
broader: nonparametric-inference
related: Regression
type: Concept
definition: The mathematical process that makes a curve smooth.

SX Arietis variable stars
broader: early-type-variable-stars
broader: magnetic-variable-stars
broader: dwarf-novae
type: Concept
broader: cataclysmic-variable-stars

Photon sphere
type: Concept
broader: black-hole-physics
definition: A surface where if a photon is emitted from one of its points the photon follows a closed orbit and returns periodically to its departure point. Such a surface exists only near sufficiently compact objects where the curvature of space-time is very important. In other words, a body can take a stable orbit around a black hole provided that it moves with the speed of light. However, only photons can have such a velocity; hence the term 'photon sphere.' For a non-rotating Schwarzschild black hole, the photon sphere has a radius of R = 3GM/c2 = 3 RS/2, where G is the gravitational constant, M is the mass, c is the speed of light, and RS is the Schwarzschild radius. For a rotating, Kerr black hole, the situation is much more complex due to the Lense-Thirring effect. In that case circular paths exist for radii whose values depend on the rotation direction. More specifically, in the equatorial plane there are two possible circular light paths: a smaller one in the direction of the rotation, and a larger one in the opposite direction.

Emission nebulae
definition: An ionized nebula whose spectrum consists of emission lines.
related: Gaseous nebulae
broader: nebulae
related: Herbig-Haro objects
type: Concept

Deprecated: Mira Variables
use instead: Mira variable stars
type: Concept
deprecated: :__

Active galactic nuclei
broader: high-energy-astrophysics
broader: active-galaxies
related: Radio sources
related: AGN host galaxies
definition: A central region of an active galaxy, which is a light-year or less in diameter and has an abnormally high luminosity. The nucleus emits high energy radiation (gamma rays, X-rays, ultraviolet) and shows variability over various time-scales, sometimes very short (hours to weeks). Emission line spectra reveal high velocity motions up to 10^4 km s^-1. AGNs are divided into two main types. Type I refers to an AGN whose nucleus is visible (the spectra has both narrow and broad emission lines), while in type II AGN, the broad line region (BLR) is obscured and the lines are very narrow. This may be due either to the viewing angle or some intrinsic difference in structure.
type: Concept
broader: galaxy-nuclei

Bootstrap
type: Concept
related: Cross-validation
broader: uncertainty-bounds
broader: astrostatistics-techniques

Dust destruction
type: Concept
broader: dust-physics

Hot dark matter
type: Concept
definition: Any form of dark matter which had a significant velocity dispersion (comparable to the velocity of light), when the Universe first became matter-dominated.
broader: dark-matter

Darkrooms
deprecated: :__
type: Concept

Warm dark matter
type: Concept
broader: dark-matter

Scattered disk objects
broader: resonant-kuiper-belt-objects
type: Concept

Convolutional neural networks
type: Concept
broader: neural-networks

Long baseline interferometry
broader: interferometry
type: Concept

Stellar faculae
related: Solar faculae
broader: stellar-photospheres
type: Concept

Solar evolution
related: Stellar evolution
broader: solar-physics
type: Concept

Gravastars
type: Concept
broader: quantum-gravity

Holographic interferometry
broader: interferometry
type: Concept

Surface ices
type: Concept
broader: planetary-surfaces

Fabry-Perot interferometers
broader: interferometers
type: Concept

Ecliptic latitude
broader: ecliptic-coordinate-system
definition: One of the two coordinates in the ecliptic system; the angle measured from the ecliptic, positive toward the north.
type: Concept

SRa variable stars
type: Concept
broader: semi-regular-variable-stars

Solar radio emission
type: Concept
broader: solar-electromagnetic-emission

Dirac cosmology
broader: cosmological-models
type: Concept

Bianchi cosmology
broader: cosmology
type: Concept

Atmospheric clouds
type: Concept
broader: planetary-atmospheres

Two-spectrum binary stars
deprecated: :__
type: Concept

Extrinsic variable stars
type: Concept
definition: A star whose variation in apparent brightness is not due to changes in the star itself but to some external cause, such as eclipsing by a companion.
broader: variable-stars

Gamma-ray sources
definition: An astronomical object that emits gamma rays.
related: Gamma-ray astronomy
broader: astronomical-radiation-sources
broader: gamma-rays
type: Concept

Blazhko effect
broader: rrab-variable-stars
broader: rrc-variable-stars
definition: A long term, generally irregular modulation of light curves of a large subclass of RR Lyrae stars. Most of the modulations occur on the time scale of some 60 periods, although the range extends from some tens to some hundreds of periods. Since its discovery over a hundred years ago, a number of explanations have been proposed for this effect, but its nature is still a matter of investigation. The explanations include: closely spaced pulsation modes, a modal 1:2 resonance, an oblique rotator model, a non-radial modal interaction, convective cycles, and nonlinear resonant mode coupling between the 9th overtone and the fundamental mode.
type: Concept

Markov chain Monte Carlo
broader: algorithms
type: Concept
broader: computational-methods

Umbra
broader: eclipses
type: Concept
definition: A dark area, especially the blackest part of a shadow from which all light is cut off.

Occultation
broader: syzygy
definition: The blocking of light from an astronomical object, such as a star, by another object, such as the Moon or an asteroid, that passes in front of it. Also, the period of time for which this blocking takes place. A solar eclipse is a particular case of occultation. Determining the moment of stellar occultation by the Moon is used to improve our knowledge of the lunar orbit. Moreover, the occultation of stars by asteroids or planetary satellites provide important data for better measuring the sizes of the occulting bodies.
related: Transits
type: Concept

Astronomical optics
type: Concept
broader: astronomical-techniques

Model selection
type: Concept
broader: astrostatistics-techniques

Plasma physics
related: Plasma astrophysics
definition: The study of the physical properties of the various forms of plasmas and their processing.
broader: laboratory-astrophysics
type: Concept

Asteroid surfaces
broader: asteroids
type: Concept

Ionization
type: Concept
definition: The process by which ions are produced, typically occurring by interaction with electromagnetic radiation ('photoionization'), or by collisions with atoms or electrons ('collisional ionization').
broader: atomic-reactions
broader: molecular-reactions

Nucleated dwarf galaxies
type: Concept
broader: dwarf-galaxies

Biosignatures
broader: astrobiology
type: Concept

Equatorial coordinate system
broader: astronomical-coordinate-systems
type: Concept
definition: An astronomical coordinate system for indicating the positions of celestial objects on the celestial sphere. The system consists of two components, right ascension and declination. Right ascension is the angle between the vernal equinox and the point where the hour circle intersects the celestial equator. The right ascension is always measured eastward from the vernal equinox, in the units of hours, minutes, and seconds. Declination is the angle between the celestial equator and the position of the star measured along the star's hour circle. It is measured in degrees, minutes, and seconds north or south of the celestial equator. By definition, the vernal equinox is located at right ascension 0h and declination 0°. Equatorial coordinates change with time due to the precession of the Earth's rotation axis.

Protogalaxies
broader: galaxies
type: Concept
related: Cosmology
definition: A huge mass of gas that by contraction and condensation becomes a galaxy of stars. A galaxy during the early phase, before it has developed its present shape and stellar/gas content.

Cosmic ray showers
broader: cosmic-rays
type: Concept

K subgiant stars
broader: k-stars
type: Concept
broader: subgiant-stars

Transition probabilities
broader: atomic-physics
type: Concept
definition: The probability that a quantum-mechanical system will make a transition from a given initial state to a given final state.

Carbon dioxide
broader: greenhouse-gases
type: Concept
definition: CO_2, also called carbonic acid gas. A colorless gas which occurs in the atmosphere playing an essential part in animal respiration and the growth of green plants. It is formed by the oxidation of carbon and carbon compounds. Carbon dioxide is the most important greenhouse gas produced by human activities, primarily through the combustion of fossil fuels. Its concentration in the Earth's atmosphere has risen by more than 30% since the Industrial Revolution. CO_2 forms a solid at -78.5 °C at atmospheric pressure, and is used as a refrigerant in this form as a dry ice for the preservation of frozen foods. As carbon dioxide gas is heavier than air and does not support combustion, it is used in fire extinguishers. CO_2 is present in the interstellar medium and is one of the main molecules in comets.

International Space Station
type: Concept
broader: space-stations

Planetary ring divisions
broader: planetary-rings
type: Concept

Dwarf planets
type: Concept
broader: solar-system
definition: A new category of astronomical objects in the solar system introduced in a resolution by the 26th General Assembly of the International Astronomical Union (IAU) on August 24, 2006. The characterizing properties are as follows: 1) It is in orbit around the Sun; 2) It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; 3) It has not 'cleared the neighbourhood' around its orbit; and 4) It is not a satellite of a planet, or other non-stellar body. The property 3 reclassified Pluto from a planet to a dwarf planet because it has not cleared the neighborhood of its orbit (the Kuiper Belt). The largest known dwarf planets are: Eris, Pluto, Ceres, Makemake, and 2015 RR245.

Interplanetary physics
related: Solar wind
related: Interplanetary magnetic fields
broader: planetary-science
type: Concept

Galactic bulge
related: Galaxy bulges
type: Concept
broader: milky-way-galaxy-physics
definition: The central galaxy bulge of the Milky Way.

Superclusters
broader: galaxy-clusters
type: Concept
definition: An aggregation of clusters of galaxies. Superclusters are typically about one hundred million (10^8) light-years in diameter and contain tens of thousands of galaxies. Some examples are the Local Supercluster, Centaurus supercluster, Laniakea supercluster, Perseus-Pisces supercluster, Shapley supercluster, Virgo supercluster.

Supernova remnants
broader: nebulae
type: Concept
broader: interstellar-medium
definition: The body of expanding gas ejected at a speed of about 10,000 km s^-1 by a supernova explosion, observed as a diffuse gaseous nebula, often with a shell-like structure. Supernova remnants are generally powerful radio sources. The evolution of the SNR can be divided into different phases according to the dominant physical processes. Simplified models are made for the first stages, to get an idea of typical time scales, expansion velocities, and sizes. The three main phases are: 1) the free expansion phase, 2) the Sedov-Taylor phase, and 3) the snowplow phase.

Near-Earth objects
type: Concept
related: Comets
related: Close encounters
related: Asteroids
broader: small-solar-system-bodies
definition: An asteroid, comet, or large meteoroid whose orbit brings it exceptionally close to the Earth, and which may therefore pose a collision danger. Most such objects are in orbits around the Sun with perihelion distance less than 1.3 astronomical units.

Ecliptic longitude
broader: ecliptic-coordinate-system
type: Concept
definition: One of the two coordinates in the ecliptic system; the angle measured eastwards along the ecliptic from 0° to 360°, with the origin at the vernal equinox.

Planet formation
related: Protoplanetary disks
type: Concept
broader: planetary-system-formation

Primordial galaxies
broader: early-universe
broader: galaxies
type: Concept
definition: A high redshift, metal-deficient galaxy that formed very early in the history of the Universe.

Milky Way magnetic fields
broader: milky-way-galaxy-physics
related: Galaxy magnetic fields
type: Concept

Radio quiet quasars
type: Concept
definition: A type of quasar with weak radio emission. These types of quasars have strong emissions in both the optical and X-ray spectra. Within the optical spectrum, both broad and narrow emission lines are present. Their host is usually an elliptical galaxy, but less commonly, it might be a spiral.
broader: quasars

Exoplanet tides
type: Concept
broader: exoplanet-dynamics

SRd variable stars
broader: semi-regular-variable-stars
type: Concept

Surface composition
type: Concept
broader: planetary-surfaces
related: Exoplanet surface composition

Interplanetary scintillation
type: Concept
broader: astronomical-optics
broader: radio-astronomy

Neutral-neutral reactions
broader: molecular-reactions
type: Concept

Dust continuum emission
broader: interstellar-emissions
type: Concept

Nuclear fission
type: Concept
broader: nuclear-astrophysics

Solar white-light flares
broader: solar-flares
type: Concept

Interstellar scattering
type: Concept
broader: interstellar-medium

Cosmic magnetic fields theory
type: Concept
broader: astrophysical-magnetism

Hydroxyl sources
type: Concept
broader: astronomical-radiation-sources
related: Hydroxyl masers
definition: An astronomical source emitting microwave radiation characteristic of the hydroxyl OH molecule, especially one showing a maser effect. OH sources are found in molecular clouds in interstellar medium and in the cool envelopes of evolved stars.
related: OH/IR stars
related: Cocoon stars

Astronomical reference materials
type: Concept
broader: astronomical-research

Compact binary stars
definition: A binary star system which is composed of a collapsed object (degenerate dwarf, neutron star, or black hole) in orbit with a low-mass (≤ 0.5 M_sol) secondary star, wherein the collapsed star accretes matter from its companion. These two objects form a binary system of overall dimensions 10^6 km with an orbital period of only hours or less.
broader: close-binary-stars
broader: interacting-binary-stars
type: Concept

Limb darkening
type: Concept
definition: An apparent decrease in brightness of the Sun near its edge as compared to its brightness toward the center. Limb darkening is readily apparent in photographs of the Sun. The reason is that when we look toward the disk's center we look into deeper and hence hotter layers along the line of sight. Toward the limb, we get radiation from higher and hence cooler and less bright layers of the photosphere. Limb darkening has been detected in the case of several other stars. A similar phenomenon occurs in eclipsing binaries where the effect of limb darkening on one or both components manifests itself in the shape of the system's light curve.
broader: solar-atmosphere

Matter density
broader: density-parameters
type: Concept

Celestial mechanics
definition: The branch of astronomy that deals with the calculation of motions of celestial bodies under the action of their mutual gravitational attractions.
type: Concept
broader: gravitation

Night sky brightness
broader: sky-brightness
type: Concept

Astrophysical dust processes
related: Solid matter physics
related: Intergalactic medium
type: Concept
broader: cosmology

Lunar rays
type: Concept
broader: lunar-features

CII region
broader: astrochemistry
type: Concept

Habitable planets
type: Concept
broader: exoplanets

Confidence interval
broader: uncertainty-bounds
type: Concept

Automatic patrol telescopes
broader: automated-telescopes
type: Concept

Summer solstice
type: Concept
broader: solstice
definition: The moment in the northern hemisphere when the Sun attains its highest declination of 23°26' (or 23°.44) with respect the equator plane. It happens when the Earth's axis is orientated directly toward the Sun, on 21 or 22 June. During the northern solstice the Sun appears to be directly overhead at noon for places situated at latitude 23.44 degrees north, known as the tropic of Cancer. The summer solstice can occur at any moment during the day. Two successive summer solstices are shifted in time by about 6 h. The summer solstice in the northern hemisphere is the winter solstice in the southern hemisphere.

Astrospheres
type: Concept
broader: stellar-interstellar-interactions

Geocorona
definition: The outermost part of Earth's atmosphere, a tenuous halo of hydrogen and some helium extending out to perhaps 15 Earth radii, which emits Lyman-alpha radiation when it is bombarded by sunlight.
type: Concept
broader: exosphere

High-energy cosmic radiation
type: Concept
broader: cosmic-rays

Hubble diagram
broader: hubble-s-law
definition: A plot of the redshift of galaxies against their distance or against their apparent magnitude.
type: Concept

Photogrammetry
broader: photographic-astrometry
type: Concept
broader: photographic-photometry

Primary mirror
broader: reflecting-telescopes
type: Concept

Helium abundance
deprecated: :__
type: Concept

Oblique rotators
type: Concept
broader: asteroseismology

Tides
broader: tidal-interaction
type: Concept

Molecular reactions
type: Concept
broader: molecular-physics

Sunspot cycle
broader: solar-cycle
type: Concept
related: Sunspots

Cosmic anisotropy
type: Concept
broader: cosmic-microwave-background-radiation

Circumstellar shells
broader: stellar-structures
type: Concept
broader: circumstellar-matter
definition: A shell of dust, molecules, and neutral gas around an evolved star resulting from an intensive mass loss phase, such as the asymptotic giant branch phase for low- and intermediate mass stars and LBVs or supernovae for massive stars.

Quiet solar corona
broader: solar-corona
type: Concept

Bok globules
type: Concept
broader: protostars
broader: dark-interstellar-clouds
broader: h-ii-regions

Space research
type: Concept
broader: astronomical-research

Filamentary nebulae
type: Concept
broader: nebulae
definition: A type of nebula, generally ionized, consisting of filament-like structures of gas, such as the Veil Nebula (NGC 6960) or the supernova remnant IC 443.
related: Interstellar filaments

Solar eclipses
broader: eclipses
type: Concept
definition: An eclipse in which the Earth passes through the shadow cast by the Moon. Solar eclipses only happen when the Moon is new and when the Moon lies close to the node of its orbit.

Intergalactic voids
type: Concept
use instead: Voids
deprecated: :__

Finite-source photometric effect
type: Concept
broader: gravitational-microlensing

Interstellar line absorption
type: Concept
broader: interstellar-absorption

Early-type emission stars
broader: early-type-stars
type: Concept
broader: emission-line-stars

Flat-spectrum radio quasars
definition: A member of a small subset of quasars consisting of bright radio galaxies whose flux of visible light output can vary by as much as 50% in a single day.
broader: blazars
type: Concept

Uranus
broader: outer-planets
definition: The seventh planet from the Sun and the third largest, discovered by William Herschel in 1781. It has a diameter about 51,100 km, four times that of the Earth, and a mass 14.5 times Earth's mass. Uranus orbits the Sun at a distance over 19 times the Earth-Sun distance. Although it takes about 84 years for Uranus to make a revolution, it completes a fast rotation in only 17 and a half hours. Unlike the other planets, its axis of rotation lies mostly in the plane of the Solar System. Uranus is internally less active than the other giant planets, which added to its larger distance from the Sun, makes it colder. It has a dense atmosphere made of mostly molecular hydrogen (83 percent) and helium (15 percent), with two percent methane and traces of acetylene and other hydrocarbons. The planet's greenish-blue color is due to light scattering as in Earth's sky and the absorption of red light by its small amount of atmospheric methane. Uranus has a ring system and 27 known satellites.
related: Uranian satellites
type: Concept

Solar coronal heating
broader: solar-corona
type: Concept

Eros
definition: An S-type asteroid about 34.4 × 11.2 × 11.2 km in size, which is the second-largest near-Earth asteroid after 1036 Ganymede. It belongs to the group of Amor asteroids. Eros was discovered on 13 August 1898 by Gustav Witt in Berlin and Auguste Charlois at Nice. It was the first asteroid orbited by an Earth probe (in 2000).
type: Concept
broader: amor-group

Stellar masses
related: Solar mass
type: Concept
broader: stellar-properties
related: Stellar mass functions

Carbon-nitrogen cycle
broader: hydrogen-burning
type: Concept

Blank fields
type: Concept
broader: field-of-view

Interplanetary discontinuities
type: Concept
broader: interplanetary-physics

Mach-Zehnder interferometers
type: Concept
broader: interferometers
broader: optical-phase-shifting-interferometry

Planetary bow shocks
broader: interplanetary-physics
type: Concept
broader: shocks

Greenhouse gases
broader: greenhouse-effect
definition: Gases responsible for the greenhouse effect. These gases include: water vapor; carbon dioxide; methane; nitrous oxide; chlorofluorocarbons; and tropospheric ozone.
type: Concept

Virgo Supercluster
broader: superclusters
type: Concept
definition: The irregular supercluster that contains the Virgo cluster and the Local Group. At least 100 galaxy groups and clusters are located within its diameter of 110 million light-years. It is one of millions of superclusters in the observable Universe.
related: Virgo Cluster

Meteor radiants
type: Concept
broader: meteor-showers

X-ray stars
broader: emission-line-stars
type: Concept

Kracht group
type: Concept
broader: sunskirters

Galaxy tides
broader: interacting-galaxies
type: Concept

Ultraviolet observatories
broader: observatories
type: Concept

F giant stars
type: Concept
broader: giant-stars
broader: f-stars

Solar active regions
broader: solar-activity
type: Concept

CNO anomaly
broader: solar-physics
type: Concept

Schmidt cameras
type: Concept
broader: catadioptric-telescopes

Asteroid satellites
broader: natural-satellites-solar-system
type: Concept
broader: asteroids

Solar system planets
type: Concept
broader: solar-system
related: Natural satellites (Solar system)
related: Exoplanets

B stars
broader: stellar-spectral-types
definition: A star of spectral type B, whose spectrum is marked by absorption lines of hydrogen; also known as B-type star. B-type optical spectra are characterized by the presence of neutral He lines (mainly He I 4471 Å) at about type B9. He I strengthens up to about B2, then decreases. Ionized helium (mainly He II 4541 Å) first appears at about B0. Most metallic lines are absent or weak, except some absorption lines for the higher ionization states of silicon, oxygen, carbon, and magnesium. These are hot stars with effective temperatures ranging from about 10,000 K at B9 to nearly 30,000 K at B0. They are between 3 and 20 solar masses. Some famous examples include Rigel, Achernar, and Hadar.
broader: early-type-stars
type: Concept
related: Stellar classification

Field of view
definition: The entire angular expanse of the sky viewed by an optical instrument.
related: Field galaxies
related: Field stars
type: Concept
broader: astronomical-techniques

Astronomy image processing
type: Concept
broader: astronomy-data-analysis

Exoplanet dynamics
type: Concept
broader: exoplanet-astronomy

Telescope properties
type: Concept
broader: telescopes

RR Lyrae variable stars
definition: A member of a large class of pulsating stars of type A2-F6 with periods less than 1 day. They are similar to Cepheids, except that their periods are much shorter and are less luminous. RR Lyrae stars belong to Population II and are often found in globular clusters (hence one of their older names cluster variables) or elsewhere in the galactic halo. They are used as distance indicators (standard candle) out to more than 200 kpc.
type: Concept
broader: pulsating-variable-stars

Interstellar plasma
type: Concept
broader: interstellar-medium

Non-thermal radiation sources
type: Concept
broader: astronomical-radiation-sources

Galactic longitude
broader: galactic-coordinate-system
definition: In the Galactic coordinate system, the angle between the Galactic Center and the projection of the object on the Galactic plane. Galactic longitude ranges from 0 degrees to 360 degrees.
type: Concept

Public domain software
type: Concept
broader: software-licensing

Stellar processes
type: Concept
broader: stellar-physics

Cosmological neutrinos
type: Concept
broader: particle-astrophysics

Planetary alignment
type: Concept
broader: planetary-science

Quasar-galaxy pairs
broader: quasars
broader: galaxies
type: Concept

Runaway stars
type: Concept
broader: high-velocity-stars
definition: A massive, young, and hot star that is moving quickly through space. Runaways are probably propelled through space from a binary star when its companion has exploded as a supernova, or ejected from a stellar cluster by the dynamical interactions in the system.

Celestial equator
type: Concept
definition: An imaginary great circle on the sky half-way between the celestial poles. It is the projection of the equator of the Earth on the sky.
broader: equatorial-coordinate-system

Radio lobes
type: Concept
definition: An extended region of diffuse radio emission, often dumbbell shaped, that surrounds a radio galaxy.
broader: active-galactic-nuclei

Radical-radical recombination
broader: recombination
type: Concept
broader: molecular-reactions

Outlier detection
broader: classification
type: Concept

Orbits
definition: The path followed by a body moving in a gravitational field. For bodies moving under the influence of a centrally directed force, without significant perturbation, the shape of the orbit must be one of the conic section family of curves (circle, ellipse, parabola, or hyperbola).
broader: celestial-mechanics
type: Concept

Morgan-Keenan classification
type: Concept
related: Stellar spectral types
broader: stellar-classification

Asteroid dynamics
type: Concept
related: Asteroids
broader: planetary-science

Molecule Formation
type: Concept
deprecated: :__

Astrophysical magnetism
type: Concept
broader: astrophysical-processes

Megalithic astronomy
broader: archaeoastronomy
related: Stone circles
type: Concept

Exosphere
definition: The outermost portion of the Earth's atmosphere. Extremely tenuous, it lies above the ionosphere from a height of about 500 km, to the edge of interplanetary space. Since the mean free path is much greater than the atmospheric scale height, the atoms or molecules never collide with each other.
type: Concept
broader: earth-atmosphere

Space stations
type: Concept
broader: space-vehicles

Stellar atmospheres
related: Solar atmosphere
related: Stellar atmospheric opacity
broader: stellar-physics
related: Astrophysical masers
type: Concept

Galaxy stellar disks
broader: galaxy-disks
type: Concept

Henry Draper catalog
broader: stellar-classification
type: Concept
broader: catalogs
definition: A catalog of stars in which every star is classified by its stellar spectrum. Though named for the astronomer Henry Draper, it was cataloged by Annie J. Cannon (225,300 stars), and later extended by Margaret W. Mayall.

Detached binary stars
definition: A binary star system whose components are not in contact and in which no significant mass exchange is occurring.
type: Concept
broader: binary-stars

Roche lobe
type: Concept
definition: The region around a star in a binary system within which orbiting material is gravitationally bound to that star. The point at which the Roche lobes of the two stars touch is called the inner Lagrangian point.
broader: three-body-problem
related: Roche lobe overflow

Herbig-Haro objects
type: Concept
broader: young-stellar-objects
related: Infrared sources
broader: intrinsic-variable-stars
related: Emission nebulae

UV Persei stars
type: Concept
broader: cataclysmic-variable-stars

FK Comae stars
broader: ellipsoidal-variable-stars
type: Concept

Cometary studies
deprecated: :__
type: Concept

Detection
definition: Any operation on an electrical or electromagnetic signal to recover information from it.
type: Concept
broader: astrostatistics-techniques

Three-body problem
broader: celestial-mechanics
broader: gravitational-interaction
type: Concept
definition: The mathematical problem of studying the positions and velocities of three mutually attracting bodies (such as the Sun, Earth and Moon) and the stability of their motion. This problem is surprisingly difficult to solve, even in the simple case, called restricted three-body problem, where one of the masses is taken to be negligibly small so that the problem simplifies to finding the behavior of the mass-less body in the combined gravitational field of the other two.

Blueshifted
broader: doppler-shift
type: Concept
definition: The apparent shift of the wavelength towards the shorter wavelength region of the radiation spectrum of an approaching object due to the Doppler effect.

Double-mode pulsation
broader: multi-periodic-pulsation
type: Concept

Philosophy of astronomy
broader: interdisciplinary-astronomy
type: Concept

Gravitational wave detectors
broader: astronomical-detectors
related: Gravitational waves
type: Concept

Companion stars
type: Concept
definition: The fainter of the two stars in a binary star system.
broader: binary-stars

Early stellar evolution
type: Concept
broader: stellar-evolution

Flash stars
broader: emission-line-stars
type: Concept

Radiative braking
type: Concept
definition: The slowing down of a star's rotation due to radiative momentum transfer caused by emission of electromagnetic radiation.
broader: stellar-rotation

Chemically peculiar stars
definition: Main sequence stars of spectral type A or B identified by the presence of anomalously strong or weak absorption lines of certain elements in their spectra. CP stars have been divided into four main classes on the basis of their spectra: 1) non-magnetic metallic-lined (CP1, Am star), magnetic (CP2, Ap star), non-magnetic mercury-manganese (CP3, HgMn star), and helium-weak (CP4, He-weak star).
broader: stellar-types
type: Concept

Solar observatories
type: Concept
broader: observatories

Naked singularities
broader: gravitational-singularities
type: Concept

Moving cluster parallax
broader: parallax
type: Concept
related: Moving clusters

Exocomets
related: Interstellar objects
type: Concept
broader: exoplanet-astronomy
related: Comets

Pulsar planets
definition: A planet orbiting a pulsar. The first such planet to be discovered was around a millisecond pulsar known as PSR 1257+12.
type: Concept
broader: exoplanets

Descending node
type: Concept
broader: orbital-nodes
definition: The point in an orbit where the orbiting body crosses a reference plane, such as the ecliptic or the celestial equator, going from north to south.

Exoplanet rings
related: Planetary rings
broader: exoplanet-structure
type: Concept

Solar radio telescopes
broader: radio-telescopes
broader: solar-telescopes
type: Concept

Two-color diagrams
broader: astrometry
broader: photometry
broader: apparent-magnitude
type: Concept
definition: A graph on which two color indices such as B-V and U-B are plotted, one along each axis, for a sample of stars or other objects, such as stars.

Radial velocity
broader: exoplanet-detection-methods
broader: astrometry
type: Concept
definition: The component of a three-dimensional velocity vector of an object directed along the line of sight. It is measured by examining the Doppler shift of lines in the spectrum of astronomical objects.

Cirrus uncinus
deprecated: :__
type: Concept

Classification systems
definition: Systems for the grouping of astronomical objects into categories on the basis of physical, morphological, or evolutionary characteristics.
type: Concept
broader: astronomical-research

Dark interstellar clouds
broader: nebulae
type: Concept
broader: interstellar-clouds

Molecular physics
broader: laboratory-astrophysics
related: Molecular spectroscopy
type: Concept

K dwarf stars
broader: dwarf-stars
broader: k-stars
broader: late-type-dwarf-stars
type: Concept

Gamma Cassiopeiae stars
broader: emission-line-stars
broader: eruptive-variable-stars
type: Concept

Sociology of astronomy
type: Concept
broader: cultural-astronomy

FK catalog
broader: catalogs
type: Concept

Galaxy winds
definition: An outflow of hot gas, analogous to the solar wind, from a galaxy that has recently undergone a high burst of star formation or has an active galactic nucleus. Galaxy winds are streams of high speed charged particles blowing out of galaxies with speeds of 300 to 3,000 km s^-1. In the case of starbursts, galactic winds are powered by stellar winds driven by massive stars and supernova explosions. Galaxy winds contain a mixture of extremely hot metal-enriched supernova ejecta and cooler entrained gas and dust. Outflowing material has been observed at great distances from galaxies (10 to 100 kpc). In some cases they escape the galaxy potential well and pollute the intergalactic medium with heavy elements. A prominent example is the superwind of the starburst galaxy M82.
broader: galaxy-processes
related: Galactic winds
type: Concept

Antumbra
definition: That part of the Moon's shadow that extends beyond the umbra. It is similar to the penumbra in that the Sun is only partially blocked by the Moon. From within the antumbra, the Sun appears larger than the Moon which is seen in complete silhouette. An annular eclipse is seen when an observer passes through the antumbra.
broader: eclipses
type: Concept

Infrared telescopes
broader: space-telescopes
definition: A telescope capable of observing infrared radiation from astronomical objects.
type: Concept

Ion-neutral reactions
broader: molecular-reactions
type: Concept

Stellar mergers
broader: interacting-binary-stars
type: Concept

Galactic radio sources
broader: radio-sources
type: Concept

Astrometric microlensing effect
type: Concept
broader: gravitational-microlensing

Astronomical seeing
broader: astronomical-optics
definition: A measure of the blurring and degradation of the image of astronomical objects caused by turbulence in the Earth's atmosphere, including the telescope environment. Seeing causes the images of stars to break up into speckle patterns, which change very rapidly with time.
type: Concept

Cosmic web
type: Concept
definition: The entire, large-scale structure of the Universe in which galaxy clusters are connected by cosmic filaments (made up of dark matter and baryons) in a spongelike geometry, while the low-density voids are connected to each other by low-density tunnels.
broader: large-scale-structure-of-the-universe

Star forming regions
type: Concept
definition: A region in the interstellar medium where processes of star formation are going on or have occurred in the past.
broader: star-formation

Vernal equinox
broader: equinox
definition: The point of intersection between the ecliptic and the celestial equator at which the Sun passes from south to north of the celestial equator during its apparent annual motion. The instant of this event. It occurs on March 20, 21 or rarely 19. At the vernal equinox, as with the autumnal equinox, night and day are equal in length world over. Several thousands years ago the vernal equinox was in Aries, but because of precession it has now slid west into Pisces. Right ascension and celestial longitude are measured from the vernal equinox.
type: Concept

Asteroid occultation
type: Concept
broader: occultation

Inflationary universe
type: Concept
broader: early-universe
definition: A class of Big Bang models of the Universe that include a finite period of accelerated expansion in their early histories. Such an event would have released enormous energy, stored until then in the vacuum of space-time. The horizon of the Universe expanded, temporarily, much faster than the speed of light.
broader: cosmological-models

Pluto
type: Concept
broader: dwarf-planets
definition: A dwarf planet in the solar system which until 2006 was known as the 9th major planet. Pluto revolves around the Sun in a highly elliptical orbit at a mean distance of 39.5 astronomical units once every about 248 years. The orbit eccentricity is 0.25 (compare with the Earth's 0.02) yielding a perihelion distance of 29.66 astronomical units and an aphelion distance of 48.87 AU. Its orbital inclination is 17 degrees, which is much higher than those of the other planets. Pluto's mass is 1.308 x 10^22 kg, that is 0.00218 Earth mass (0.177 Moon mass), its equatorial radius is 0.19 Earth radius, and its rotation period is equal to 6.39 Earth days. It has five known satellites, in order of distance from Pluto: Charon, Styx, Nix, Kerberos, and Hydra. Pluto's radius is estimated to be about 1150 km (0.18 Earths). Pluto is smaller than seven of the solar system's satellites (the Moon, Io, Europa, Ganymede, Callisto, Titan, and Triton). Pluto's surface has an estimated temperature of 37.5 K and is composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide.

Warm neutral medium
broader: interstellar-phases
type: Concept

Galaxy photometry
related: Galaxies
type: Concept
broader: photometry

Rockets
type: Concept
broader: space-vehicles

O giant stars
broader: o-stars
type: Concept
broader: giant-stars

Planetary surfaces
broader: planetary-structure
related: Exoplanet surfaces
type: Concept

Winter solstice
definition: The moment in the northern hemisphere when the Sun attains its lowest declination of -23°26' (or -23°.44) with respect the equator plane. It happens when the Earth's axis is orientated directly away from the Sun, on 21 or 22 December. During the northern winter solstice the Sun appears to be directly overhead at noon for places situated at latitude 23.44 degrees south, known as the tropic of Capricorn. The winter solstice can occur at any moment during the day. Two successive winter solstices are shifted in time by about 6 h. The winter solstice in the northern hemisphere is the summer solstice in the southern hemisphere.
type: Concept
broader: solstice

Caustic crossing
type: Concept
broader: caustic-curve

Gunn photometry
broader: broad-band-photometry
type: Concept

All-sky cameras
broader: wide-field-telescopes
type: Concept

Finding lists
broader: astronomical-reference-materials
type: Concept

Weak-line T Tauri stars
type: Concept
broader: t-tauri-stars
definition: A T Tauri star that lacks strong emission lines in its optical spectrum, and lacks both strong stellar wind and infrared excess. These objects are believed to be pre-main sequence stars without obvious signs for disk accretion. Weak-line T Tauri stars result from the evolution of classical T Tauri stars.

Autumnal equinox
definition: One of the two points where the ecliptic crosses the celestial equator. At the autumnal equinox the Sun appears to be moving across the equator from the northern celestial hemisphere to the southern celestial hemisphere. The instant of the event.
broader: equinox
type: Concept

Solar coronal loops
definition: An arc-like structure in the Sun's corona that is found around sunspots and in active regions. These structures are associated with the closed magnetic field lines that connect magnetic regions on the solar surface. The loops are sometimes as high as 10,000 km with their two ends situated in photosphere regions of opposite magnetic polarity. This implies that the coronal loops are tubes of magnetic flux filled with hot plasma. They last for days or weeks but most change quite rapidly.
broader: solar-corona
related: Stellar coronal loops
type: Concept

Solar extreme ultraviolet emission
type: Concept
broader: solar-electromagnetic-emission

Eruptive binary stars
deprecated: :__
type: Concept

Occulting disks
type: Concept
broader: occultation

Siderostats
broader: stellar-tracking-devices
type: Concept
definition: A mirror arrangement with clock drive that reflects light from a celestial body to a fixed position.

Stellar convective zones
related: Solar convective zone
type: Concept
broader: stellar-structures

Solar radiation
definition: All the constituents making up the Sun's emission: photons, electrons, protons, neutrinos, and atomic nuclei.
broader: solar-physics
type: Concept

Plutinos
type: Concept
broader: resonant-kuiper-belt-objects

Jupiter trojans
type: Concept
broader: trojan-asteroids
definition: The family of asteroids that share Jupiter's orbit and lie in elongated, curved regions around the two Lagrangian points 60° ahead and behind of Jupiter. The Lagrangian points L4 and L5 host several thousands of them.

Planetary dynamics
broader: planetary-science
type: Concept

Planetary probes
broader: space-probes
type: Concept

Large Magellanic Cloud
definition: The larger of the two Magellanic Cloud galaxies visible in the southern hemisphere at about 22 degrees from the South Celestial Pole. It is approximately on the border between the constellations Dorado and Mensa in a region of faint stars. The center of the LMC is approximately RA: 5h 23m 35s, dec: -69° 45' 22''. The LMC shines with a total apparent visual magnitude of approximately zero. It spans an area of the sky about 9 by 11 degrees, corresponding to about 30,000 light-years across in the longest dimension, for a distance of some 162,000 light-years. It has a visible mass of about one-tenth that of our own Galaxy (10^10 M_sun). The LMC and its twin, the Small Magellanic Cloud, are two of our most prominent Galactic neighbors. The LMC is classified as a disrupted barred spiral galaxy of type SBm, the prototype of a class of Magellanic spirals. The galaxy is characterized by a prominent offset stellar bar located near its center with the dominant spiral arm to the north with two "embryonic" arms situated to the south. The metallicity in the LMC is known to be lower than in the solar neighborhood by a factor 2 or more. Based on 20 eclipsing binary systems, the distance to the LMC is measured to one percent precision to be 49.59±0.09 (statistical) ±0.54 (systematic) kpc.
type: Concept
broader: magellanic-clouds

Microlensing optical depth
broader: gravitational-microlensing
type: Concept

Earth's seasons
related: Seasonal phenomena
broader: earth-atmosphere
type: Concept

RC Aurigae stars
type: Concept
deprecated: :__

Microgravity
definition: The state or condition where the force of gravity is very weak, e.g. the weightlessness experienced inside an orbiting spacecraft.
broader: space-research
type: Concept

Supergranulation
type: Concept
broader: solar-granulation

Mars trojans
broader: trojan-asteroids
definition: The family of asteroids located at either of the stable Lagrangian points (L4 or L5) of the orbit of Mars.
type: Concept

Interstellar dust processes
related: Interstellar dust
type: Concept
broader: astrophysical-dust-processes
related: Solid matter physics

Megamasers
type: Concept
broader: astrophysical-masers

Galactic center
type: Concept
related: Oort constants
definition: The rotational center of the Milky Way galaxy located in the direction of the Sagittarius constellation at a distance of 7.62 ± 0.32 kpc. Its equatorial coordinates (J2000 epoch) are: R.A. 17h45m40.04s, Dec. -29° 00' 28.1''. The Sun orbits around the Galactic center once every 200 million years at a speed of 220 km per second. It is believed that there is a supermassive black hole at the Galactic center.
broader: milky-way-galaxy-physics

Solar abundances
broader: solar-physics
broader: chemical-abundances
related: Stellar abundances
type: Concept

Stellar oscillations
type: Concept
related: Solar oscillations
broader: asteroseismology
broader: stellar-properties

Einstein universe
definition: A cosmological model in which a static (neither expanding nor collapsing) Universe is maintained by introducing a cosmological repulsion force (in the form of the cosmological constant) to counterbalance the gravitational force.
broader: cosmological-models
type: Concept

Very long baseline interferometers
broader: radio-interferometers
type: Concept
related: Very long baseline interferometry

Metal line absorbers
type: Concept
broader: intergalactic-clouds

Laser guide stars
broader: astronomical-optics
type: Concept

M subdwarf stars
broader: m-stars
type: Concept

Cosmochemistry
related: Cosmic abundances
broader: astrochemistry
definition: The study of the chemical composition of the universe and the processes that produced those compositions. Cosmochemistry is an interdisciplinary science that overlaps with geochemistry, geology, astronomy, astrophysics, and geophysics.
broader: cosmology
type: Concept

Interstellar atomic gas
type: Concept
broader: interstellar-medium

Kerr-Newman black holes
type: Concept
broader: rotating-black-holes
broader: charged-black-holes
definition: A rotating charged black hole. Compare with the Kerr black hole and the Reissner-Nordstrom black hole.

Mineral physics
broader: solid-matter-physics
type: Concept

Galaxy properties
broader: galaxy-physics
type: Concept

Ozone layer
type: Concept
definition: An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 15 to 20 kilometers above the Earth's surface.
broader: earth-atmosphere

Solar coronal holes
related: Stellar coronal holes
broader: solar-corona
definition: An area in the solar corona which appears dark in X-rays and ultraviolet light. The gas density in these areas are very low, about 100 times less than that of coronal active regions. The magnetic field lines in a coronal hole extend out into interplanetary space rather than returning to the Sun's surface, as they do in other parts of the Sun (open magnetic field line). Ionized hot gas can escape easily along such a path, and this brings about high speed solar wind streams.
type: Concept

Stellar evolution
related: Solar evolution
broader: stellar-physics
type: Concept
related: Stellar evolutionary models
related: Schonberg-Chandrasekhar limit
definition: The gradual changes in physical state (spectrum, luminosity, temperature) and chemical composition that occurs during the life of a star.
related: Young stellar objects

Radio galaxies
broader: active-galaxies
definition: A galaxy that is extremely luminous at radio wavelengths between 10 MHz and 100 GHz. The radio luminosity of a strong radio galaxy (10^37 - 10^39 watts) can be up to a million times greater than the radio output of an ordinary galaxy and up to a hundred times greater than the optical luminosity of a galaxy such as the Milky Way. The optical counterparts of radio galaxies are usually an elliptical galaxy. Radio galaxies often exhibit jet structure from a compact nucleus. They typically display two radio lobes that are often approximately aligned with the jets observed in the optical and that may extend for millions of light-years.
related: Radio sources
type: Concept

Slow solar wind
type: Concept
broader: solar-wind

Interstellar medium
definition: The environment containing the interstellar matter (the gas and dust that exists in open space between the stars), consisting of gas (mostly hydrogen) and dust. Even at its densest phase, the interstellar medium is emptier than the best vacuum man can create in the laboratory, but because space is so vast, the interstellar medium still adds up to a huge amount of mass.
type: Concept

Galaxy kinematics
broader: galaxy-processes
type: Concept

Discrete radio sources
broader: radio-sources
type: Concept

Elemental abundance
use instead: Chemical abundances
type: Concept
deprecated: :__

High energy astrophysics
type: Concept
definition: A branch of astrophysics that deals with objects emitting highly energetic radiation, such as X-ray astronomy, gamma-ray astronomy, and extreme ultraviolet astronomy, as well as neutrinos and cosmic rays.

Hypernovae
broader: supernovae
definition: A highly energetic supernova explosion. This phenomenon, which is more violent than a typical supernova event, is accompanied by a gamma-ray burst.
type: Concept

Cool cores
broader: x-ray-sources
type: Concept

Density parameters
broader: cosmological-parameters
type: Concept

Lunar geochronology
type: Concept
broader: lunar-surface

Intergalactic medium phases
broader: intergalactic-medium
type: Concept

Spatial point processes
type: Concept
broader: astrostatistics-techniques

Meridian
broader: horizontal-coordinate-system
type: Concept
definition: An imaginary great circle on the celestial sphere that passes through its poles and the observer's zenith.

Symbiotic binary stars
broader: emission-line-stars
broader: interacting-binary-stars
definition: A stellar object whose optical spectrum displays lines characteristic of gases of two very different temperatures, typically of an M star (3500 K) and a B star (20 000 K) superimposed. A symbiotic star is in fact a close binary system.
type: Concept

Absolute magnitude
definition: The magnitude a star would have if it were at a distance of 10 parsecs in a void space, without interstellar absorption. The absolute magnitude is usually deduced from the visual magnitude, measured through a V filter (UBV system), when it is written as M_V. If it is defined for another wavelength, it gets another index (U, B, etc). If the radiation on all wavelengths is included, it becomes absolute bolometric magnitude, M_bol. The Sun has the absolute magnitude +4.8. Most of the stars have absolute magnitudes ranging between -9 (supergiants) and +19 (red dwarfs).
type: Concept
broader: magnitude

Red giant clump
broader: red-giant-branch
type: Concept

Walraven photometry
type: Concept
definition: A photometric system with five wavelength ranges that does not use filters. Instead it uses prisms and lenses (spectroscopy) to select the bands simultaneously. The wavelengths and the bandwidths are: W, 3250 and 140 Å; U, 3630 and 240 Å; L, 3840 and 230 Å; B, 4320 and 450 Å; and V, 5470 and 720 Å. The Walraven photometer was unique in design and remained literally unique as copies were never built. In addition, during its whole life the photometer was mounted permanently on the same telescope that had been built specifically for this instrument, the 91 cm 'Lightcollector' reflector, which started in 1958 at the Leiden Southern Station in Broederstroom, South-Africa. After 20 years in South-Africa the telescope and photometer were moved to the European Southern Observatory La Silla observatory in Chile. The photometric observations were resumed in March 1979 and continued for another 12 years until the decommissioning of the photometer in 1991.
broader: medium-band-photometry

Milky Way mass
broader: milky-way-galaxy-physics
type: Concept
related: Galaxy masses

Space debris
definition: Man-made objects in orbit around the Earth that no longer serve any useful purpose. The estimated number of debris include about 22,000 tractable objects larger than 10 cm in all orbits, of which 2,200 are dead satellites and last stages of the rocket that put them in orbit. There are also left-overs from spacecraft and mission operations, such as bolts, lens caps, clamp bands, auxiliary motors, etc. The debris presents a threat because of their high speeds, which ranges between 15 and 20 km/sec. Also called space junk, space waste, orbital debris.
type: Concept
broader: space-vehicles

Alfven waves
type: Concept
broader: space-plasmas
definition: A magnetohydrodynamic wave in a magnetized plasma, arising as a result of restoring forces associated with the magnetic field. It is a transverse wave which propagates in the direction of the magnetic field. Also called magnetohydrodynamic wave.
related: Magnetohydrodynamics
broader: solar-chromosphere

Regression
type: Concept
definition: A method for fitting a curve through a set of points using some goodness-of-fit criterion. The most common type of regression is linear regression.
related: Smoothing
broader: astrostatistics-techniques

Exoplanet atmospheric composition
type: Concept
related: Atmospheric composition
broader: exoplanet-atmospheres

DDO photometry
broader: medium-band-photometry
type: Concept

Analemma
broader: orbital-motion
type: Concept

Exoplanet atmospheres
type: Concept
related: Planetary atmospheres
broader: exoplanet-structure

Noctilucent clouds
deprecated: :__
type: Concept

Spectrophotometric standards
broader: spectrophotometry
type: Concept

Spectral energy distribution
type: Concept
broader: photometry
definition: A plot showing the energy emitted by a source as a function of the radiation wavelength or frequency. It is used in many branches of astronomy to characterize astronomical sources, in particular mainly in near infrared and middle infrared to study protostars or young stellar objects. The SED of these objects is divided in four classes. Class 0 in which the SED represents a very embedded protostar, where the mass of the central core is small in comparison to the mass of the accreting envelope. The SED is characterized by the blackbody radiation of the envelope and peaks at submillimeter wavelengths. Class I objects possess a SED that peaks in the far infrared and is characterized by a weak contribution of the blackbody of the central protostar (detected in near infrared) and the emission of a thick disk and dense envelope. These objects have less mass in the envelope and more massive central cores with respect to Class 0. Class II objects are the classical T Tauri stars with a SED due to the emission of a thin disk and the central star. They have accumulated most of their final mass and have dispersed almost completely their circumstellar envelope. Finally, Class III objects have pure photospheric spectra. Their SED is peaked in the optical and is well approximated by a blackbody emission with a faint infrared excess due to the presence of a residual optically thin disk that may be the origin of planetesimals. This classification scheme can be made more quantitative by defining a spectral index.

Lunar occultation
type: Concept
broader: occultation

Refracting telescopes
type: Concept
definition: A telescope in which an image is formed by the refraction of light through a lens or lens system.
broader: optical-telescopes

Dynamical friction
definition: The gravitational interaction between a relatively massive body and a field of much less massive bodies through which the massive body travels. As a result, the moving body loses momentum and kinetic energy. An example of dynamical friction is the sinking of massive stars to the center of a star cluster, a process called mass segregation. Dynamical friction plays an important role in stellar dynamics. It was first quantified by Chandrasekhar (1943).
broader: gravitational-interaction
type: Concept

Photometer
broader: astronomical-instrumentation
definition: An instrument for measuring the amount of light.
type: Concept

Chemical enrichment
definition: The process by which the relative abundance of a given chemical element or species in an astrophysical object is increased. For example the increase of the heavy element content of the interstellar medium due to stellar evolution.
type: Concept
broader: galaxy-chemical-evolution

Semimajor axis
broader: elliptical-orbits
type: Concept
definition: Half the length of the major axis of an ellipse; a standard element used to describe an elliptical orbit.

Glaciation
type: Concept
related: Glaciers
broader: planetary-dynamics

Partial eclipses
broader: eclipses
type: Concept
definition: An eclipse that is not total.

Astronomical simulations
type: Concept
broader: astronomy-software
definition: The construction of a mathematical model to reproduce the characteristics of an astronomical phenomenon, system, or process, often using a computer, in order to infer information or solve problems.

Radio astrometry
broader: radio-astronomy
type: Concept
broader: astrometry

Zero-age main sequence stars
broader: main-sequence-stars
type: Concept
related: Hertzsprung Russell diagram
related: Main sequence

19521 Chaos (Kuiper Belt object)
deprecated: :__
type: Concept

Astrosphere interstellar medium interactions
broader: stellar-phenomena
type: Concept
related: Heliosphere

Spectrometers
definition: A spectrograph in which the spectrum is recorded by electronic means so that wavelength, intensity, etc. can be measured. An instrument for determining the distribution of energies in a beam of particles.
broader: astronomical-instrumentation
type: Concept

Penumbra
broader: eclipses
type: Concept
definition: The portion of a shadow in which light from an extended source is partially but not completely cut off by an intervening body; the area of partial shadow surrounding the umbra. The lighter part of a shadow during an eclipse. An observer standing in the penumbra of a Solar eclipse will see a partial eclipse.

Far infrared astronomy
type: Concept
broader: infrared-astronomy

Mesogranulation
broader: solar-granulation
type: Concept

Recombination
definition: The capture of an electron by a positive ion. It is the inverse process to ionization.
broader: atomic-reactions
type: Concept

Type Ic supernovae
broader: core-collapse-supernovae
type: Concept
definition: A Type I supernova that shows weak or no helium lines and no strong silicon (Si II) absorption feature near 6150 Å. Type Ic supernovae are believed to result from the evolution of massive stars.

Luminosity classification
broader: classification-systems
type: Concept
definition: A classification of stellar spectra according to luminosity for a given spectral type. The luminosity class is an indication of a star's surface gravity. It is shown by a Roman numeral as follows: I (supergiants), II (bright giants), III (normal giants), IV (subgiants), and V (dwarf stars, or main-sequence stars). Luminosity classes VI (subdwarfs) and VII (white dwarfs) are rarely used. Subclasses a, b, and c are especially used for supergiants, while the most luminous hypergiants are assigned luminosity class Ia-0.

Galaxy planes
type: Concept
broader: galaxy-kinematics

Declination
broader: equatorial-coordinate-system
type: Concept
definition: Angular distance (symbol δ) from the celestial equator. One of the coordinates, with right ascension, that defines the position of a heavenly body in the equator system. Declination is measured positively (+) north or negatively (-) south of the celestial equator from 0° to 90°.

Young disk Cepheid variable stars
type: Concept
broader: cepheid-variable-stars

Fornax dwarf spheroidal galaxy
type: Concept
broader: dwarf-spheroidal-galaxies

Astrophotography
broader: astronomical-methods
type: Concept
definition: The photography of stars, other celestial bodies, and stellar fields.

Magnitude
definition: A measure of brightness in astronomy on a logarithmic scale in which a difference of five magnitudes represents a difference of 100 times in brightness. In this scale the lower a magnitude, the brighter the object. The faintest magnitude reached by unaided eye is 6.
type: Concept
broader: astrometry
related: Photometric systems

Urca process
broader: neutrino-astronomy
definition: A cycle of nuclear reactions, primarily among the iron group of elements, accompanied by a high rate of neutrino formation. Neutrinos carry away energy quickly leading to the cooling of the stellar core and a dramatic drop in the pressure. Due to this process in pre-supernova stars, the core becomes unable to support the weight of the overlying layers. These layers collapse onto the core and subsequently rebound and escape in the supernova explosion.
broader: particle-astrophysics
type: Concept

Solar physics
related: Solar neutrinos
related: Stellar physics
type: Concept
definition: The branch of astrophysics concerned with the study of the physical properties of the Sun based on the most detailed observations which can be obtained for a star.
related: The Sun

Late-type stars
broader: stellar-types
type: Concept
definition: A star of spectral type K, M, S, or C, with a surface temperature lower than that of the Sun.

Neptunian satellites
related: Neptune
type: Concept
broader: natural-satellites-solar-system

Close encounters
related: Near-Earth objects
broader: celestial-mechanics
definition: In a star cluster, coming across of two stars so closely that their orbits alter by their mutual gravitational attractions. As regards an asteroid or comet, a situation when it crosses the Moon's orbit and approaches the Earth with a risk of collision.
type: Concept

Reflecting telescopes
broader: optical-telescopes
type: Concept
definition: A telescope in which the image is produced by reflection of light by a concave mirror.

Field stars
related: Field of view
related: Star clusters
broader: stellar-types
type: Concept
definition: A star that does not belong to a stellar cluster, but happens to be adjacent to it.

Metallicity
broader: chemical-abundances
type: Concept
definition: In a star, nebula, or galaxy, the proportion of the material that is made up of metals, that is elements heavier than helium. It is generally denoted by Z. The term 'metallicity' is a misnomer used in astrophysics. In practice, the metallicity of stars is usually expressed by the number ratio of iron atoms to hydrogen atoms per unit volume, with respect to the solar values: [Fe/H] = log10(NFe/NH)star - log10(NFe/NH)Sun, where NFe and NH are the numbers of iron and hydrogen atoms per unit volume. In fact it is taken to be equal to the iron abundance with respect to the solar value. The solar logarithmic iron abundance is 7.50 ± 0.04, with respect to that of hydrogen which, by convention, is 12.00. Stellar metallicity is often expressed in mass fraction. Nebular metallicity is often defined as the relative abundance of oxygen: (NO/NH)neb/(NO/NH)Sun, where N_O and N_H represent the numbers of oxygen and hydrogen atoms per unit volume.

Atomic data
broader: atomic-physics
type: Concept

Scale height
definition: The height within which some parameter, such as pressure or density, decreases by a factor of e. For example, an atmospheric scale height of 100 km means that the value at 100 km is 1/e the value at the surface.
type: Concept
broader: planetary-atmospheres

Hubble Space Telescope
broader: optical-observatories
broader: space-observatories
broader: ultraviolet-observatories
type: Concept
definition: A telescope of 2.4 m in diameter, a joint NASA and ESA project, launched in 1990 into a low-Earth orbit 600 km above the ground. It was equipped with a collection of several science instruments that worked across the entire optical spectrum (from infrared, through the visible, to ultraviolet light). During its lifetime Hubble has become one of the most important science projects ever.

Direct absorption
type: Concept
broader: microwave-spectroscopy
broader: millimeter-wave-spectroscopy

Rotation powered pulsars
type: Concept
broader: pulsars

Underabundances
type: Concept
broader: chemical-abundances
definition: The abundance of a chemical element being lower than a reference value, in particular compared to that of the Sun.

Spectral index
type: Concept
broader: astrometry
broader: radio-astronomy

High altitude balloons
broader: astronomical-instrumentation
type: Concept

Cocoon stars
related: Red supergiant stars
type: Concept
related: Red giant stars
definition: A star hidden in a dense envelope of gas and dust which is a strong source of infrared emission.
broader: protostars
related: Hydroxyl sources
related: OH/IR stars
related: Infrared sources

Equatorial mounts
broader: telescope-mounts
definition: A telescope mounting consisting of a polar axis pointed toward the celestial pole, and a declination axis supporting the instrument at right angles to the polar axis.
type: Concept

Kepler's laws
broader: celestial-mechanics
type: Concept

Comet tail streamers
type: Concept
broader: comet-tails

Atmospheric variability
broader: planetary-atmospheres
type: Concept
related: Exoplanet atmospheric variability

Electric discharge
type: Concept
broader: production-techniques

Bolometric correction
definition: The difference between the visual magnitude and bolometric magnitude.
type: Concept
broader: astrometry

Eruptive phenomena
broader: stellar-evolution
type: Concept

Atmospheric extinction
related: Earth atmosphere
type: Concept
definition: The decrease in the intensity of light from a celestial body due to absorption and scattering by Earth's atmosphere. It increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they do at the zenith.
broader: extinction

Pulsars
type: Concept
broader: neutron-stars
definition: A rotating neutron star that emits a radio beam that is centered on the magnetic axis of the neutron star. As the magnetic axis and hence the beam are inclined to the rotation axis, a pulse is seen every time the rotation brings the magnetic pole region of the neutron star into view. In this way the pulsar acts much as a light house does, sweeping a beam of radiation through space. The pulse or spin periods range from 1.4 milliseconds to 8.5 seconds. As neutron stars concentrate an average of 1.4 solar masses on a diameter of only 20 km, pulsars are exceedingly dense and compact, representing the densest matter in the observable Universe. The pulsar radiation, chiefly emitted in radio frequencies (0.1-1 GHz), is highly polarized. The exact mechanism by which a pulsar radiates is still a matter of vigorous investigation. Simply put, an enormous electric field is induced by the rotation of a magnetized neutron star. The force of this field exceeds gravity by ten to twelve orders of magnitudes. Charged particles are whereby pulled out from the stellar surface resulting in a dense, magnetized plasma that surrounds the pulsar (magnetosphere). The charged particles flow out of the magnetic polar caps of the neutron star, following the open magnetic field lines. The acceleration of the charged particles along the curved magnetic field lines will cause them to radiate.
related: Pulsar wind nebulae

Milky Way stellar halo
broader: milky-way-galaxy-physics
related: Galaxy stellar halos
type: Concept

Interstellar radiation field
definition: A global ionizing radiation in the interstellar medium provided by various sources all together.
type: Concept
broader: interstellar-medium

Parsec
definition: A basic unit of astronomical distances, corresponding to a trigonometric parallax of one second of arc. In other words, it is the distance at which one astronomical unit (the mean radius of the Earth's orbit) subtends an angle of 1 arcsecond. 1 pc = 3.2616 light-years = 206,265 astronomical units = 30.857 x 10^12 km.
type: Concept
broader: distance-measure

Opik theory
broader: close-encounters
type: Concept

Galaxy dark matter halos
definition: A vast region surrounding a galaxy where dynamical tracers reveal a large amount of hidden mass. The halo has considerable mass but relatively low luminosity, suggesting the presence of a lot of dark matter.
related: Milky Way dark matter halo
broader: galaxy-structure
type: Concept

Hydrodynamical simulations
broader: astronomical-simulations
related: Hydrodynamics
type: Concept

Stellar granules
related: Solar granules
broader: stellar-photospheres
type: Concept

Cosmological perturbation theory
type: Concept
related: Perturbation methods
broader: big-bang-theory

Amorphous irregular galaxies
type: Concept
broader: irregular-galaxies

Henyey track
definition: A nearly horizontal path on the Hertzsprung-Russell diagram that a pre-main sequence star of small mass follows in an early stage of evolution after leaving the Hayashi track and before reaching the main sequence. During this stage the pre-main sequence star remains almost wholly in radiative equilibrium.
related: Hertzsprung Russell diagram
type: Concept
broader: stellar-evolutionary-tracks

Faber-Jackson distance
broader: stellar-distance
broader: astronomical-techniques
type: Concept

Rovers
type: Concept
broader: space-probes

Astrophysical masers
related: Molecular clouds
broader: extragalactic-astronomy
related: Stellar atmospheres
type: Concept
broader: interstellar-medium
related: Comets
related: Planetary atmospheres

Plasma clouds
type: Concept
broader: interstellar-clouds

Complex organic molecules
broader: molecular-physics
type: Concept

Merged-beams
broader: molecular-beams
type: Concept

X-ray binary stars
broader: compact-binary-stars
definition: A binary star system where one of the stars has evolved and collapsed into an extremely dense body such as a white dwarf, a neutron star, or a black hole. The enormous gravitational attraction of the massive, dense, but dim component pulls material from the brighter, less massive star in an accretion disk. The gravitational potential energy of the accreted matter is converted to heat by viscosity and eventually to high-energy photons in the X-ray range. The brightest X-ray binary is Scorpius X-1.
broader: semi-detached-binary-stars
related: X-ray sources
type: Concept

Ocean planets
type: Concept
broader: exoplanets
definition: A hypothetical exoplanet covered by a water envelope. The presence of such a planet stems from the implicit assumption of Habitable Zone temperatures and a liquid water surface.

Non-Gaussianity
type: Concept
broader: cosmology
related: Cosmic microwave background radiation

Laser interferometry
type: Concept
broader: interferometry

German equatorial mounts
deprecated: :__
type: Concept

Brown dwarfs
definition: A star-like object whose mass is too small to sustain hydrogen fusion in its interior and become a star. Brown dwarfs are substellar objects and occupy an intermediate regime between those of stars and giant planets. With a mass less than 0.08 times that of the Sun (about 80 Jupiter masses), nuclear reactions in the core of brown dwarfs are limited to the transformation of deuterium into Helium-3. The reason is that the cores of these objects are supported against gravitational collapse by electron degeneracy pressure (at early spectral types) and Coulomb pressure (at later spectral types). Brown dwarfs, as ever cooling objects, will have late M dwarf spectral types within a few Myrs of their formation and gradually evolve as L, T and Y dwarfs brown dwarf cooling. As late-M and early-L dwarfs, they overlap in temperature with the cool end of the stellar main sequence (M dwarf, L dwarf, T dwarf, Y dwarf). In contrast to the OBAFGKM sequence, the M-L-T-Y sequence is an evolutionary one.
type: Concept
broader: stellar-evolutionary-types

Missing mass
broader: intergalactic-medium
type: Concept
definition: The unseen mass whose gravitational influence is needed to account for the way galaxies rotate, and also to bind clusters of galaxies together. It is thought to consist, in part, of giant halos of dark matter that surround the visible portions of galaxies, and similar material that invisibly occupies the intergalactic voids.

Sunspots
broader: solar-physics
broader: solar-active-regions
related: Starspots
type: Concept
related: Sunspot cycle
definition: An area seen as a dark patch on the Sun's surface. Sunspots appear dark because they are cooler (of about 4000 °C) than the surrounding photosphere (about 6000 °C). They range in size from a few hundred kilometers to several times the Earth's diameter and last from a few hours to a few months. Very small sunspots are called pores. The number of sunspots varies from maximum to minimum in about 11 years, the sunspot cycle. Their appearance during a cycle follows the Sporer law. A typical spot has a central umbra surrounded by a penumbra, although either features can exist without the other. Sunspots are associated with strong magnetic fields of 0.2 to 0.4 tesla. A given sunspot has a single magnetic polarity. The opposite polarity may be found in other sunspots or in the bright and diffuse facular region adjacent to the sunspot. The first recorded naked-eye sightings of sunspots were by Chinese astronomers in the first century B.C. Johannes Fabricius (1587-1617) was the first to argue that sunspots are areas on the solar surface.

Brane cosmology
type: Concept
broader: cosmology

Crossed-beams
broader: molecular-beams
type: Concept

Turnoff point
definition: The point on the Hertzsprung-Russell diagram of a star cluster at which stars begin to leave the main sequence and move toward the red giant branch. The main-sequence turnoff is a measure of age. In general, the older a star cluster, the fainter the main-sequence turnoff.
type: Concept
broader: hertzsprung-russell-diagram

Cape RI photometry
type: Concept
broader: broad-band-photometry

Equation of the center
broader: orbital-motion
type: Concept

Lyman-alpha galaxies
broader: galaxies
type: Concept

Degenerate matter
type: Concept
definition: Highly compressed matter in which the normal atomic structure has broken down and which, because of quantum-mechanical effects, exerts a pressure that is independent of temperature. Bodies with masses less than Chandrasekhar's limit (1.4 solar masses) are supported by electron degeneracy pressure and have densities of about 10^6 kg/m^3. In collapsed stars of mass above 1.4 solar masses, gravity will overwhelm electron degeneracy and further collapse ensues. Electrons combine with protons to form neutrons, so producing a neutron star. Because neutrons, like electrons, are fermions and therefore subject to the Pauli exclusion principle, at high enough densities, about 10^14 kg/m^3, neutron degeneracy pressure prevents further collapse of the star. For masses larger than 2-3 solar masses, even neutron degeneracy cannot prevent further collapse, and a black hole is formed.
broader: stellar-remnants

BL Lacertae objects
type: Concept
broader: active-galaxies

Lunar craters
definition: A bowl-like depression on the rigid surface of the Moon, usually caused by the high-speed impact of a colliding object.
broader: lunar-impacts
broader: lunar-features
type: Concept
related: Craters

Large-scale structure of the universe
type: Concept
broader: cosmology

Solar activity
definition: The general term for all forms of short-lived phenomena on the Sun, including solar flares, sunspots, prominences, etc., indicating that the Sun is an active star.
related: Stellar activity
type: Concept
broader: solar-physics

Eclipses
type: Concept
definition: The passage of the shadow of a celestial body over the surface of another. The maximum number of solar and lunar visible eclipses occurring annually is seven; the minimum number is two, both being solar. Solar eclipses take place when the new Moon is close to an orbital node and on the same longitude with the Sun. At that moment either the umbra, antumbra, or the penumbra touches the Earth's surface. For an observer located in the umbra the eclipse is total, while for one placed in the antumbra it is annular. Annular eclipses occur around lunar apogee. An observer situated in the penumbra sees only a partial eclipse. A total or annular eclipse can be seen from a band with a width of 270 km at the most, around which, the much larger partiality zone extends. The Moon's shadow crosses the Earth from west to east at about 3,200 km/h. During total eclipses the Sun's disk is entirely covered and the solar corona can be seen. A solar eclipse can last up to 3h (between the first and the fourth contacts). Totality has a theoretical maximum duration of 7m 31s, but it is usually shorter. A lunar eclipse can be seen from any place on Earth where the Moon is above the horizon; it occurs when the full Moon passes through the central dark shadow of the Earth. The Earth's shadow is much wider than the Moon and this is why the lunar eclipses can last up to four hours (between the first and the fourth contact).
broader: syzygy

Atmospheric scintillation
broader: astronomical-optics
type: Concept

Galaxy interactions
broader: galaxy-physics
type: Concept

Neutrino decoupling
broader: early-universe
type: Concept

K stars
type: Concept
definition: An orange-red star of spectral type K with a surface temperature of about 3600-5000 K. The spectra of K stars are dominated by the H and K lines of calcium and lines of neutral iron and titanium, with molecular bands due to cyanogen (CN) and titanium dioxide (TiO). Examples are Arcturus and Aldebaran.
broader: late-type-stars
broader: stellar-spectral-types
related: Stellar classification

Supersonic expansion
broader: molecular-beams
type: Concept

Natural satellites (Solar system)
related: Solar system planets
type: Concept
related: Natural satellites (Extrasolar)
definition: A solar system object that revolves around a primary body and is not man made.
related: Natural satellite formation
related: Natural satellite atmospheres
related: Natural satellite dynamics
related: Natural satellite surfaces
broader: solar-system

Baryon density
broader: density-parameters
type: Concept

Quaoar
deprecated: :__
type: Concept

Prior distribution
type: Concept
broader: bayesian-statistics

Seyfert's sextant
deprecated: :__
type: Concept

Hot intergalactic medium
broader: intergalactic-medium-phases
type: Concept

UV Ceti stars
related: Variable radiation sources
type: Concept
broader: flare-stars

Planetary rings
related: Exoplanet rings
type: Concept
broader: solar-system

Ephemeral active regions
type: Concept
broader: solar-active-regions
broader: sunspots

Natural satellites (Extrasolar)
related: Exoplanets
related: Natural satellites (Solar system)
definition: A natural satellite orbiting an extrasolar planet.
broader: exoplanet-astronomy
type: Concept

Markarian galaxies
broader: emission-line-galaxies
broader: active-galaxies
type: Concept

Sampling distribution
type: Concept
broader: astrostatistics-distributions

Exoplanet atmospheric structure
broader: exoplanet-atmospheres
related: Atmospheric structure
type: Concept

Isothermal sphere profile
broader: large-scale-structure-of-the-universe
type: Concept

Makemake
type: Concept
deprecated: :__

Quenched galaxies
broader: galaxies
type: Concept
related: Galaxy quenching

Two-point correlation function
type: Concept
broader: spatial-point-processes

Rich galaxy clusters
broader: galaxy-clusters
type: Concept
definition: A galaxy cluster with a particularly large number of galaxies.

Exoplanet systems
definition: A planetary system consisting of exoplanets orbiting a star other than Sun.
type: Concept
broader: exoplanet-astronomy
related: Solar system

Hubble constant
broader: cosmological-parameters
definition: The Hubble parameter for the present epoch. It is the constant of proportionality between the recession velocities of galaxies and their distances from each other. The latest determinations using the Hubble Space Telescope observations of Cepheids give H_0 = 72 ± 8 km s^-1 Mpc^-1, the WMAP observations yield 70.4 ± 1.3 km s^-1 Mpc^-1, and the Planck Satellite observations give 67.3 ± 1.2 km s^-1 Mpc^-1. More recently, the Hubble constant was derived by a team of astronomers, using the NASA/ESA Hubble Space Telescope, with a 2.4% accuracy. The new value, 73.2 km s^-1 Mpc^-1, suggests that the Universe is expanding between five and nine percent faster than previously calculated. The Hubble law is only applicable for large distances (> 20 Mpc), when the proper motions of galaxies in groups and clusters cannot confuse the recession due to expansion.
type: Concept
broader: observational-cosmology

Galaxy stellar content
type: Concept
broader: galaxy-properties

Galaxy radial velocities
type: Concept
broader: galaxy-properties

Jets
definition: Fountain-like formations of gas and/or dust gushing out from compact regions of some astronomical objects.
type: Concept
broader: high-energy-astrophysics

Aeronomy
broader: atmospheric-science
type: Concept
definition: The study of the upper atmosphere, especially of regions of ionized gas.

VY Sculpturis stars
type: Concept
broader: nova-like-variable-stars

Milky Way dynamics
related: Galaxy dynamics
broader: milky-way-galaxy-physics
type: Concept

Solar telescopes
definition: A telescope designed so that heating effects produced by the Sun do not distort the images.
type: Concept
broader: telescopes

Lunar composition
broader: lunar-interior
type: Concept

Heliocentric orbit
type: Concept
broader: orbital-motion

Multi-color photometry
broader: photometric-systems
type: Concept

Cosmological models
broader: cosmology
type: Concept

Hydrodynamics
related: Hydrodynamical simulations
type: Concept
definition: The branch of physics dealing with the motion, energy, and pressure of neutral fluids.
broader: astrophysical-fluid-dynamics

Astrostatistics techniques
type: Concept
broader: astrostatistics

Brightest cluster galaxies
type: Concept
broader: giant-galaxies

Ionospheric scintillation
type: Concept
related: Earth ionosphere
broader: atmospheric-scintillation

Maksutov telescopes
type: Concept
broader: catadioptric-telescopes

Navarro-Frenk-White profile
related: Dark matter
broader: galaxy-mass-distribution
type: Concept

Aperture synthesis
type: Concept
definition: The method of combining the signals received by several smaller telescopes distributed over a very large area or baseline to provide the high angular resolution of a much large telescope.
broader: interferometry

Faint blue stars
type: Concept
broader: early-type-stars

Solar storm
type: Concept
broader: solar-activity

Blue large-amplitude pulsators
type: Concept
broader: pulsating-variable-stars

pp waves
broader: gravitational-waves
type: Concept

Nuclear abundances
type: Concept
broader: chemical-abundances

Comet interiors
broader: comet-nuclei
type: Concept

Interacting binary stars
type: Concept
broader: close-binary-stars

Sundials
type: Concept
broader: early-astronomical-instruments

Argument of perigee
definition: The angular distance between the ascending node of an object orbiting the Earth and its perigee, measured from the Earth.
broader: orbital-elements
type: Concept

WZ Sagittae stars
broader: dwarf-novae
type: Concept
definition: A subclass of dwarf novae with an extremely long outburst period, comparable with that of a recurrent nova.
broader: cataclysmic-variable-stars

Bright nebulae
definition: In contrast to a dark nebula, a bright cloud of interstellar gas and dust. The term designates both emission nebulae and reflection nebulae.
broader: nebulae
type: Concept

Parametric hypothesis tests
type: Concept
broader: astrostatistics-strategies

Collapsing clouds
broader: molecular-clouds
type: Concept

Semiminor axis
type: Concept
broader: elliptical-orbits

Celestial sphere
definition: An imaginary sphere, of large but indefinite dimension, used as a basis to define the position coordinates of celestial bodies. The center can be the Earth, the observer, or any other point which plays the role of origin for a given system of coordinates. Seen from the Earth, the celestial sphere rotates around the celestial axis every 23h 56m 04s (the sidereal day), as a result of the Earth's rotation. Two important circles on the celestial sphere are the celestial equator and the ecliptic. The angle between them, about 23.40 degrees, is known as the obliquity of the ecliptic. The celestial equator and the ecliptic intersect at two points, vernal equinox and autumnal equinox. The positions of the celestial poles and therefore that of the celestial equator move gradually on the celestial sphere, due to precession.
broader: equatorial-coordinate-system
type: Concept

Sloan photometry
broader: broad-band-photometry
type: Concept

Planetary theory
type: Concept
broader: planetary-science

Solar flares
type: Concept
broader: solar-activity
definition: A bright eruption form the Sun's chromosphere in the vicinity of a sunspot. Solar flares are caused by tremendous explosions on the surface of the Sun. In a matter of just a few minutes they heat the material to many millions of degrees and release as much energy as a billion megatons of TNT.
related: Stellar flares
broader: solar-physics
broader: solar-storm

Circumgalactic medium
broader: galaxy-structure
broader: extragalactic-astronomy
type: Concept

Monte Carlo methods
broader: reaction-models
type: Concept

Limiting magnitude
type: Concept
definition: The faintest magnitude reachable by an instrument.
broader: apparent-magnitude

Polarimeters
type: Concept
broader: astronomical-instrumentation

Flyby missions
type: Concept
definition: A mission in which a trajectory takes a space probe close to a Solar System body (planet, satellite, asteroid, comet) but does not permit it to enter an orbit about the body.
broader: space-probes

Galaxy abundances
broader: galaxy-properties
broader: galaxy-physics
broader: chemical-abundances
related: Galactic abundances
type: Concept

Neutron stars
type: Concept
broader: compact-objects
broader: nuclear-astrophysics

Stellar accretion disks
type: Concept
broader: late-stellar-evolution
related: Stellar accretion

Z Andromedae stars
broader: emission-line-stars
broader: close-binary-stars
type: Concept
broader: symbiotic-binary-stars

Sculptor dwarf elliptical galaxy
broader: dwarf-elliptical-galaxies
definition: A dwarf elliptical galaxy that is a satellite of our Milky Way. It lies about 285,000 light-years away in the constellation Sculptor, and has an absolute magnitude of -11.28 and a mass of about 3 million solar masses. The Sculptor Dwarf is a metal-deficient galaxy containing only 4 percent of the oxygen and carbon elements in our own Galaxy.
type: Concept

Nebulae
definition: A cloud of gas and dust in the interstellar space.
type: Concept
broader: interstellar-medium

Primordial magnetic fields
broader: magnetic-fields
type: Concept

Galaxy evolution
related: Milky Way evolution
type: Concept
broader: galaxy-processes
broader: cosmological-evolution

Fisher's Information
broader: astrostatistics-tools
type: Concept

Ocean-atmosphere interactions
type: Concept
broader: earth-atmosphere

Nonrotating black holes
type: Concept
broader: classical-black-holes

Exoplanet astrometry
use instead: Astrometry
deprecated: :__
type: Concept

Microlensing event rate
type: Concept
broader: gravitational-microlensing

Comet volatiles
broader: comet-nuclei
type: Concept

Solar instruments
type: Concept
definition: An instrument especially designed for solar observations.
broader: astronomical-instrumentation

Meridian circles
definition: A telescope with a graduated vertical scale, used to measure the declinations of heavenly bodies and sometimes to determine the time of meridian transits.
type: Concept
broader: telescopes
broader: transit-instruments

Silicon burning
definition: The nucleosynthetic process taking place in the interior of massive stars whereby silicon is transmuted into iron, nickel, and neighboring nuclei collectively called the iron peak elements.
type: Concept
broader: nucleosynthesis

Wilson-Bappu Effect
deprecated: :__
type: Concept

Galaxy mass distribution
type: Concept
broader: galaxy-masses

Radio transient sources
related: Radio bursts
broader: radio-sources
broader: transient-sources
type: Concept

Prime focus
type: Concept
broader: telescope-properties

High time resolution astrophysics
type: Concept
broader: astronomical-techniques

Gamma-ray bursts
definition: An intense discharge of gamma rays, which range in duration from tenth of a second to tens of seconds and occur from sources widely distributed over the sky. The radio wave afterglow from the burst can last more than a year, making long-term observations of the sources possible. The favored hypothesis is that they are produced by a relativistic jet created by the merger of two compact objects (specifically two neutron stars or a neutron star and a black hole). Mergers of this kind are also expected to create significant quantities of neutron-rich radioactive species, whose decay should result in a faint transient, known as a kilonova, in the days following the burst. Indeed, it is speculated that this mechanism may be the predominant source of stable r-process elements in the Universe. Recent calculations suggest that much of the kilonova energy should appear in the near-infrared spectral range, because of the high optical opacity created by these heavy r-process elements.
related: Gamma-ray transient sources
broader: gamma-ray-sources
type: Concept
broader: burst-astrophysics
related: Gamma-ray bursters

Gravitational disruption
type: Concept
broader: newtonian-gravitation

Ring singularities
broader: gravitational-singularities
related: Kerr black holes
type: Concept

Stellar convective shells
type: Concept
broader: stellar-convection-envelopes

Solar nebulae
broader: solar-evolution
definition: The cloud of interstellar gas and dust from which the Sun and the rest of the solar system initially formed.
related: Solar system formation
type: Concept

Big Bang nucleosynthesis
broader: nucleosynthesis
type: Concept
broader: big-bang-theory

Quintessence
definition: In cosmology, a hypothetical new 'element,' distinct from any normal matter (either baryonic or not) or radiation, intended to explain the observed ever accelerating expansion of the Universe. Quintessence can have several types and differs from the cosmological constant in that it can vary in space and time. In modern physics, the four known "elements" are the baryons (proton, neutron, etc.), the leptons (neutrinos, electrons, etc.), the photon, and the hypothetical non-baryonic matter, which is thought to be 80% of the total matter in the Universe. The quintessence field is a possibility which can be confirmed or disproved by measurements of the dark energy value at different redshifts. In some models, the quintessence is fine-tuned to explain both the cosmological constant problem and the inflation in the very early Universe.
broader: dark-energy
type: Concept

Planetary climates
type: Concept
related: Planetary atmospheres
broader: planetary-science

T Tauri stars
broader: irregular-variable-stars
definition: A member of a class of young stellar objects of roughly 1 solar mass showing strong infrared excess emission attributed to circumstellar disks and found within or close to molecular clouds. T Tauri stars are protostars in the final stages of formation to become a stable main sequence star. The nuclear reactions in their core have not yet stabilized and the stars are known for the variability of their brightness.
broader: young-stellar-objects
type: Concept
broader: emission-line-stars
related: FU Orionis stars

Impact gardening
type: Concept
broader: impact-phenomena

Mixture model
broader: classification
type: Concept

Astronomy software
broader: computational-astronomy
definition: A general term used to describe a collection of computer programs, procedures, and documentation that perform some astronomy related tasks on a computer system.
type: Concept

Diurnal parallax
definition: The apparent difference between the position of a celestial object measured from the Earth's surface and the position that would be recorded by a hypothetical observer at the center of the Earth.
broader: parallax
type: Concept

Stellar bridges
type: Concept
broader: interstellar-medium

Albedo
type: Concept
broader: planetary-science

Wavelet analysis
type: Concept
broader: astrostatistics-techniques
definition: A refinement of Fourier analysis which enables to simplify the description of a complicated function in terms of a small number of coefficients. The formal history of wavelet theory began in the early 1980s when Jean Morlet, a French geophysicist, introduced the concept of wavelet and studied wavelet transform as a new tool for scientific signal analysis. In 1984, his collaboration with Alex Grossmann yielded a detailed mathematical study of the continuous wavelet transforms and their various applications. Although similar results had already been obtained 20-50 years earlier by several other researchers, the rediscovery of the old concepts provided a new method for decomposing functions.

Photoelectric photometry
type: Concept
definition: A photometry in which the magnitudes are obtained using a photoelectric photometer.
broader: photometry

Optical astronomy
related: Visible sources
broader: astronomical-methods
definition: Optical astronomy is the part of astronomy that uses optical instruments (mirrors, lenses, and solid-state detectors) to observe light from near-infrared to near-ultraviolet wavelengths. Visible-light astronomy, using wavelengths detectable with the human eyes (about 400–700 nm), falls in the middle of this spectrum.
type: Concept

Galaxy cluster counts
broader: cosmic-web
broader: galaxy-physics
type: Concept

H beta photometry
type: Concept
broader: narrow-band-photometry

Rapid bursts
broader: burst-astrophysics
type: Concept

Gravitational equilibrium
definition: The condition in a celestial body when gravitational forces acting on each point are balanced by some outward pressure, such as radiation pressure or electron degeneracy pressure, so that no vertical motion results.
type: Concept
broader: newtonian-gravitation

Lenticular galaxies
type: Concept
related: De Vaucouleurs Sandage classification
related: Hubble classification scheme
broader: disk-galaxies

Angular separation
type: Concept
definition: Between two points A and B, the angle subtended by lines drawn from an observing point O to A and B.
broader: astronomical-location

Ice composition
type: Concept
broader: ice-physics

Bayes factor
broader: astrostatistics-tools
type: Concept

B subdwarf stars
broader: subdwarf-stars
broader: b-stars
type: Concept

Interstellar absorption
type: Concept
broader: interstellar-medium
definition: The absorption of light from stars and other objects by intervening gas and dust in interstellar space.

Minor planets
type: Concept
deprecated: :__

Cometary nebulae
broader: protostars
broader: nebulae
type: Concept

Borasisi
deprecated: :__
type: Concept

Australites
type: Concept
broader: tektites

Focal length
type: Concept
broader: telescope-properties

Gravitational fields
broader: newtonian-gravitation
type: Concept
definition: The region of space in which gravitational attraction exists.

Fanaroff-Riley radio galaxies
broader: radio-galaxies
definition: Analyzing a sample of 57 radio galaxies from the 3CR catalogue, which were clearly resolved at 1.4 GHz or 5 GHz, Fanaroff & Riley (1974) discovered that the relative positions of regions of high and low surface brightness in the lobes of extragalactic radio sources are correlated with their radio luminosity. They divided the sample into two classes using the ratio RFR of the distance between the regions of highest surface brightness on opposite sides of the central galaxy or quasar, to the total extent of the source up to the lowest brightness contour in the map. Fanaroff-Riley Class I (FR-I) , Fanaroff-Riley Class II (FR-II). The boundary between the two classes is not very sharp, and there is some overlap in the luminosities of sources classified as FR-I or FR-II on the basis of their structures. The physical cause of the FR-I/II dichotomy probably lies in the type of flow in the radio jets.
type: Concept

Planetary boundary layers
broader: planetary-atmospheres
type: Concept

Spectropolarimetry
broader: polarimetry
related: Solar electromagnetic emission
type: Concept
definition: A technique of observation in astrophysics which combines spectroscopy and polarization measurements. Spectropolarimetry has a wide range of applications in astrophysics, including stellar magnetic field studies.

Aurorae
definition: A phenomenon consisting of luminous colorful arcs, rays, and streamers that appear in a planet's upper atmosphere during the night with the greatest frequency in the northern and southern polar magnetic zones. This non-thermal radiation is caused by the emission of light from atoms excited by electrons accelerated along the planet's magnetic field lines at the magnetic poles. Fluorescent emission from atomic oxygen at 5557 Å results in a greenish glow, and there is a weaker effect from the red line at 6300 Å. Blue and purple colors are emitted by atomic and molecular nitrogen.
type: Concept
broader: planetary-magnetosphere

S-process
broader: stellar-nucleosynthesis
broader: nucleosynthesis
type: Concept
definition: A nucleosynthesis process by which chemical elements heavier than copper are formed through a slow flux of neutrons absorbed by atomic nuclei (neutron-capture element). The capture of neutrons occurs on time scales that are long enough to enable unstable nuclei to decay via the emission of a beta particle before absorbing another neutron. Prominent s-process elements include barium, zirconium, and yttrium.

Trigonometric parallax
type: Concept
definition: The parallax of a nearby star (less than 300 light-years) against the background of more distant stars resulting from the motion of the Earth in its orbit around the Sun.
broader: parallax

Black hole thermodynamics
type: Concept
broader: black-hole-physics

Galactic and extragalactic astronomy
type: Concept

Gamma-ray telescopes
type: Concept
broader: telescopes
broader: space-telescopes

Event horizons
broader: gravitational-singularities
broader: black-hole-physics
type: Concept
definition: The surface surrounding a black hole with the property that any light ray emitted inside it cannot escape to the outer space because of the strength of the gravitational field. The radius of the event horizon is called the Schwarzschild radius.

Kullback-Leibler distance
broader: astrostatistics-tools
type: Concept

S stars
broader: giant-stars
definition: A red giant of spectral type S whose spectrum is dominated by molecular bands arising from zirconium oxide (ZrO). S stars also have strong cyanogen bands and contain spectral lines of lithium and technetium. Almost all S stars are long-period variables.
broader: late-type-stars
type: Concept
broader: heavy-metal-stars
broader: stellar-spectral-types

Timing variation methods
type: Concept
broader: exoplanet-detection-methods

Flat space
broader: cosmological-parameters
type: Concept

Exoplanet surface composition
broader: exoplanet-surface-characteristics
related: Surface composition
type: Concept

Mesosphere
broader: earth-atmosphere
definition: The layer of the atmosphere located between the stratosphere and the ionosphere, where temperature drops rapidly with increasing height. It extends between 17 to 80 kilometers above the Earth's surface.
type: Concept

X-ray active galactic nuclei
broader: active-galactic-nuclei
type: Concept

Solar active region velocity fields
broader: solar-active-regions
type: Concept

Bekenstein-Hawking entropy
broader: black-hole-physics
type: Concept

Periodic variable stars
type: Concept
broader: variable-stars

Hypervelocity stars
broader: high-velocity-stars
definition: A star whose velocity is so great that it will escape the gravitational potential of our Galaxy. Depending on the location and direction of motion, this criterion typically corresponds to a stellar velocity in the Galactic rest frame larger than 400 km s^-1, and up to about 1200 km s^-1. The nature of the hypervelocity stars (HVSs) spans a wide range of types from OB stars, to metal-poor F-type stars and G/K dwarfs. While there is evidence from many late-type B HVSs in the halo to originate from the Galactic supermassive black hole (SMBH), other HVSs seem to originate from the galactic disk. HVSs can obtain their large velocities from a number of different processes: 1) Tidal disruption of close binary stars by the central SMBH of the Milky Way. In this process one star is captured by the SMBH while the other is ejected at high speed via the gravitational slingshot mechanism. 2) Exchange encounters in other dense stellar environments between hard binaries and massive stars may cause stars to be ejected and escape our Galaxy. 3) Disruption of close binaries via supernova explosions. The runaway velocities of both ejected stars can reach large values when asymmetric supernovae are considered, i.e. when the newborn neutron star receives a momentum kick at birth.
type: Concept

Interstellar filaments
related: Filamentary nebulae
type: Concept
broader: interstellar-medium

Natural Decay
type: Concept
deprecated: :__

Milky Way evolution
broader: milky-way-galaxy-physics
type: Concept
related: Galaxy evolution

Maunder minimum
broader: sunspots
definition: A period from about 1645 to 1715 when the number of sunspots was unusually low. This solar activity minimum is attested also through the increased content of carbon-14 in tree rings in that period. The reason is that the cosmic rays which produce carbon-14 reach the Earth in a greater number when there is weak solar activity. The Maunder minimum occurred during a period of cooling of the Earth, called the Little Ice Age. The Maunder minimum is one of a number of periods of low solar activity, including the Dalton minimum, the Sporer minimum, the Wolf minimum, and the Oort minimum.
type: Concept

Dirty snowball model
type: Concept
definition: A model for a cometary nucleus proposed by Fred Whipple (1950-51), according to which the nucleus is a solid body (a few kilometers across) made up of various ices (frozen water, methane, ammonia, carbon dioxide, and hydrogen cyanide) in which dust is embedded. Dust particles are liberated when the ices vaporize as the comet approaches the Sun, and they get blown away by solar radiation pressure, often forming impressive, gently curved dust tails.
broader: comets

Me variable stars
broader: me-stars
broader: m-giant-stars
type: Concept

Time domain astronomy
broader: astronomical-methods
type: Concept
related: Transient sources
related: Variable stars
related: Proper motions
related: Time series analysis

Burst astrophysics
broader: high-energy-astrophysics
broader: stellar-phenomena
type: Concept
related: Supernovae

Expectation-Maximization Algorithm
broader: algorithms
type: Concept

Newtonian cosmology
definition: The use of Newtonian mechanics to derive homogeneous and isotropic solutions of Einstein's field equations, which represent models of expanding Universe. The Newtonian cosmology deviates from the prediction of general relativity in the general case of anisotropic and inhomogeneous models.
broader: cosmological-models
type: Concept

G supergiant stars
broader: supergiant-stars
broader: g-stars
type: Concept

Ion trapping
broader: experimental-techniques
type: Concept

Bayesian information criterion
broader: model-selection
type: Concept
broader: astrostatistics-tools

CMBR detectors
type: Concept
broader: astronomical-detectors
related: Cosmic microwave background radiation

F layer
type: Concept
broader: ionosphere

Double galaxies
broader: galaxies
type: Concept

Importance sampling
type: Concept
broader: markov-chain-monte-carlo

CCD photometry
type: Concept
broader: photometry

Solid body tides
type: Concept
broader: tides

Dumbbell galaxies
type: Concept
broader: double-galaxies

Akaike information criterion
type: Concept
broader: model-selection

Recurrent novae
type: Concept
broader: novae
definition: A cataclysmic variable star that undergoes outbursts similar to those found in novae.

Planetary thermal histories
broader: planetary-science
type: Concept

Schonberg-Chandrasekhar limit
broader: gravitational-collapse
type: Concept
related: Stellar evolution
definition: During the main sequence stage, a star burns the hydrogen in its core and transforms it into helium. When the helium mass amounts to about 10% of the initial stellar mass, the star can no longer maintain the hydrostatic equilibrium in its core; the star increases its volume and leaves the main sequence in order to become a red giant.

Nonparametric inference
type: Concept
related: Nonparametric hypothesis tests
broader: astrostatistics-strategies

Astronomical coordinate systems
type: Concept
broader: astrometry
definition: Values in a reference system used to relate the position of a body on the celestial sphere. Four main coordinate systems are utilized in astronomy: the equatorial, horizontal, ecliptic, and galactic coordinates systems.

Astrostatistics strategies
type: Concept
broader: astrostatistics

Population II stars
type: Concept
definition: A member of a population of relatively old stars, containing a small fraction of metals, found mainly in the halo of the Galaxy and in globular clusters.
broader: stellar-populations

Direct detection interferometry
type: Concept
broader: interferometry

Radio continuum emission
broader: interstellar-emissions
type: Concept
definition: A continuum emission with frequencies in the radio range of the electromagnetic spectrum.
broader: radio-sources

Milky Way Galaxy physics
broader: galaxy-physics
related: Milky Way Galaxy
type: Concept

Primordial black holes
type: Concept
broader: astrophysical-black-holes
definition: A black hole formed following the Big Bang event due to incredibly violent turbulence that squeezed concentrations of matter to high densities. These black holes, first suggested by Stephen Hawking, are expected to have a mass comparable to that of a mountain and a size as small as an atom.

Hydrogen burning
broader: stellar-nucleosynthesis
broader: nucleosynthesis
type: Concept
definition: A series of thermonuclear reactions, taking place mainly in low-mass stars, such as the Sun, which transforms four hydrogen nuclei (protons) into one helium (Helium-4) nucleus and thereby generates energy in the stellar core. First, two protons (Helium-1) combine to form a deuterium nucleus (Helium-2) with the emission of a positron and a neutrino. The deuterium nucleus then rapidly captures another proton to form a Helium-3 nucleus, while emitting a gamma ray. There are three alternatives for the next step. In the proton-proton chain, occurring in 86% of the cases, two Helium-3 nuclei fuse to a final Helium-4 nucleus while two protons are released. The mass of the resulting Helium-4 nucleus is less than the total mass of the four original protons. The difference, ~ 0.7% of the total mass of the protons, is converted into energy and radiated by the Sun. In this process, the Sun loses some 4 million tons of its mass each second.

E layer
broader: ionosphere
type: Concept

Water vapor
broader: greenhouse-gases
type: Concept
definition: Water in the gaseous state. Atmospheric water in vapor form; one of the most important components of the atmosphere.
broader: planetary-atmospheres

Chthonian planets
type: Concept
broader: exoplanets

Newtonian gravitation
type: Concept
broader: gravitation

Lunar node regression
broader: lunar-theory
broader: orbital-motion
type: Concept

Shell burning
definition: The nuclear reactions in a shell around a star's core that continue after the fuel in the core itself has been exhausted. As the fuel is progressively exhausted, the shell moves outward until it enters regions too cool for the reactions to continue. For example, after the exhaustion of hydrogen in the core, helium burning might take place in the core with a shell of hydrogen burning surrounding it. Stars may have more than one region of shell burning during their stellar evolution, each shell with its own nuclear reactions.
broader: circumstellar-shells
type: Concept

Center of mass
definition: A fixed point in a body through which the resultant force of gravitational attraction acts.
broader: orbital-elements
type: Concept

Ionized coma gases
type: Concept
broader: comae

N-body simulations
related: N-body problem
broader: astronomical-simulations
broader: gravitational-interaction
type: Concept

Polar tides
broader: ocean-tides
type: Concept

Planetary polar regions
broader: planetary-structure
type: Concept

Curved space
broader: cosmological-parameters
type: Concept

Lunar theory
type: Concept
broader: celestial-mechanics
broader: lunar-science

Technetium stars
broader: heavy-metal-stars
broader: chemically-peculiar-stars
type: Concept

Evaporating gaseous globules
type: Concept
broader: interstellar-clouds

Tidal distortion
broader: tidal-interaction
type: Concept

Solar coronal radio emission
type: Concept
broader: solar-corona

Milky Way dark matter halo
type: Concept
related: Galaxy dark matter halos
broader: milky-way-galaxy-physics

Supernova dynamics
broader: stellar-dynamics
related: Supernovae
related: Explosive nucleosynthesis
related: Stellar remnants
type: Concept

Quantum-chemical calculations
type: Concept
broader: theoretical-techniques

Interstellar extinction
type: Concept
broader: extinction

Oblateness
definition: The property possessed by a round body that is flattened at the poles.
broader: planetary-structure
type: Concept

Ellipsoidal variable stars
broader: close-binary-stars
broader: extrinsic-variable-stars
type: Concept

Solar rotation
type: Concept
broader: solar-physics
definition: The motion of the Sun around an axis which is roughly perpendicular to the plane of the ecliptic; the Sun's rotational axis is tilted by 7.25° from perpendicular to the ecliptic. It rotates in the counterclockwise direction (when viewed from the north), the same direction that the planets rotate (and orbit around the Sun). The Sun's rotation is differential, i.e. the period varies with latitude on the Sun (differential rotation). Equatorial regions rotate in about 25.6 days. The regions at 60 degrees latitude rotate more slowly, in about 30.9 days.
related: Stellar rotation

Experimental techniques
type: Concept
broader: laboratory-astrophysics

Stellar F corona
related: Solar F corona
type: Concept
broader: stellar-coronae

Milky Way formation
broader: milky-way-galaxy-physics
related: Galaxy formation
type: Concept

Radio receivers
type: Concept
broader: astronomical-detectors

Photoheliographs
type: Concept
broader: early-astronomical-instruments

Dynamical evolution
broader: cosmological-evolution
type: Concept

X-ray quasars
type: Concept
broader: quasars

A subdwarf stars
broader: subdwarf-stars
broader: a-stars
type: Concept

Lunar seismology
type: Concept
broader: lunar-interior

Gregorian telescopes
type: Concept
broader: reflecting-telescopes

Chondrules
definition: Millimeter-sized grains of silicate sometimes found in large numbers in chondrite meteorites. They are essentially glassy beads made by a violent but brief heating event that caused dust grains to form melt droplets. However, the cause of the heating remains unknown.
type: Concept
broader: chondrites

Galaxy ages
broader: galaxy-properties
type: Concept

Interstellar thermal emission
broader: interstellar-emissions
type: Concept

Technosignatures
type: Concept
broader: search-for-extraterrestrial-intelligence

Gamma-ray spectroscopy
type: Concept
broader: spectroscopy

Solar radio flares
broader: solar-flares
type: Concept

Altitude angle
type: Concept
broader: horizontal-coordinate-system

Blue cloud galaxies
broader: galaxy-colors
type: Concept

Long baseline interferometers
type: Concept
broader: radio-interferometers

Super Earths
definition: An extrasolar planet more massive than the Earth but less massive than 10 Earth masses. The first discovered super-Earth orbits an M4 V star named GJ 876. Its estimated mass is 7.5±0.7 Earth masses and it has an orbital period of 1.94 days. It is close to the host star, and the surface temperature is calculated to lie between 430 and 650 K.
broader: exoplanets
type: Concept

Solar spicules
type: Concept
broader: solar-chromosphere

Interstellar masers
type: Concept
definition: A maser phenomenon created by young stars and protostars in the surrounding dense molecular clouds of gas and dust.
broader: interstellar-emissions

Young star clusters
broader: open-star-clusters
type: Concept

Cosmological parameters from large-scale structure
broader: cosmological-parameters
type: Concept

Shell stars
broader: chemically-peculiar-stars
type: Concept
definition: A main-sequence star, usually of spectral class B to F, whose spectrum shows bright emission lines superimposed on the normal absorption lines. The emission spectrum is explained by the presence of a circumstellar shell of gas surrounding the star at the equator. Shell stars are fast rotators.

Solar fibrils
type: Concept
definition: A linear pattern in the chromosphere of the Sun, as seen through an H-alpha filter, occurring near strong sunspots and plages or in filament channels. They are magnetically confined tubes of hot plasma. Individually, they are about 10,000 km long and last for 10 to 20 minutes.
broader: solar-spicules

Sunspot number
type: Concept
broader: sunspots
definition: A quantity which gives the number of sunspots at a given time. It is defined by the relationship R = k(10g + f), where R is the sunspot number, k is a constant depending on the observation conditions and the instrument used, g is the number of the groups and f is the number of individual spots that can be counted.

Radio active galactic nuclei
broader: active-galactic-nuclei
type: Concept

Galaxy colors
broader: galaxy-properties
type: Concept

F stars
related: Stellar classification
broader: stellar-spectral-types
type: Concept
broader: intermediate-type-stars

Infrared dark clouds
definition: A dark cloud characterized by a visual extinction A_v ≥ 10^2 mag. Infrared dark clouds are opaque even at 8 μm, and can be seen in silhouette against the bright diffuse mid-infrared emission in the interstellar medium.
broader: dark-interstellar-clouds
broader: molecular-clouds
type: Concept

Radiative magnetohydrodynamics
broader: astrophysical-fluid-dynamics
type: Concept

X-ray photometry
broader: waveband-photometry
type: Concept

Surface gravity
broader: planetary-science
type: Concept
definition: The rate at which a small object in free fall near the surface of a body is accelerated by the gravitational force of the body: g = GM / R^2, where G is the gravitational constant, and M and R are the mass and radius of the object. The surface gravity of Earth is equal to 980 cm s^-2.

Ice formation
type: Concept
broader: ice-physics

Lunar science
type: Concept
related: The Moon
related: Lunar probes
broader: solar-system-astronomy

Solar system terrestrial planets
broader: solar-system-planets
related: Extrasolar rocky planets
definition: The planets Mercury, Venus, Earth and Mars collectively. They are also known as the terrestrial planets.
type: Concept

Interstellar emissions
type: Concept
broader: interstellar-medium

Galaxy environments
type: Concept
broader: extragalactic-astronomy

Gravitational waves
related: Gravitational wave detectors
broader: cosmic-inflation
definition: A space-time oscillation created by the motion of matter, as predicted by Einstein's general relativity. When an object accelerates, it creates ripples in space-time, just like a boat causes ripples in a lake. Gravitational waves are extremely weak even for the most massive objects like supermassive black holes. They had been inferred from observing a binary pulsar in which the components slow down, due to losing energy from emitting gravitational waves. Gravitational waves were directly detected for the first time on September 14, 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO). Since then several other events have been detected by LIGO and Laser Interferometer Space Antenna (LISA).
type: Concept
related: Gravitational wave astronomy
broader: particle-astrophysics
broader: general-relativity

Lunar interior
broader: selenology
type: Concept

Long period variable stars
broader: periodic-variable-stars
definition: A type of variable star in which variations in brightness occur over long time-scales of months or years. The term generally refers to Mira variable types.
type: Concept
broader: pulsating-variable-stars

X-ray detectors
broader: astronomical-detectors
type: Concept

Lunar rilles
definition: A long, narrow, straight or sinuous trench or valley with steep walls and roughly parallel sides on the surface of the Moon.
broader: lunar-features
type: Concept

Hawking radiation
type: Concept
broader: black-hole-physics
definition: The radiation produced by a black hole when quantum mechanical effects are taken into account. According to quantum physics, large fluctuations in the vacuum energy occurs for brief moments of time. Thereby virtual particle-antiparticle pairs are created from vacuum and annihilated. If pair production happens just outside the event horizon of a black hole, as soon as these particles are formed they would both experience drastically different gravitational attractions due to the sharp gradient of force close to the black hole. One particle will accelerate toward the black hole and its partner will escape into space. The black hole used some of its gravitational energy to produce these two particles, so it loses some of its mass if a particle escapes. This gradual loss of mass over time means the black hole eventually evaporates out of existence.

Magnetic anomalies
broader: magnetic-fields
type: Concept

Two-body problem
broader: celestial-mechanics
broader: gravitational-interaction
type: Concept
definition: In classical mechanics, the study concerned with the dynamics of an isolated system of two particles subject only to the Newtonian gravitational force between them. The problem can be separated into two single-particle problems with the following solutions. The equation of the center of mass is governed by the equation of the same form as that for a single particle. Moreover, the motion of either particle, with respect to the other as origin, is the same as the motion with respect to a fixed origin of a single particle of reduced mass acted on by the same internal force.

Pulsar wind nebulae
broader: nebulae
related: Pulsars
broader: supernova-remnants
definition: A bubble of shocked relativistic particles, produced when a pulsar's relativistic wind interacts with its environment.
type: Concept

Greenhouse effect
type: Concept
broader: earth-atmosphere

Metropolis-Hastings
broader: markov-chain-monte-carlo
type: Concept

Early-type galaxies
broader: galaxies
type: Concept
definition: In the Hubble classification, galaxies on the left part of the Hubble sequence. Early-type galaxies tend to have redder colors, higher average surface brightnesses, and lower neutral hydrogen content than late-type galaxies. This terminology is based on the obsolete and erroneous idea that elliptical and lenticular galaxies might be evolutionary precursors to spiral and barred spiral galaxies.

Nuclear physics
type: Concept
broader: laboratory-astrophysics
related: Nuclear astrophysics
definition: The branch of physics which is concerned with the study of atomic nuclei, subatomic particles, and their exploitation.

Narrow band photometry
type: Concept
broader: photometric-systems

Time series analysis
related: Time domain astronomy
broader: astrostatistics-techniques
type: Concept

High-luminosity active galactic nuclei
broader: active-galactic-nuclei
type: Concept

Individual planetary nebulae
type: Concept
deprecated: :__

Linear regression
definition: In statistics, a regression method that establishes a linear relationship between two random variables.
broader: regression
type: Concept

Nonparametric hypothesis tests
broader: astrostatistics-strategies
related: Nonparametric inference
type: Concept

Speckle interferometry
definition: A technique for generating a clear composite image of a celestial object blurred by atmospheric turbulence in which a large number of short-exposure photographs are mathematically correlated by a computer. By comparing the behavior of the speckles in a series of images it is possible to approach the theoretical resolution of the telescope.
type: Concept
broader: interferometry

Carbon burning
definition: The stage in the evolution of a star after helium burning when the core of the star consists mainly of carbon and oxygen. In stars of mass greater than about 8 solar masses, whose cores reach a temperature above 5 x 10^8 K and density above 3 x 10^9 kg m^-3, carbon burning can begin via reactions such as the following: 12C + 12C → 20Ne + 4He or 12C + 12C → 23Na + p or 12C + 12C → 23Mg + n. The time-scale for this phase of nucleosynthesis is of order of five hundred years.
broader: nucleosynthesis
type: Concept

X-ray transient sources
broader: x-ray-sources
broader: transient-sources
definition: An X-ray source that enhances its emission well above its usual level for a period typically longer than a week. Recurrences can be periodic or aperiodic, but there is no obvious correlation between recurrence time and the luminosity amplitude of the outburst. X-ray transients seem to be divided into two classes: those associated with high-mass X-ray binary (HMXB) stars and those associated with low-mass X-ray binary (LMXB) stars. The HMXBs contain a neutron star or black hole paired with a massive star. Often, the stellar companion is a Be star, Be star, which sometimes sheds material from its equatorial region. In these systems, the transient event is characterized by having more higher energy X-rays in the spectrum. The LMXBs contain a neutron star or black hole orbiting around a cooler, low-mass star. These transient events often generate more lower-energy X-rays.
type: Concept
related: X-ray bursts

Deep space probes
type: Concept
broader: space-probes

Transient detection
broader: time-series-analysis
type: Concept

Tertiary stars
broader: companion-stars
type: Concept

Deucalion
deprecated: :__
type: Concept

Millimeter astronomy
broader: radio-astronomy
type: Concept

Keplerian orbit
broader: orbital-motion
definition: The orbit of a spherical object of a finite mass around another spherical object, also of finite mass, governed by their mutual gravitational forces only.
type: Concept

Galactic abundances
related: Galaxy abundances
type: Concept
broader: milky-way-galaxy-physics
broader: chemical-abundances

Deprecated: Far Infrared Astronomy
use instead: Far infrared astronomy
deprecated: :__
type: Concept

Triton
broader: neptunian-satellites
type: Concept
definition: The seventh and the largest of Neptune's satellites. It has a diameter of 2,700 km and orbits its planet at a mean distance of 354,760 km every 5.877 days. Triton was discovered by William Lassell in 1846 scarcely a month after Neptune was discovered. Triton is colder than any other measured object in the solar system with a surface temperature of -235° C. It has an extremely thin atmosphere. Nitrogen ice particles might form thin clouds a few kilometers above the surface. The atmospheric pressure at Triton's surface is about 15 microbars, 0.000015 times the sea-level surface pressure on Earth. Triton is the only large satellite in the solar system to circle a planet in a retrograde motion, that is in a direction opposite to the rotation of the planet.

Planetary migration
broader: planetary-science
type: Concept
related: Exoplanet migration

R stars
type: Concept
broader: giant-stars
broader: stellar-spectral-types
broader: carbon-stars
broader: late-type-stars

Blue compact dwarf galaxies
definition: An small irregular galaxy undergoing violent star formation activity. These objects appear blue by reason of containing clusters of hot, massive stars which ionize the surrounding interstellar gas. They are chemically unevolved since their metallicity is only 1/3 to 1/30 of the solar value.
type: Concept
broader: compact-dwarf-galaxies

Me stars
related: Mira variable stars
broader: emission-line-stars
broader: m-stars
type: Concept

Hickson compact group
broader: galaxy-groups
type: Concept

Stellar pulsations
broader: stellar-phenomena
type: Concept

South galactic pole
broader: galactic-poles
type: Concept

Affine invariant
broader: markov-chain-monte-carlo
type: Concept

Regolith
broader: planetary-surfaces
related: Lunar regolith
type: Concept

Ring nebulae
broader: nebulae
type: Concept

Time-of-flight mass spectrometry
broader: mass-spectrometry
type: Concept

Stellar remnants
type: Concept
broader: stellar-astronomy
related: Supernova dynamics

Galaxy luminosities
broader: galaxy-properties
type: Concept

Neptune trojans
type: Concept
broader: trans-neptunian-objects
broader: trojan-asteroids

Phase defect
broader: astrometry
type: Concept

Atmospheric composition
related: Exoplanet atmospheric composition
type: Concept
broader: planetary-atmospheres

Neutron interferometry
type: Concept
broader: interferometry

Infrared galaxies
broader: starburst-galaxies
definition: A galaxy that emits most of its energy in the infrared region of the spectrum. Such galaxies are thought to have unusually high rates of star formation and are also described as starburst galaxies.
type: Concept
related: Infrared sources

Mesopause
type: Concept
broader: earth-atmosphere

Emerging flux tubes
type: Concept
broader: astrophysical-magnetism

Chirped-pulse Fourier-transform
broader: microwave-spectroscopy
broader: millimeter-wave-spectroscopy
type: Concept

Europa
broader: galilean-satellites
type: Concept
definition: The sixth of Jupiter's known moons and the fourth largest; it is the second of the Galilean satellites. With a diameter of 3140 km, Europa is slightly smaller than Earth's Moon. Its mass is 4.80 × 10^22 kg, i.e. 1.5 times less massive than Earth Moon. Its distance to Jupiter is 670,900 km, or about 9 Jovian radii. Its orbital period is 3.55 Earth days which equals its rotation period. Europa's density is 3.0 g cm^-3, typical of a mixture of rocks including ice. Its high albedo (0.67) suggests that its surface is mostly water ice. The surface temperature of Europa ranges between about 125 K (-150 °C) at the equator and about 50 K (-220 °C) at the poles. There are few impact craters on Europa, because its surface is too active and therefore young. The most striking features of Europa's surface are structures called lineae and lenticulae. The thickness of the ice crust could range between a few kilometers to a few tens of kilometers. It is now believed that there is an ocean of salty water, up to 100 km deep, flowing under Europa's ice. Europa's ocean is kept liquid due to tidal heating by Jupiter.

Main sequence
definition: An evolutionary stage in the life of a star when it generates its energy by the conversion of hydrogen to helium via nuclear fusion in its core. Stars spend 90% of their life on the main sequence. On the Hertzsprung-Russell diagram it appears as a track running from top left (high temperature, high luminosity, high mass) to lower right (low temperature, low luminosity, low mass).
type: Concept
related: Dwarf stars
related: Zero-age main sequence stars
related: Main sequence stars
broader: hertzsprung-russell-diagram

Longitude of the ascending node
type: Concept
definition: One of the orbital elements used to specify the orbit of an object in space. It is the angle from the reference direction, called the origin of longitude, to the direction of the ascending node, measured in the reference plane.
broader: orbital-elements

Galaxy components
use instead: Galaxy structure
deprecated: :__
type: Concept

D galaxies
type: Concept
broader: elliptical-galaxies

H I line emission
broader: interstellar-line-emission
type: Concept

Vilnius photometry
type: Concept
broader: medium-band-photometry

Galaxy bars
related: Galactic bar
broader: barred-spiral-galaxies
type: Concept

Lyman-break galaxies
type: Concept
broader: galaxies

Expanding stellar envelopes
type: Concept
broader: stellar-properties

IRAS galaxies
deprecated: :__
type: Concept

Molecular structural parameters
type: Concept
broader: molecular-data

Nuclear fusion
type: Concept
broader: nuclear-astrophysics

Galaxies
definition: Generally, a large body of gas, dust, and stars held together by their mutual gravitational attraction and ranging in mass from about 10^6 to 10^13 M_sun. If a galaxy also contains dark matter its mass will be much larger. Galaxies are grouped into three main categories: spiral galaxies, elliptical galaxies, and irregular galaxies.
type: Concept
related: Galaxy photometry
related: Galaxy classification systems
broader: galactic-and-extragalactic-astronomy
related: Galaxy spectroscopy

Twotinos
type: Concept
broader: resonant-kuiper-belt-objects

Silicon stars
broader: ap-stars
type: Concept

Chemical thermodynamics
type: Concept
broader: quantum-chemical-calculations

Gravitational wave astronomy
broader: astronomical-methods
type: Concept
related: Gravitational waves

Planetary mineralogy
broader: planetary-geology
type: Concept

Period search
broader: time-series-analysis
type: Concept

Transit duration variation method
type: Concept
broader: timing-variation-methods

Solar neighborhood
broader: milky-way-galaxy-physics
broader: solar-system
type: Concept
definition: That part of the Milky Way galaxy lying near the Sun. In fact there is no definition of the exact radius of this region. It is referred to the immediate solar neighborhood (within about 5 pc), the solar neighborhood (within about 25 pc), and the extended solar neighborhood (within a few hundred pc).

Asteroseismology
type: Concept
broader: stellar-astronomy
related: Helioseismology
definition: The study of the internal structure of stars through the interpretation of their pulsation periods (stellar pulsation). The radial pulsations are the result of sound waves resonating in the stars interior. Different pulsation modes penetrate to different depths inside a star. If a large number of pulsation modes occurs, then the stellar interior, which is not directly observable, can be probed from oscillation studies because the modes penetrate to various depths inside the star. Using a complex mathematical analysis, very detailed investigations of the structure of the star's interior can be carried out. Applied to the Sun, it is called helioseismology.

Prograde orbit
type: Concept
broader: orbital-motion

Cosmochronology
broader: cosmochemistry
type: Concept

Solar magnetic fields
broader: helioseismology
type: Concept
broader: solar-physics
definition: The Sun's magnetic field which is probably created by the differential rotation of the Sun together with the movement of charged particles in the convective zone. Understanding how the solar magnetic field comes about is the fundamental problem of Solar Physics. The solar magnetic field is responsible for all solar magnetic phenomena, such as sunspots, solar flares, coronal mass ejections, and the solar wind. The solar magnetic fields are observed from the Zeeman broadening of spectral lines, polarization effects on radio emission, and from the channeling of charged particles into visible coronal streamers. The strength of Sun's average magnetic field is 1 gauss (twice the average field on the surface of Earth, around 0.5 gauss), and can be as strong as 4,000 Gauss in the neighborhood of a large sunspot.
related: Stellar magnetic fields

Gamma-ray detectors
type: Concept
broader: astronomical-detectors

Stellar M coronal regions
deprecated: :__
type: Concept

Gamma Doradus variable stars
type: Concept
broader: pulsating-variable-stars

Ap stars
type: Concept
definition: Stars of spectral type A in which lines of ionized metals and rare-earth elements are abnormally enhanced. Such stars have unusually strong magnetic fields, thousands of times stronger than the Sun's typical surface field. Ap stars are generally slow rotators because of magnetic braking.
broader: chemically-peculiar-stars
broader: a-stars

Earth-moon system
broader: solar-system-astronomy
type: Concept
definition: A physical system composed on the Earth and the Moon in which both objects directly influence each other. The total energy in the Earth-Moon system is conserved. The most notable influence that the two objects have on each other is tides.

Astronomical unit
definition: A unit of length equal to 149,597,870,700 m exactly, with symbol 'au' (re-definition at the International Astronomical Union's 28th General Assembly in Beijing, China, August 20-31). The astronomical unit equals 1.5813 x 10^-5 light-years and 4.8481 x 10^-6 parsecs.
broader: distance-measure
type: Concept

Astrostatistics tools
broader: astrostatistics
type: Concept

RR Telescopii stars
type: Concept
deprecated: :__

Clustering
type: Concept
broader: astrostatistics-techniques
definition: Grouping of a number of similar astronomical objects.

Hierarchical cosmology
broader: cosmology
type: Concept
definition: A cosmology characterized by clustering of galaxy clusters in increasingly larger systems.

Stellar ages
broader: stellar-properties
type: Concept

Extrasolar ice giants
type: Concept
broader: exoplanets

High resolution spectroscopy
type: Concept
broader: molecular-spectroscopy

Stellar structures
broader: stellar-physics
type: Concept

Lagrangian points
type: Concept
broader: three-body-problem
definition: Five locations in space where the centrifugal force and the gravitational force of two bodies (m orbiting M) neutralize each other. A third, less massive body, located at any one of these points, will be held in equilibrium with respect to the other two. Three of the points, L1, L2, and L3, lie on a line joining the centers of M and m. L1 lies between M and m, near to m, L2 lies beyond m, and L3 on the other side of M beyond the orbit. The other two points, L4 and L5, which are the most stable, lie on either side of this line, in the orbit of m around M, each of them making an equilateral triangle with M and m. L4 lies in the m's orbit approximately 60° ahead of it, while L5 lies in the m's orbit approximately 60° behind m.

B giant stars
broader: b-stars
broader: giant-stars
type: Concept

Experimental data
type: Concept
broader: laboratory-astrophysics

Reionization
broader: big-bang-theory
definition: The ionization of neutral hydrogen atoms formed during recombination epoch in the early Universe.
broader: early-universe
type: Concept

Extreme carbon stars
broader: giant-stars
related: Infrared sources
type: Concept

Galaxy nuclei
type: Concept
broader: galaxy-structure

Lambda density
type: Concept
broader: density-parameters

Holmberg radius
type: Concept
broader: galaxy-distances

B supergiant stars
type: Concept
related: Alpha Cygni variable stars
broader: b-stars
broader: early-type-supergiant-stars

Galactic archaeology
broader: stellar-astronomy
type: Concept
definition: Study of galactic evolution through study of stellar populations.
broader: milky-way-galaxy-physics

Penumbral filaments
type: Concept
deprecated: :__

Solar coronal mass ejection shocks
broader: shocks
broader: solar-particle-emission
type: Concept

Lunar maria
type: Concept
definition: Areas on the surface of the Moon that appears darker and smoother than their surroundings. Once thought to be seas, lunar maria are now known to be basaltic basins created by volcanic lava floods.
broader: lunar-features

High contrast spectroscopy
broader: high-contrast-techniques
type: Concept

Nonlinear regression
type: Concept
broader: regression

Centaur group
definition: An asteroid whose orbit around the Sun lies typically between the orbits of Jupiter and Neptune (5 to 30 astronomical units). The first Centaur, called Chiron, was discovered in 1977, but since then more than 100 roughly similar objects have been found. Three centaurs, Chiron, 60558 Echeclus, and 166P/NEAT 2001 T4, have been found to display cometary comas. Chiron and 60558 Echeclus are now classified as both asteroids and comets. Most of the Centaur asteroids are probably dormant comets from the Kuiper belt which have been pulled in by the gravity of outer planets.
type: Concept
broader: asteroids

Geosynchronous satellites
type: Concept
definition: A satellite that has a circular orbit around the Earth identical to a geostationary orbit except that the satellite's orbit does not necessarily lie in the Earth's equatorial plane.
broader: artificial-satellites

Transient sources
related: Time domain astronomy
type: Concept
broader: high-energy-astrophysics

Of stars
type: Concept
broader: o-stars
definition: An O star whose spectrum displays strong N III 4634-4640-4642 emission and strong He II 4686 emission. The N III lines are always much stronger than C III 4647-4650-4651 when the latter are present. Historically, Of stars were considered to belong to the peculiar category, hence the f notation (see below). In his thesis work, Walborn removed them from that category and established them as the normal O-type supergiants. He also used the notation ((f)), (f), and f to describe the progression from strong He II 4686 absorption, through weakened/absent, to emission, respectively, correlated with increasing N III emission strength, subsequently showing that it is a luminosity sequence -- the first such for stars earlier than O9.
broader: early-type-emission-stars

Photographic photometry
type: Concept
broader: photometry

Circumstellar gas
broader: circumstellar-matter
type: Concept

Eclipsing binary stars
definition: A binary star in which one of the two stars passes in front of the other so that the system's total light periodically fades. The most famous eclipsing binary is Algol.
broader: binary-stars
broader: extrinsic-variable-stars
type: Concept

Stellar diffusion
type: Concept
broader: stellar-processes

Molecular clouds
related: Astrophysical masers
broader: interstellar-clouds
type: Concept

Radial pulsations
type: Concept
broader: stellar-pulsations

SU Ursae Majoris stars
broader: dwarf-novae
type: Concept

Solar core
type: Concept
related: Stellar cores
broader: solar-interior

Lunar gravitational field
type: Concept
broader: selenology

Galactic winds
broader: milky-way-galaxy-physics
related: Galaxy winds
type: Concept

Space astrometry
type: Concept
broader: astrometry

Planetary ionospheres
type: Concept
broader: planetary-atmospheres

Mass ratio
broader: three-body-problem
type: Concept

Atmospheric refraction
definition: The shift in apparent direction of a celestial object caused by the bending of light while passing through the Earth's atmosphere. Since the density of the atmosphere decreases with altitude, the starlight will bend more as it continues down through the atmosphere. As a result, a star will appear higher in the sky than its true direction.
type: Concept
broader: astronomical-optics

Variable star period change
type: Concept
broader: apparent-magnitude

Ultraluminous x-ray sources
type: Concept
definition: An X-ray source that is not in the nucleus of a galaxy, and is more luminous than 10^39 ergs s^-1, brighter than the Eddington luminosity of a 10 solar mass black hole. In general, there is about one ULX per galaxy in galaxies which host ULXs. The Milky Way contains no such objects. ULXs are thought to be powered by accretion onto a compact object. Possible explanations include accretion onto neutron stars with strong magnetic fields, onto stellar black holes (of up to 20 solar masses) at or in excess of the classical Eddington limit, or onto intermediate-mass black holes (10^3 - 10^5 solar masses). NGC 1313X-1, NGC 5408X-1, and NGC 6946X-1 are three ULXs with X-ray luminosities up to ~10^40 erg s^-1.
broader: x-ray-sources

FU Orionis stars
related: T Tauri stars
broader: young-stellar-objects
broader: flare-stars
related: Infrared sources
type: Concept
broader: emission-line-stars

Individual quasars
type: Concept
deprecated: :__

Celestial objects catalogs
type: Concept
broader: catalogs

Luminous infrared galaxies
broader: infrared-galaxies
definition: A galaxy that emits most of its energy in the infrared and whose infrared luminosity (in the 8-1000 µm range) is more than 10^11 solar luminosities.
type: Concept

Common envelope evolution
broader: multiple-star-evolution
type: Concept
related: Common envelope binary stars

Neutral coma gases
type: Concept
broader: comae

Magellanic Stream
type: Concept
broader: magellanic-clouds
broader: high-velocity-clouds
definition: A thin trail of gas stretching from the Magellanic System toward our own Galaxy over about 150° on the sky, corresponding to hundreds of thousands of light-years. This gas consists primarily of neutral hydrogen and is thought to have originated from the Large and Small Magellanic Clouds as a result of tidal interactions with the Milky Way.

Roche limit
definition: The smallest distance at which a satellite under the influence of its own gravitation and that of a central mass about which it is describing a Keplerian orbit can be in equilibrium. This does not, however, apply to a body held together by the stronger forces between atoms and molecules. At a lesser distance the tidal forces of the primary body would break up the secondary body. The Roche limit is given by the formula d = 1.26 R_M (ρ_M/ρ_m)^(1/3), where R_M is the radius of the primary body, ρ_M is the density of the primary, and ρ_m is the density of the secondary body. This formula can also be expressed as: d = 1.26 R_m (M_M/M_m)^(1/3), where R_m is the radius of the secondary. As an example, for the Earth-Moon system, where R_M = 6,378 km, ρ_M = 5.5 g cm^(-3), and ρ_m = 2.5 g cm^(-3) is 1.68 Earth radii.
broader: tidal-interaction
broader: three-body-problem
type: Concept

Fast novae
type: Concept
broader: novae

Transits
type: Concept
related: Occultation
broader: syzygy
definition: An event where one astronomical object appears to move across the face of another. As seen from Earth, the planets Venus and Mercury are seen to transit the Sun. We can also observe natural satellites transit the face of their host planet. Similarly exoplanets have been observed to transit their host stars.

Helmholtz-Kelvin contraction
broader: stellar-evolution
related: Pre-main sequence
type: Concept

Radio occultation
broader: occultation
type: Concept

Gravitational lensing shear
type: Concept
broader: gravitational-lensing

Theoretical data
type: Concept
broader: laboratory-astrophysics

Ae stars
broader: early-type-emission-stars
broader: a-stars
type: Concept

Interferometers
related: Interferometry
broader: astronomical-instrumentation
definition: An arrangement of two or more separate telescopes placed at some distance from one another, each one receiving electromagnetic radiation (optical, infrared, or radio) from a celestial object and brought together to form an interference pattern.
type: Concept

Eye pieces
broader: optical-telescopes
type: Concept

Massive stars
broader: stellar-types
type: Concept
definition: A star whose mass is larger than approximately 10 solar masses. The spectral types of massive stars range from about B3 (B star) to O2 (O star) and include Wolf-Rayet stars as well as Luminous Blue Variables. Massive stars are very rare; for each star of 20 solar masses there are some 100,000 stars of 1 solar mass. Despite this rarity, they play a key role in astrophysics. They are major sites of nucleosynthesis beyond oxygen and, therefore, are mainly responsible for the chemical evolution of galaxies. Due to their high ultraviolet flux and powerful stellar winds, they bring about interesting phenomena in the interstellar medium, like H II regions, turbulence, shocks, bubbles, and so on. Massive stars are progenitors of supernovae (type Ia, type Ic and type II), neutron stars, and black holes. The formation processes of massive stars is still an unresolved problem. For massive stars the accretion time scale is larger than the Kelvin-Helmholtz time scale. This means that massive stars reach the main sequence while accretion is still going on.

Diffuse radiation
type: Concept
broader: earth-atmosphere

Holmium stars
broader: ap-stars
type: Concept

Secondary cosmic rays
type: Concept
broader: cosmic-rays
definition: A burst of secondary charged and neutral particles arising when primary cosmic rays collide with the atmospheric oxygen or nitrogen nuclei in the upper atmosphere. The collision produces mostly pions (π), along with some kaons (K), antiprotons, and antineutrons. Neutral pions very quickly decay, usually into two gamma rays. Charged pions also decay but after a longer time. Therefore, some of the pions may collide with yet another nucleus of the air before decaying, which would be into a muon and a neutrino. The fragments of the incoming nucleus also interact again, also producing new particles.

Chemical abundances
broader: astrochemistry
type: Concept
definition: The relative amount of a given chemical element or chemical compound with respect to another element or compound in a given sample.

Comet Halley
type: Concept
definition: The most famous comet orbiting the Sun once about every 75 years. The last time it appeared was in 1986, and it is predicted to return in 2061. Its earliest recorded sighting is traced back to 240 BC in China. In 1705 Edmond Halley used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time. He found that the bright comets of 1531, 1607, and 1682 had almost the same orbits. He concluded that these appearances must belong to a single recurring comet, and predicted its return for 1758. Halley's comet is the first known periodic comet, hence its designation 1P/Halley.
broader: comets

Ice destruction
type: Concept
broader: ice-physics

Orbiters
type: Concept
broader: space-probes
definition: A spacecraft or satellite designed to orbit a planet or other solar system body.

Visible sources
broader: astronomical-radiation-sources
type: Concept
related: Optical astronomy

Cape photometry
broader: broad-band-photometry
type: Concept

Astronomical radiation sources
broader: cosmology
type: Concept

Molecular gas
type: Concept
broader: interstellar-phases

Tidal radius
broader: tidal-interaction
type: Concept

Titan
definition: The largest and the sixth moon of Saturn discovered by Christiaan Huygens in 1655. Called also Saturn VI. Titan has a diameter of 5,150 km, about half the size of Earth and almost as large as Mars. It orbits Saturn at a mean distance of 1,221,830 km every 15.945 days. It is the only moon known to have an atmosphere. Its surface temperature is -179 °C, which makes water as hard as rocks and allows methane to be found in its liquid form. Its surface pressure is slightly higher than Earth's pressure (1.6 bars against 1 bar at sea level). The Huygens probe released from Cassini-Huygens landed on Titan on December 25, 2004. From the data obtained by Cassini-Huygens, we know that Titan is a world with lakes and seas composed of liquid methane and ethane near its poles, with vast, arid regions not made of silicates as on Earth, but of solid water ice coated with hydrocarbons that fall from the atmosphere. Titan's icy dunes are gigantic, reaching, on average, 1 to 2 km wide, hundreds kilometers long and around 100 m high. Titan is the only other place in the solar system known to have an Earth-like cycle of liquids flowing across its surface as the planet cycles through its seasons. Each Titan season lasts about 7.5 Earth years. The Huygens probe made the first direct measurements of Titan's lower atmosphere. Huygens also directly sampled aerosols in the atmosphere and confirmed that carbon and nitrogen are their major constituents. Cassini followed up Huygens' measurements from orbit, detecting other chemicals that include propylene and poisonous hydrogen cyanide, in Titan's atmosphere. Cassini's gravity measurements of Titan revealed that this moon is hiding an internal, liquid water and ammonia ocean beneath its surface. Huygens also measured radio signals during its descent that strongly suggested the presence of an ocean 55 to 80 km below the moon's surface.
type: Concept
broader: saturnian-satellites

Luminous blue variable stars
broader: emission-line-stars
broader: early-type-variable-stars
broader: eruptive-variable-stars
type: Concept
definition: A high-luminosity variable star, which represents a transition phase in the life of a massive star when it evolves off the main sequence to become a supernova. Only about a dozen confirmed luminous blue variable stars are presently known in our Galaxy.
related: Alpha Cygni variable stars

High-redshift galaxy clusters
type: Concept
broader: galaxy-clusters

Nadir
type: Concept
broader: horizontal-coordinate-system
definition: The point on the celestial sphere blocked from view by Earth and diametrically opposite to the zenith.

SRc variable stars
type: Concept
broader: semi-regular-variable-stars

Circumstellar envelopes
broader: circumstellar-matter
definition: A very extensive envelope of cold gaseous materials surrounding evolved cool stars, notably red giants, red supergiants (Mira variables), or asymptotic giant branch stars. The typical size of such envelopes is several thousand times that of the stellar radius and their temperature ranges from 1000 to 10 K. Circumstellar envelopes result from mass loss from the central star (10-7 to 10-4 solar masses per year) and expand with moderate velocities (10 to 15 km sec^-1). The low temperature of the envelope is at the origin of the formation of molecules, which in certain conditions provide maser emission (H_{2}O, OH, SiO). Similarly, dust grains form in the envelope produce an infrared excess emission.
type: Concept

Perihelion
definition: The nearest point to the Sun in an orbit around the Sun; opposite of aphelion.
broader: apsides
type: Concept

Doppler imaging
type: Concept
broader: astronomical-techniques

Radio loud quasars
type: Concept
definition: A quasar that has the same characteristics as a radio-quiet quasar with the addition of having strong radio emissions.
broader: quasars

Surface photometry
type: Concept
broader: photometry

Electron impact ionization
type: Concept
broader: ionization

Abell clusters
type: Concept
definition: Galaxy clusters listed in the Abell catalog, which is a catalog of 4073 rich galaxy clusters grouped by constellation and by catalog number. Published first in 1958, it contained the clusters visible from the northern hemisphere. A supplement for the southern hemisphere survey was published in 1988.
broader: galaxy-clusters

Lunar phase
broader: lunar-science
type: Concept
definition: One of the various changes in the apparent shape of the Moon, because as the Moon orbits the Earth different amounts of its illuminated part are facing us. The phases of the Moon include: the new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent, and new moon again.

Baryogenesis
type: Concept
definition: The hypothetical mechanism of creating the baryon asymmetry in the Universe. Explaining the observed matter asymmetry is an important open question in physical cosmology.
broader: early-universe

Solar neutrino problem
type: Concept
broader: neutrino-astronomy
definition: A major discrepancy between the flux of neutrinos detected at Earth from the solar core and that predicted by current models of solar nuclear fusion and our understanding of neutrinos themselves. The problem, lasting from the mid-1960s to about 2002, was a considerably lesser detected number of neutrons compared with theoretical predictions. The discrepancy has since been resolved by new understanding of neutrino physics, requiring a modification of the standard model of particle physics, in particular neutrino oscillation.

Solar prominences
broader: solar-activity
related: Solar filaments
type: Concept

Hypergiant stars
broader: evolved-stars
type: Concept
definition: A high luminosity star with absolute visual magnitude around -10, about 10^6 times as luminous as the Sun. Hypergiant stars are evolved massive stars belonging to the luminosity class Ia+ or Ia0. Their spectra show very broadened emission and absorption lines resulting from the high luminosity and low surface gravity which favor strong stellar wind.

Quasinormal modes
type: Concept
broader: black-hole-physics

Quantum cosmology
type: Concept
broader: cosmology

Sedna
deprecated: :__
type: Concept

Baker-Nunn cameras
broader: catadioptric-telescopes
type: Concept
broader: wide-field-telescopes

Chondrites
type: Concept
definition: The most common type of meteorites containing chondrules. These stony meteorites make up about 86% of all meteorites. An important feature of the chondrites is that, with the exception of a few highly volatile elements, they have the same composition as the Sun.
broader: meteorites

Luminous blue variables
deprecated: :__
type: Concept

Planetary-disk interactions
type: Concept
broader: planetary-science

G subgiant stars
broader: subgiant-stars
broader: g-stars
type: Concept

Planetary atmospheres
related: Exoplanet atmospheres
type: Concept
related: Planetary climates
broader: planetary-science
related: Atmospheric science
related: Astrophysical masers

Radial velocity methods [Exoplanets]
type: Concept
use instead: Radial velocity
deprecated: :__

Oort cloud objects
broader: oort-cloud
type: Concept

Plutonian satellites
broader: natural-satellites-solar-system
type: Concept

Emission line galaxies
broader: galaxies
type: Concept

Solar atmospheric motions
broader: solar-motion
type: Concept

Light curves
definition: A curve showing the behavior of the light from a variable star over a period of time.
broader: photometry
related: Light curve classification
type: Concept

Cosmic rays
definition: Extremely energetic atomic nuclei which travel through the Universe at practically the speed of light and strike the Earth from all direction. Almost 90% of all the incoming primary cosmic rays are protons, about 9% are helium nuclei (alpha particles) and about 1% are electrons (beta minus particles). Some cosmic rays come from the Sun (mainly due to solar flares), most come from galactic supernovae, and a few with the highest energy are suspected to originate from outside the Milky Way. As for their flux, about 1 charged particle per second per cm^2 impacts the Earth. The typical kinetic energy of these particles is about 10 MeV/nucleon to several GeV/nucleon, although there are some at higher energies. In fact, the cosmic ray with the highest energy has been measured above x 10^20 eV. These ultra-high energy cosmic rays are suspected to be extragalactic, as there is no plausible mechanism of acceleration to these energies by a supernova, for example. Again, compare these energies to those of solar neutrinos that have only 0.26 MeV. Cosmic rays may be divided into primary cosmic rays and secondary cosmic rays. Their energy ranges from 10^9 to 10^20 electron-volts.
type: Concept
related: Cosmic ray astronomy
broader: particle-astrophysics
broader: astronomical-radiation-sources

Rotational spectroscopy
broader: spectroscopy
type: Concept

Polarimetry [Exoplanets]
type: Concept
deprecated: :__
use instead: Polarimetry

Interferometric correlation
broader: interferometry
type: Concept

Pareto distribution
broader: astrostatistics-distributions
type: Concept

Wolf diagram
type: Concept
broader: astrometry

Extrasolar radiation
type: Concept
broader: astronomical-radiation-sources

Solar coronal plumes
type: Concept
definition: A coronal feature of the Sun, which appears as long, thin streamers that project outward from the Sun's north and south poles.
related: Stellar coronal plumes
broader: solar-corona

Critical curve
broader: gravitational-lensing
type: Concept

Molecule Destruction
deprecated: :__
type: Concept

North celestial pole
type: Concept
broader: celestial-poles
definition: The point in the northern hemisphere where the rotation axis of Earth touches the celestial sphere. The star Polaris, also called the Pole Star, is located very near this point, at an angular separation of 42 arcminutes (about 1.4 lunar diameters).

Upper atmosphere
type: Concept
broader: planetary-atmospheres
definition: The general term applied to the atmosphere above the troposphere.

Stellar evolutionary models
related: Stellar evolution
type: Concept
broader: astronomical-research

Initial mass function
type: Concept
broader: stellar-evolution

Comet ion tails
broader: comet-tails
type: Concept

Celestial poles
definition: The point of the sky, north or south, where the projection of the Earth's axis of rotation intersects the celestial sphere. They are at 90° relative to the celestial equator. Because of precession, the celestial poles describe a circle around the ecliptic's poles every 25,800 years.
broader: equatorial-coordinate-system
type: Concept

Radar observations
type: Concept
broader: radar-astronomy

Open universe
type: Concept
definition: A Freidmann-Lemaitre cosmological model in which space is infinite and of negative curvature or Euclidean, and which expands forever.
broader: cosmological-models

Cosmic abundances
type: Concept
broader: chemical-abundances
related: Cosmochemistry

Light pollution
broader: night-sky-brightness
type: Concept

Secular parallax
type: Concept
definition: The angle subtended at a star by a baseline that is the distance the Sun moves in a given interval of time with respect to the local standard of rest (4.09 AU per year).
broader: parallax

Stellar coronae
related: Solar corona
type: Concept
broader: stellar-atmospheres

Wide binary stars
type: Concept
broader: binary-stars
definition: A binary system with semi-major axis as large as 10,000 astronomical units.

Gravitational singularities
type: Concept
related: Black holes
broader: general-relativity

Planetesimals
type: Concept
broader: protoplanetary-disks
broader: debris-disks

Interstellar dust
related: Interstellar dust processes
broader: interstellar-clouds
type: Concept
definition: An aggregation of dust grains in the space between stars. Interstellar dust absorbs, scatters, and polarizes the light from distant stars, causing the interstellar extinction. Large dark regions in the plane of the Milky Way and other galaxies are caused by intervening clouds of dust. The conclusive proof for the presence of a general and selective interstellar absorption is due to R. J. Trumpler (1930).
related: Interstellar dust extinction
related: Solid matter physics

Interstellar dynamics
type: Concept
broader: interstellar-medium

Debris disks
broader: circumstellar-disks
type: Concept

Stellar jets
broader: stellar-interstellar-interactions
related: Stellar mass loss
type: Concept
broader: jets

Astronomical location
broader: astrometry
type: Concept

Dense interstellar clouds
type: Concept
broader: interstellar-clouds

Galaxy classification systems
related: Galaxies
related: Elliptical galaxies
broader: classification-systems
related: Galaxy physics
type: Concept

Stellar kinematics
broader: galaxy-physics
related: Stellar associations
type: Concept

Cosmogony
type: Concept
broader: cosmology
definition: A philosophical, religious, or mythical story of the creation or origin of the Universe, usually referring to the solar system.
related: Solar system

LIGO
type: Concept
broader: laser-interferometry
definition: A facility dedicated to the detection and measurement of cosmic gravitational waves. It consists of two widely separated installations, or detectors, within the United States, operated in unison as a single observatory. One installation is located in Hanford (Washington) and the other in Livingston (Louisiana), 3,000 km apart. Funded by the National Science Foundation (NSF), LIGO was designed and constructed by a team of scientists from the California Institute of Technology, the Massachusetts Institute of Technology, and by industrial contractors. Construction of the facilities was completed in 1999. Initial operation of the detectors began in 2001. Each LIGO detector beams laser light down arms 4 km long, which are arranged in the shape of an 'L.' If a gravitational wave passes through the detector system, the distance traveled by the laser beam changes by a minuscule amount -- less than one-thousandth of the size of an atomic nucleus (10^-18 m). Still, LIGO should be able to pick this difference up. LIGO directly detected gravitational waves for the first time from a binary black hole merger (GW150914) on September 14, 2015. The Nobel Prize in physics 2017 was awarded to three physicists (Rainer Weiss, Barry C. Barish, and Kip S. Thorne) for decisive contributions to the LIGO detector and the observation of gravitational waves. LIGO had a prominent role in the detection of GW170817, the first event with an electromagnetic counterpart.

Pulsar timing method
type: Concept
broader: timing-variation-methods

CEMP stars
broader: chemically-peculiar-stars
definition: A star that presents very low iron abundances [Fe/H] < -4 but an anomalous richness in carbon. CEMP stars have been defined as a subset of metal-poor stars that exhibit elevated [C/Fe] ≥ +1.0. It has been recognized that ~15-20% of stars with [Fe/H] < -2.0 are carbon enhanced. This fraction rises to 30% for [Fe/H] < -3.0, to 40% for [Fe/H] < -3.5, and ~75% for [Fe/H] < -4.0. This increasing trend of CEMP-star frequency with declining [Fe/H] is confirmed by the observation of many thousands of CEMP stars.
type: Concept

Theoretical techniques
type: Concept
broader: laboratory-astrophysics

Stellar x-ray flares
broader: stellar-flares
type: Concept

Circumstellar masers
broader: circumstellar-matter
type: Concept
definition: Maser emission from molecules in the circumstellar envelopes of red giants, and also from regions around protostars.

Flash spectra
type: Concept
broader: solar-eclipses
definition: The spectrum of the solar chromosphere obtained during a solar eclipse in the instant before or after totality. In the flash spectrum the usual solar absorption lines are replaced with bright emission lines. This is because in that very short interval only the photosphere is eclipsed by the Moon, and not the chromosphere. The American astronomer Charles A. Young was the first to observe it during the solar eclipse of 1870 (December 22) in Spain.

Solar wind
definition: A mass outflow, consisting of protons, electrons, and other subatomic particles, expelled constantly from the solar corona at about 500 km per second. The solar mass-loss rate in this phenomenon amounts to about 2 x 10^-14 solar masses per year, or about 106 tons per second.
related: Interplanetary physics
broader: solar-physics
related: Stellar winds
type: Concept

Meteorite composition
type: Concept
broader: meteorites

Natural satellite dynamics
related: Natural satellites (Solar system)
type: Concept
broader: planetary-science

Relativistic jets
type: Concept
broader: radio-jets

Starspots
type: Concept
definition: A phenomenon similar to a sunspot but occurring on the surface of a star other than Sun. Due to spatial resolution constraints, starspots so far observed are in general much larger than those on the Sun, up to about 30% of the stellar surface may be covered, corresponding to sizes 100 times greater than those on the Sun.
broader: stellar-physics
related: Sunspots

Mean matter density
broader: large-scale-structure-of-the-universe
type: Concept

Stellar luminosities
type: Concept
broader: stellar-properties

Statistical parallax
type: Concept
definition: The mean parallax of a group of stars that are all at approximately the same distance, as determined from their radial velocities and proper motions.
broader: parallax

Irregular natural satellites (Extrasolar)
definition: A satellite whose orbit around its exoplanet is eccentric, inclined with respect to the equatorial plane, and relatively far from the planet. Strong stellar perturbations cause the orbit to precess.
related: Irregular satellites
broader: natural-satellites-extrasolar
type: Concept

Solar properties
deprecated: :__
type: Concept

Solar motion
type: Concept
broader: solar-physics
related: Stellar motion

Intrinsic variable stars
broader: variable-stars
type: Concept
definition: A variable star whose fluctuations in brightness are due to natural changes in the luminosity of the star itself, not by external causes, such as in extrinsic variable stars.

Extrasolar gaseous planets
type: Concept
broader: exoplanets

Spacetime metric
type: Concept
broader: general-relativity

D layer
type: Concept
broader: ionosphere

Circumstellar matter
definition: Dust, gas and plasma around stars, generally present in the form of stellar winds or nebulae ejected by the stars.
broader: stellar-astronomy
type: Concept

Search for extraterrestrial intelligence
broader: astrobiology
type: Concept
definition: The scientific attempt to detect intelligent extraterrestrial life by surveying the sky to find the existence of transmissions, especially radio waves or light, from a civilization on a distant planet. The SETI Institute, that carries out the project, is a private non-profit center founded in 1984. There are many methods that SETI scientific teams use to search for extraterrestrial intelligence. Many of these search billions of radio frequencies that reach Earth from all over the Universe, looking for an intelligent radio signal. Other SETI teams search by looking for signals in pulses of light emanating from the stars.

Observatories
broader: astronomical-instrumentation
type: Concept
related: Astronomical site protection

Asteroid rotation
broader: asteroid-dynamics
type: Concept

Gravitational deflection
type: Concept
broader: geodesics

Doppler shift
type: Concept
definition: Effect of the relative motion of a wave source (light, sound) and the observer. If the source is moving away, the wavelength is stretched (shifted toward lower frequencies). If the source is approaching, the wavelength is compressed (shifted toward higher frequencies). These effects, known as Doppler shifts, are in the case of light waves called redshift and blueshift, respectively.
broader: astronomical-techniques

Ultraluminous infrared galaxies
definition: A galaxy that emits more than 90% of its energy in the infrared (8-1000 µm) and whose infrared luminosity exceeds 10^12 solar luminosities. Quasars can also have such high or even higher bolometric luminosities. However LIRGs and ULIRGs emit the bulk of their energy in the infrared. Most of ULIRGs are found in merging and interacting galaxy systems. It is thought that their luminosity results from galactic collisions, which increase the rate of star formation.
broader: infrared-galaxies
type: Concept

Double periodic variable stars
broader: multi-periodic-variable-stars
type: Concept
broader: binary-stars

Coronagraphic imaging
broader: astronomical-techniques
broader: high-contrast-techniques
type: Concept

Cloud computing
broader: computational-methods
type: Concept

OB supergiant stars
broader: ob-stars
type: Concept
broader: early-type-supergiant-stars

Helium burning
type: Concept
definition: The stage in the evolution of a star, after the exhaustion of hydrogen, when the star produces its energy by the fusion of helium into carbon and oxygen.
broader: stellar-nucleosynthesis
broader: nucleosynthesis

Coma dust
type: Concept
broader: comae

Io
definition: The fifth of Jupiter's known moons and the third largest. It is the innermost of the Galilean satellites. With a diameter of 3630 km, Io is slightly larger than Earth's Moon. It revolves at a mean distance of 422,000 km from Jupiter. Its mass is 8.93 x 10^22 kg (about 1.2 Earth Moons) and its orbital period 1.8 Earth days. The mean surface temperature of Io is -155 °C. Io's yellow color derive from sulfur and molten silicate rock. The unusual surface of Io is kept very young by its system of active volcanoes. The intense tidal force of Jupiter stretches Io and damps wobbles caused by Jupiter's other Galilean moons. The resulting friction greatly heats Io's interior, causing molten rock to explode through the surface. Io's volcanoes are so active that they are effectively turning the whole moon inside out. Some of Io's volcanic lava is so hot it glows in the dark.
type: Concept
broader: galilean-satellites

Interferometry
type: Concept
definition: An observational technique that uses the interference phenomenon to substantially increase the resolving power of telescopes.
broader: astronomical-techniques
related: Interferometers

Solar granules
related: Solar granulation
broader: solar-photosphere
related: Stellar granules
definition: One of the convective cells constituting the solar granulation.
type: Concept

Lithium abundance
type: Concept
deprecated: :__

Lyman limit systems
broader: intergalactic-clouds
type: Concept

Line intensities
type: Concept
broader: atomic-spectroscopy
broader: molecular-spectroscopy

Ejecta
broader: stellar-remnants
type: Concept
related: Supernovae
related: Solar coronal mass ejections
related: Stellar coronal mass ejections
definition: Material, in solid, liquid, or gaseous form, thrown out by a stellar body, especially as a result of supernova explosion.

Be stars
broader: emission-line-stars
broader: b-stars
broader: early-type-emission-stars
type: Concept
definition: A hot star of spectral type B showing Balmer lines in emission. Be stars are fast rotators (spinning at about 200 km/sec) and have strong stellar winds with important mass loss.

Spin-orbit resonances
type: Concept
broader: orbits

Radio jets
broader: active-galactic-nuclei
broader: galaxies
type: Concept
broader: jets
definition: An astrophysical jet appearing in the radio wavelengths of the electromagnetic spectrum.

Fork mounts
type: Concept
deprecated: :__

Horizontal parallax
definition: The angle under which the radius of the Earth at the place of observation would be seen from a celestial body when it is in the horizon (at the instant of rising or setting). The amount varies with the latitude since the Earth is not exactly spherical, and is greatest at equator.
broader: diurnal-parallax
type: Concept

Amor group
definition: A near-Earth asteroid (NEA) with perihelion distances between 1.017 and 1.3 astronomical units. The Amor asteroids approach the orbit of the Earth from beyond, but do not intersect it. Most Amors do cross the orbit of Mars. It is estimated that 32% of the total number of NEAs are Amors. One of the larger Amors is Eros.
type: Concept
broader: near-earth-objects

Astronomical site protection
related: Observatories
type: Concept
broader: astronomical-research

High latitude field
type: Concept
broader: field-of-view

Vertical circle
type: Concept
broader: horizontal-coordinate-system
definition: The greater circle on the celestial sphere which passes through zenith, nadir, and the star and whose plane is perpendicular to the plane of horizon.

Red giant bump
broader: red-giant-branch
type: Concept

Comparison stars
definition: A non-variable star used to monitor the variations of another nearby star.
type: Concept
broader: standard-stars

Earth's clouds
definition: A visible mass of water droplets and/or ice particles in the atmosphere above the Earth's surface.
type: Concept
broader: earth-atmosphere
broader: atmospheric-clouds

Very small grains
type: Concept
broader: interstellar-dust

Optical bursts
related: Novae
broader: burst-astrophysics
related: Binary stars
type: Concept

Ecliptic
broader: ecliptic-coordinate-system
type: Concept

Geostationary satellites
broader: geosynchronous-satellites
definition: A satellite that orbits in the plane of the Earth's equator and 35,880 km above it, at which distance the satellite's period of rotation matches the Earth's and the satellite always remains fixed in the same spot over the Earth.
type: Concept

T dwarfs
type: Concept
broader: dwarf-stars
definition: A type of brown dwarf with an effective temperature between about 1200 K and 500 K, i.e. colder than the preceding type L dwarf. The spectra of T dwarfs are characterized by the presence of methane bands in the near infrared. The presence of these bands, broad H_{2}O features, and H_2 collision-induced absorption radically alter the spectral energy distributions of T dwarfs compared to a black body at the same temperature. Hence near-infrared colors become increasingly blue (J - K ~ 0) as compared to L dwarfs. The first T dwarf, called Gl 229B, was discovered by Nakajima et al. The spectral classification scheme (subtypes T0 to T9) currently used was defined by Burgasser et al.
broader: brown-dwarfs
broader: late-type-dwarf-stars

Early-type variable stars
broader: early-type-stars
type: Concept

Chemically peculiar giant stars
type: Concept
broader: giant-stars
broader: chemically-peculiar-stars

Interstellar reddening
type: Concept
definition: The dimming of light during its travel in the interstellar medium due to absorption by intervening dust grains. Since shorter wavelengths are particularly affected, the spectrum of the light is increasingly dominated by the long wavelength end of the spectrum. As a result, the light is 'reddened' as it travels through space. Robert J. Trumpler (1886-1956), a Swiss-American astronomer, was the first to produce a definite evidence of the existence of interstellar extinction and to estimate its magnitude (1930).
broader: interstellar-extinction

Ring galaxies
definition: A galaxy with a ring-like appearance around the central luminous center. The ring consists of massive, relatively young bright stars. It is believed that ring galaxies result from the head-on collision of two different galaxies.
type: Concept
broader: disk-galaxies

Automated telescopes
type: Concept
broader: telescopes

Penrose process
definition: A hypothetical means of extracting energy from a rotating black hole. If a particle spirals into the ergosphere of a black hole in a direction counter to the rotation of the black hole, and if the particle then breaks up into two fragments inside the ergosphere, one of the fragments can escape with energy greater than the energy of the original particle.
broader: black-hole-physics
type: Concept

Bayesian statistics
type: Concept
broader: astrostatistics-strategies

Transmission spectroscopy
broader: spectroscopy
type: Concept

Gravitation
related: Stellar physics
type: Concept
definition: The universal phenomenon of attraction between material bodies.
broader: astrophysical-processes
related: Relativity

Draco dwarf galaxy
type: Concept
definition: A dwarf elliptical galaxy that is a satellite of our Galaxy and lies at a distance of about 250,000 light-years from the Galactic center. Its diameter is only about 3,500 light-years, and its absolute magnitude -8.6, making it the least luminous galaxy known.
broader: dwarf-galaxies

Galaxy tails
type: Concept
broader: galaxy-structure

Collimation
type: Concept
definition: To pack and align photons or atomic particles parallel to a particular direction.
broader: astronomical-optics

Eclipsing binary minima timing method
type: Concept
broader: timing-variation-methods

Exoplanet migration
related: Planetary migration
broader: exoplanet-astronomy
type: Concept

Galaxy anticenters
related: Galactic anticenter
broader: galaxy-kinematics
type: Concept

Galactic cosmic rays
broader: cosmic-rays
type: Concept

Globular star clusters
definition: A spherical aggregate of stars made up of thousands to a few million stars which is an orbiting satellite of a galaxy. There are over 150 globular clusters orbiting our galaxy. Globular clusters are gravitationally bound systems, highly concentrated to the center (up to a few 10^3 stars per cubic light-years), with a volume ranging from a few dozen up to more than 300 light-years in diameter. They are generally old and metal-poor and are among the first objects to be formed in a galaxy. There is also strong evidence that they form in major galaxy interactions and mergers. The stars in a globular cluster are thought to have a common origin and thus a single age and chemical abundance; with some exceptions such as Omega Centauri and NGC 2808, which exhibit multiple populations. The presence of various sub-populations within a globular cluster is interpreted as indicating distinct epochs of mass accretion and/or major star formation. The Milky Way hosts about 200 globular clusters. They are spherically distributed about the Galactic Center up to a radius of 350 light-years, with a maximum concentration toward the Galactic center. All but the smallest dwarf galaxies possess globular clusters. Some galaxies, e.g. M87, contain several thousands of them. There are, however, important differences. While all the globular clusters in our Galaxy and in M31 are old (ages of about 10 billion years, at least), there are galaxies, such as the two Magellanic Clouds and M33, that host much younger globular clusters (ages of a few billion years, or less).
type: Concept
broader: star-clusters

Nonstandard evolution
type: Concept
broader: stellar-evolution

Zodiac
broader: ecliptic-coordinate-system
type: Concept

Diffuse molecular clouds
definition: A type of molecular cloud in which the interstellar radiation field is sufficiently attenuated, so that the local fraction of molecular hydrogen becomes substantial (> 0.1). However, enough interstellar radiation is still present to photoionize any atomic carbon, or to photodissociate carbon monoxide (CO) such that carbon is predominantly still in the form of C+ (> 0.5). In steady state, diffuse molecular clouds must necessarily be surrounded by diffuse atomic gas, in order to provide the shielding of radiation. This means that most sightlines that cross a diffuse molecular cloud will also cross diffuse atomic gas.
type: Concept
broader: molecular-clouds

WCN stars
type: Concept
broader: wolf-rayet-stars

Yerkes classification
type: Concept
deprecated: :__

Ground-based astronomy
definition: Astronomical study carried out using a telescope on Earth, as opposed to that from an orbiting satellite.
broader: astronomical-methods
type: Concept

Optical flares
broader: stellar-flares
type: Concept

Anomalous Cepheid variable stars
type: Concept
broader: cepheid-variable-stars

Alternatives to cosmic inflation
broader: cosmic-inflation
type: Concept

Spherical aberration
broader: optical-aberration
type: Concept

Meteor showers
broader: meteors
type: Concept
definition: An increased number of meteors all appearing to diverge from the direction of a single point, called radiant. Meteor showers occur annually on the same dates, when the Earth crosses through a meteoroid stream. Meteor showers are named after the constellation in which the radiant is located. For example, the Perseids's radiant lies near the top of the constellation Perseus. Most meteor showers are caused by comets. As a comet orbits the Sun it sheds an icy, dusty debris stream along its orbit. When the Earth's orbit intersects the dust trail, more meteors are seen as the cometary debris encounters our planet's atmosphere. In the case of the Geminids and Quadrantids, those meteor showers come from the debris scattered by orbiting asteroids. Typical meteor showers show 15 to 100 meteors per hour at their peak. On very rare occasions, during a meteor storm, thousands of meteors fall per hour. Prominent meteor showers are: Quadrantids, Lyrids, Eta Aquariids, Delta Aquariids, Perseids, Orionids, Taurids, Leonids, Geminids, Ursids, Alpha Capricornids.

Millimeter-wave spectroscopy
type: Concept
broader: rotational-spectroscopy

Meteoroids
definition: A solid object in interplanetary space before it reaches the Earth's atmosphere. Meteoroids are of silicate and/or metallic matter having a size from tiniest grains up to that of the smallest asteroids.
type: Concept
broader: solar-system

Rotating shells
broader: circumstellar-shells
type: Concept

Physical properties (Ice)
type: Concept
broader: ice-physics

Poor galaxy clusters
type: Concept
broader: galaxy-clusters

Plutoids
broader: trans-neptunian-objects
type: Concept

Grazing incidence telescopes
broader: telescopes
type: Concept

Neutrino oscillations
broader: particle-astrophysics
type: Concept
broader: nuclear-astrophysics

Radio bursts
definition: A burst of emission in the radio frequencies of the electromagnetic spectrum.
broader: radio-sources
related: Radio transient sources
broader: burst-astrophysics
type: Concept

Ultraviolet photometry
type: Concept
broader: waveband-photometry

Initial conditions of the universe
type: Concept
broader: early-universe

Photographic magnitude
type: Concept
definition: The apparent magnitude of a star as determined by measuring its brightness on a photographic plate. The photographic magnitude scale is now considered obsolete.
broader: apparent-magnitude

Lunar probes
related: Lunar science
definition: A probe for exploring and reporting on conditions on or about the Moon.
type: Concept
broader: space-probes

Interstellar molecules
definition: Any molecule that occurs naturally in clouds of gas and dust in the interstellar medium. So far more than 140 species have been discovered, many of which nonexistent on Earth.
type: Concept
broader: interstellar-medium

Molecular beams
broader: experimental-techniques
type: Concept

Solar filaments
broader: solar-activity
type: Concept
related: Solar prominences

Astrometric binary stars
type: Concept
definition: A binary star in which the presence of an unresolved companion is revealed by small oscillations in the movement of the visible component caused by the gravitational influence of the unseen component.
broader: binary-stars

Z Camelopardalis stars
broader: dwarf-novae
type: Concept

High-redshift galaxies
type: Concept
broader: galaxies

Magnetic storms
broader: planetary-magnetosphere
type: Concept

Dark adaption
type: Concept
broader: astronomical-optics
definition: The automatic adjustment of the iris and retina of the eye to allow maximum vision in the dark, following exposure of the eye to a relatively brighter illumination.

Minor axis
type: Concept
definition: The axis of an ellipse that is perpendicular to the major axis at a point equidistant from the foci.
broader: elliptical-orbits

Heterodyne interferometry
type: Concept
broader: interferometry

Molecular spectroscopic constants
broader: molecular-data
type: Concept

Pulsation modes
type: Concept
definition: The way in which pulsations occur in a star due to the fact that stars act as resonant cavities, as studied in asteroseismology. A star may pulsate either with approximately spherical symmetry (radial pulsation), or as a series of waves running across the surface (non-radial pulsation). Pulsation may occur in a single mode or in multiple modes, depending on the type of star. Three different modes of pulsations have been detected through the helioseismology of the Sun: p mode, g mode, and f mode, generated by acoustic, gravity, and surface gravity waves respectively.
broader: stellar-pulsations

Earth (planet)
broader: inner-planets
type: Concept
related: The Moon
definition: The third planet from the Sun. At perihelion, it is 147,099,590 km from the Sun, and at aphelion it is 152,096.150 km, whereas its mean distance from the Sun (astronomical unit) is 149,598 x 10^6 km. Its orbital period is 365.2563 days (sidereal year) and its eccentricity 0.017. Other characteristics: axial inclination 23.44°; rotation period 23.934 h (sidereal day); mean density 5.52 g/cm^3; mass 5.974 x 10^24 kg; escape velocity 11.18 km/s; average albedo 0.37. The Earth's atmosphere consists of 78% nitrogen, 21% oxygen and 0.9% argon, plus carbon dioxide, hydrogen, and other gases in much smaller quantities. The atmospheric pressure at sea level is about 1,000 mbar. The surface average temperature is 15° C, but it varies, on the average, between -50° C (winter, Siberia) and up to + 40° C (summer, Sahara). Liquid water covers 71% of the surface. Over 5,000 active volcanoes have been registered throughout man's known history. The outer layer of the planet, the lithosphere, is covered with the crust. In the upper mantle and beneath the lithosphere, lies asthenosphere. Convection in the asthenosphere has caused plate tectonics motions and continent drifts. The densest layer of the Earth is its core, about 3000 km to 6400 km beneath the surface, consisting primarily of iron and nickel. This core is believed to be at the origin of the magnetic field, which reaches about 3 x 10^-5 tesla near the equator. It has only one natural satellite, the Moon.

Pulsating variable stars
definition: A type of variable star that changes its brightness by changing its volume through expansion and contraction. Classical pulsating stars, including Cepheids, RR Lyrae, and Delta Scuti variables, are located in a quite narrow almost vertical region in the H-R diagram, known as instability strip.
broader: intrinsic-variable-stars
type: Concept

Kolmogorov-Smirnov test
broader: nonparametric-hypothesis-tests
type: Concept

DQ stars
broader: white-dwarf-stars
type: Concept
definition: A white dwarf whose spectrum shows carbon features of any kind.

Principal component analysis
type: Concept
broader: multivariate-analysis

Photometry
type: Concept
broader: astronomical-techniques
definition: In astronomy, the measurement of the light of astronomical objects, generally in the visible or infrared bands, in which a wavelength band is normally specified.

Spectroscopy
type: Concept
broader: astronomical-techniques
definition: The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the physical and chemical properties of astronomical objects.
broader: experimental-techniques

Periastron
broader: apsides
type: Concept
definition: In a binary system, the point in the orbit where a component is nearest the other component; opposite of apastron.

OB stars
type: Concept
broader: stellar-spectral-types
related: Stellar classification
definition: A collective designation for massive O and B stars.
broader: early-type-stars

Gamma-ray lines
broader: gamma-rays
type: Concept

Photodissociation reactions
related: Photodissociation regions
broader: molecular-reactions
type: Concept

Photoionization
definition: The physical process in which an incident high-energy photon ejects one or more electrons from an atom, ion, or molecule.
type: Concept
broader: ionization

Lomb-Scargle periodogram
type: Concept
broader: period-search
definition: An algorithm for detecting and characterizing periodic signals in unevenly-sampled data. The Lomb-Scargle periodogram has a particularly wide use within the astronomy community. This method allows efficient computation of a Fourier-like power spectrum estimator from such unevenly-sampled data, resulting in an intuitive means of determining the period of oscillation.

Quiet solar chromosphere
type: Concept
broader: solar-chromosphere

Annual parallax
broader: parallax
type: Concept
definition: The difference in position of a star as seen from the Earth and Sun, i.e. the angle subtended at a star by the mean radius of the Earth's orbit around the Sun. Same as heliocentric parallax. Because the Earth revolves around the Sun, we observe the sky from a constantly moving position in space. Therefore, we should expect to see an annual effect, in which the positions of nearby objects appear to oscillate back and forth in response to our motion around the Sun. This does in fact happen, but the distances to even the nearest stars are so great that we need to make careful observations with a telescope to detect it. The annual parallax of the nearest star, Proxima Centauri, is 0.762 arcsec, which is too small for our acuity of vision.

Planetary cores
broader: planetary-structure
type: Concept

Galaxy spheroids
type: Concept
broader: galaxy-structure

Tidal friction
type: Concept
broader: tidal-interaction
definition: The friction exerted on a primary body (Earth) because of the phase lag between the tides and the gravitational attraction of the secondary body (Moon). The Earth's rotation is faster than the Moon's orbital motion; therefore the Earth's tidal bulges lead the Moon on its orbit. This has two important effects: The Earth is being pulled slightly 'back' from its sense of rotation. So the Earth's rotation slows (by about 1 second every 50,000 years). Moreover, the Moon is being pulled slightly 'forward' on its orbit. So it is harder for the Earth to hold it in place, and it moves further away from the Earth (by about 3-4 cm per yr). Tidal friction tends to synchronize the rotation period of a close-in companion with the period of its orbital motion around the primary.

Dwarf galaxies
definition: A small, low luminosity galaxy that is associated with a larger spiral galaxy and may make up part of a galactic halo. There are many of them in the Local Group, and often orbit around larger galaxies such as the Milky Way and the Andromeda Galaxy. There are three main types of them: dwarf elliptical galaxy (dE), dwarf irregular galaxy (dI), and dwarf spiral galaxy (dSA).
broader: galaxies
type: Concept

Starburst galaxies
type: Concept
definition: A galaxy showing a short-lived intense period of star formation that is unsustainable over the Hubble time due to the limited supply of gas within a galaxy. Starburst galaxies were first classified by Searle & Sargent (1972) and Searle et al. (1973), based on the blue colors produced by the massive stars formed during the burst. In the local Universe, starbursts create approximately 10% of the radiant energy and 20% of the massive stars. At z = 1, starburst characteristics are found in 15% of galaxies, presumably attributable to the greater amounts of gas typically present in young galaxies and increased galactic interactions. The starburst's impact on a galaxy and the surrounding intergalactic medium is primarily due to the consumption of gas that fuels the burst and the feedback from massive stars formed in the burst.
broader: active-galaxies
related: Post-starburst galaxies
broader: galaxies

Solar transition region
broader: solar-corona
broader: solar-chromosphere
type: Concept

UX Ursae Majoris stars
type: Concept
broader: nova-like-variable-stars

Alpha Cygni variable stars
related: A supergiant stars
related: Luminous blue variable stars
type: Concept
broader: pulsating-variable-stars
related: B supergiant stars

Craters
type: Concept
related: Lunar craters
broader: impact-phenomena

Giant stars
related: Giant branch
broader: evolved-stars
definition: A high-luminosity star that has evolved off the main sequence and lies above the main sequence on the Hertzsprung-Russell diagram. A member of the giant branch.
type: Concept

SRb variable stars
type: Concept
broader: semi-regular-variable-stars

Hall thrusters
type: Concept
broader: space-vehicles

Giant branch
type: Concept
definition: A conspicuous family of stars in the Hertzsprung-Russell diagram composed of red, evolved stars with large sizes.
broader: hertzsprung-russell-diagram
related: Giant stars

Circumpolar
type: Concept
broader: equatorial-coordinate-system

Mutual neutralization
broader: collision-processes
type: Concept

Contact binary stars
definition: A binary star system in which the two components are so close that they exchange gases in a complex manner. Their overlapping gravitational fields form a 'peanut' shaped equipotential surface.
broader: interacting-binary-stars
type: Concept

Osculatory elements
type: Concept
broader: orbital-elements

Sky noise
type: Concept
broader: astrometry

Dwarf irregular galaxies
definition: An irregular galaxy that is much smaller than other irregulars. Dwarf irregulars are generally metal poor and have relatively high fractions of gas. They are thought to be similar to the earliest galaxies that populated the Universe, and are therefore important to understand the overall evolution of galaxies.
type: Concept
broader: dwarf-galaxies
broader: irregular-galaxies

Interacting galaxies
definition: Galaxies that are close enough for their mutual gravitational attraction to produce perturbed shapes or extruded filaments of stellar material, called tidal tails. Most galaxies are in clusters, and gravitational interactions between them are common.
broader: galaxies
type: Concept

Black hole spin-flip
broader: black-hole-physics
type: Concept

Hydrogen deficient stars
type: Concept
broader: chemically-peculiar-stars

Geodesics
definition: The shortest distance between two points in space (or space-time). A geodesic on a sphere is an arc of a great circle. In the theory of general relativity, freely falling bodies follow geodesic paths in space-time.
type: Concept
broader: general-relativity

Intercloud medium
type: Concept
broader: interstellar-medium
definition: A medium in which several molecular clouds are situated.

Distance measure
type: Concept
broader: astrometry

Lenses
broader: refracting-telescopes
type: Concept

Extragalactic radio sources
broader: radio-sources
type: Concept

Ellerman Bombs
broader: solar-chromosphere
type: Concept

Zirconium stars
broader: s-stars
broader: chemically-peculiar-stars
type: Concept

Constellations
type: Concept
broader: celestial-sphere
definition: A grouping of conspicuous stars that, when seen from Earth, form an apparent pattern. The sky is divided into 88 constellations.

Lyman-alpha forest clouds
type: Concept
broader: intergalactic-clouds

Hungaria group
type: Concept
broader: asteroids

Galactic latitude
broader: galactic-coordinate-system
type: Concept
definition: In the Galactic coordinate system, the angle between the line of sight to an object and the Galactic equator. Galactic latitude ranges from +90 degrees to -90 degrees.

Stellar E corona
type: Concept
broader: stellar-coronae
related: Solar E corona

Trans-Neptunian objects
broader: kuiper-belt
type: Concept
broader: small-solar-system-bodies

Atmospheric structure
broader: planetary-atmospheres
type: Concept
related: Exoplanet atmospheric structure

Stellar colors
type: Concept
broader: stellar-properties

Astronomical sextants
broader: early-astronomical-instruments
definition: An instrument used mainly in nautical astronomy to determine the angular distances of celestial bodies above the horizon. The sextant consists of a 60° graduated arc, or limb, a small telescope, and two mirrors. Only half of one of the mirrors, the horizon glass, is silvered and, like the telescope, it is fastened to the frame supporting the limb. The other mirror, the index mirror, moves with an index arm pivoted at the center of the arc. The index arm is equipped with a vernier which moves along the limb. An object in direction S can be observed in the telescope through the un-silvered portion of the horizon glass. By moving the index arm, the second object S' is made to coincide with S in the telescope. According to the law of reflection, the angle between S and S' is double the angle between the mirrors. The angular distance between the objects can therefore be obtained from the measurement of angles between the planes of the mirrors.
type: Concept

Exoplanet atmospheric variability
type: Concept
broader: exoplanet-atmospheres
related: Atmospheric variability

Radio cores
broader: galaxy-structure
broader: active-galactic-nuclei
type: Concept
related: Radio sources

Astronomy data reduction
definition: The process of converting crude observational data into usable information for scientific interpretation, by correcting, rearranging, ordering, and simplifying.
broader: astronomy-data-analysis
type: Concept

Exoplanet atmospheric dynamics
related: Atmospheric dynamics
type: Concept
broader: exoplanet-atmospheres

Solar neutrinos
related: Solar particle emission
type: Concept
definition: A neutrino generated in the Sun. The main source of solar neutrinos is the proton-proton chain of reactions, in which an energy of +28 MeV is shared between the reaction products. These are called low-energy neutrinos. There are less important reactions in the Sun yielding a smaller flux of higher energy neutrinos. The solar neutrino flux can be estimated from the solar luminosity.
related: Solar physics
broader: nuclear-astrophysics
broader: particle-astrophysics

North ecliptic pole
type: Concept
broader: ecliptic-poles

Tidal disruption
definition: The disruption of an extended astronomical object under the action of the tidal forces exerted by another nearby object.
broader: gravitational-disruption
broader: tidal-interaction
type: Concept

Weak G band stars
type: Concept
broader: giant-stars

Photon decoupling
type: Concept
broader: big-bang-theory
broader: early-universe

Natural satellite formation
broader: planetary-science
type: Concept
related: Natural satellites (Solar system)

Sunspot groups
type: Concept
broader: sunspots

Interstellar abundances
type: Concept
broader: chemical-abundances

Dwarf spheroidal galaxies
broader: dwarf-galaxies
type: Concept
definition: A subtype of dwarf ellipticals (dwarf elliptical galaxy), which are companion to the Milky Way and other similar galaxies. The first example of such objects was discovered by Harlow Shapley (1938) in the constellation Sculptor. 22 such galaxies are known currently to orbit the Milky Way and at least 36 exist in the Local Group of galaxies. Nearby galaxy clusters such as the Virgo, Fornax, Centaurus, and Coma clusters contain hundreds to thousands of individual dSph galaxies. These galaxies have very low surface brightnesses, as low as only 1% that of the sky background. They are also among the smallest, least luminous galaxies known. Most of the radiation from dSph galaxies is emitted by stars in the optical portion of the electromagnetic spectrum. The lack of strong emission lines, infrared, or radio emission suggests that these galaxies are generally devoid of interstellar medium. The velocities of stars within dSph galaxies are so high that them must be disrupting. However, the bulk of mass in these galaxies might be undetected. Dynamical models that include dark matter do adequately explain the velocity dispersion of the stars in all dSph systems. In the most extreme cases, only 1% of the mass of the galaxy is visible. Many of the Local Group dSph galaxies show evidence for star formation more recent than 10 Gyr.

Telescope mounts
type: Concept
broader: telescopes

Astrophysical fluid dynamics
broader: interdisciplinary-astronomy
type: Concept

Voronoi tessellation
broader: spatial-point-processes
type: Concept

Galaxy voids
type: Concept
use instead: Voids
deprecated: :__

Cold Neptunes
broader: exoplanets
type: Concept

Saros cycle
broader: lunar-eclipses
type: Concept
definition: The period of 223 synodic month, equaling 6585.32 days or 18 years, 11.33 days, after which the Sun, Earth, and Moon return to approximately the same relative geometry. When two eclipses are separated by a period of one Saros, they occur at the same node with the Moon at nearly the same distance from Earth and at the same time of year. Thus, the Saros is a useful tool for organizing eclipses into families or series. Each series typically lasts 12 or 13 centuries and contains 70 or more eclipses.

Meteoroid dust clouds
type: Concept
broader: interplanetary-dust

Type II supernovae
related: Chandrasekhar limit
broader: core-collapse-supernovae
definition: A supernova type whose spectrum contains hydrogen lines. Compared with Type I supernovae, its light curve has a broader peak at maximum and dies away more rapidly. The magnitudes are smaller, ranging from M_V = -12 to -13.5, and the ejecta have lower velocities (about 5,000 km/sec). These supernovae, which result from the final evolution of massive stars, have three main divisions: Type II-P, Type II-L, and Type II-n.
type: Concept

Nuclear astrophysics
type: Concept
related: Nuclear physics
broader: astrophysical-processes

Publicly available software
broader: software-licensing
type: Concept

Solar surface
broader: solar-physics
related: Stellar surfaces
type: Concept

Computational methods
broader: computational-astronomy
type: Concept

Deceleration parameter
definition: A parameter designating the rate at which the expansion of the Universe would slow down owing to the braking gravitational effect of the matter content of the Universe.
broader: cosmological-parameters
type: Concept

Shocks
broader: plasma-astrophysics
definition: A sharp change in the properties of a gas (density, pressure, temperature).
type: Concept

Reaction models
broader: theoretical-techniques
type: Concept

Silicon monoxide masers
type: Concept
broader: astrophysical-masers

Stellar abundances
type: Concept
broader: chemical-abundances
broader: stellar-physics
related: Solar abundances

Natural satellite atmospheres
type: Concept
broader: planetary-science
related: Natural satellites (Solar system)

Synodic period
definition: For planets, the mean interval of time between successive conjunctions of two planets, as observed from the Earth. For satellites, the mean interval between successive conjunctions of a satellite with the Sun, as observed from the satellite's primary.
broader: orbital-elements
type: Concept

Nautical almanacs
broader: almanacs
type: Concept

X-ray bright point
type: Concept
broader: x-ray-astronomy

Catalogs
definition: A list or record of items systematically arranged with descriptive details.
broader: astronomical-reference-materials
type: Concept

Neutral hydrogen clouds
type: Concept
broader: interstellar-clouds

Multiple mirror telescopes
broader: optical-telescopes
type: Concept

Mini Neptunes
broader: exoplanets
type: Concept

T subdwarfs
type: Concept
broader: brown-dwarfs
broader: subdwarf-stars

Moving clusters
broader: star-clusters
type: Concept
related: Moving cluster parallax
definition: A group of stars dynamically associated so that they have a common motion with respect to the local standard of rest.

O clusters
type: Concept
broader: open-star-clusters

HST photometry
broader: broad-band-photometry
type: Concept

Stellar mass black holes
type: Concept
broader: astrophysical-black-holes

Perigee
definition: The point in the orbit of a body revolving around the Earth at which it is nearest to the Earth; opposite of apogee.
broader: apsides
type: Concept

Delta Cepheid variable stars
definition: The prototype of classical Cepheid variables, which is a pulsating yellow supergiant. John Goodricke was the first in 1784 to discover its variability. The star shows a quick and sharp rise from minimum to maximum, and slowly declines to its minimum again. The changes in brightness are accompanied by and principally caused by changes in stellar temperature and also by changes in radius. δ Cephei was actually the second Cepheid variable to be discovered. The first one, Eta Aquilae, had been discovered earlier the same year by Edward Pigott. δ Cephei varies with a period of 5.366341 days (or 5 days 8 hours 37.5 minutes) from magnitude 3.48, spectral type F5 Ib in its maximum to magnitude 4.37, spectral type G2 Ib in its minimum. It lies at a distance of 1,340 light-years.
broader: cepheid-variable-stars
type: Concept

Classical novae
type: Concept
broader: novae

Distance indicators
type: Concept
broader: stellar-distance

Low surface brightness galaxies
definition: A member of a particularly faint population of galaxies with a central surface brightness below the brightness of the background sky. The central regions of many of them resemble a dwarf galaxy, but most of the mass is contained in a large gaseous disk of low density that is observable only with long-exposure optical images or at radio wavelengths. Some are as massive as a large spiral galaxy, for example Malin 1. The proportion of LSBGs relative to normal galaxies is unknown. They may however represent a significant fraction of mass in the Universe. LSBGs are thought to be primitive systems because they have total masses similar to normal galaxies, but have typically converted less than 10% of their gas into stars. Spiral LSBGs do not obey Freeman's law.
type: Concept
broader: galaxies

Magnetic poles
broader: planetary-magnetosphere
type: Concept

Meteors
type: Concept
related: Meteorites
definition: A streak of light caused when a meteoroid enters Earth's atmosphere and becomes incandescent, mostly from friction with the air at high speed. Meteors are also referred to as shooting stars. Very bright meteors are called fireball or bolide. Most of visible meteors arise from particles ranging in size from about that of a small pebble down to a grain of sand, and generally weigh less than 1-2 grams. The brilliant flash of light from a meteor is mainly caused by the meteoroid's high level of kinetic energy as it collides with the atmosphere at high speeds (11-72 km/s). The increase in the number of meteors visible toward the end of the night results from the fact that the Earth rotates about its axis in the same direction as it orbits the Sun. This means that the leading edge (morning side) of the Earth encounters more meteoroids than the trailing edge (evening side). In general, 2 to 3 times as many meteors can be seen in the hour or so just before morning twilight, than can be seen in the early evening. Moreover, the numbers of random, or sporadic, meteors vary from season to season, due to the tilt of the Earth on its axis and other factors.
broader: meteoroids

Gegenschein
definition: A faint glow of light in the night sky seen exactly opposite the Sun. The gegenschein is sunlight back-scattered off millimeter-sized dust particles along the ecliptic. In comparison with zodiacal light (forward-scattered sunlight), which is triangular in shape and found near the horizon, the gegenschein is roughly circular.
broader: night-sky-brightness
type: Concept

Ecliptic coordinate system
type: Concept
broader: astronomical-coordinate-systems
definition: Coordinate system with the ecliptic as the fundamental plane.

L dwarfs
broader: brown-dwarfs
broader: late-type-dwarf-stars
type: Concept
definition: A type of brown dwarf with an effective temperature ranging from about 2200 K to about 1300 K, corresponding to luminosities about 4 x 10^-4 to 3 x 10^-5 times that of the Sun. L dwarfs are intermediate in temperature between M and T dwarfs. Their spectra in the optical show weak titanium oxide (TiO) and vanadium oxide (VO) absorption lines and strong metallic hydrides CrH (8611 and 9969 Å) and FeH (8692 and 9896 Å). Also are present strong neutral atomic lines of alkali metals Na I (8183, 8195 Å), K I (7665, 7699 Å), Rb I (7800, 7948 Å), Cs I (8521, 8943 Å), and sometimes Li I (6708 Å). The prototype of the L-dwarf class is GD 165B.

Apsides
type: Concept
definition: The point of greatest or least distance of the orbit of a celestial body from a center of attraction. The closest point is the periapsis, the further point the apoapsis.
broader: orbital-elements

F supergiant stars
type: Concept
broader: f-stars
broader: supergiant-stars

Galilean telescopes
broader: refracting-telescopes
type: Concept

53 Persei stars
broader: pulsating-variable-stars
type: Concept
broader: b-stars

Tidal interaction
broader: gravitational-interaction
type: Concept

Phase error
broader: astrometry
type: Concept

Lunar highlands
type: Concept
broader: lunar-features
definition: A light color area on the Moon, as contrasted with lunar maria.

Equinox correction
deprecated: :__
type: Concept

H alpha photometry
broader: narrow-band-photometry
type: Concept

Periodic orbit
broader: orbital-motion
type: Concept

Molecular spectroscopy
type: Concept
broader: spectroscopy
related: Molecular physics

Magnetic fields
broader: astrophysical-magnetism
type: Concept
definition: A field of force that is generated by electric currents, or, equivalently, a region in which magnetic forces can be observed.

Detached objects
broader: trans-neptunian-objects
type: Concept

Equinox
broader: equatorial-coordinate-system
type: Concept
definition: One of the two points on the celestial sphere where the celestial equator intersects the ecliptic, that is when the apparent ecliptic longitude of the Sun is 0° or 180°. At equinox, the length of the day and the night are equal all over the globe. The equinox is not a fixed point; it moves due to precession and nutation. If only precession is considered, we deal with the mean equinox of date. If nutation is also taken into account, then we are concerned with the true equinox.

Wormholes
type: Concept
broader: gravitational-singularities
definition: A hypothetical topological feature, based on general relativity, that connects two different points like a 'tunnel' in space-time. The most common concept of a wormhole is an Einstein-Rosen bridge. A trip through the wormhole could take much less time than a journey between the same starting and ending points in normal space. Wormholes have various types, intra-universe wormholes (connecting two distant regions of our Universe with each other) and inter-universe wormholes (that connect our Universe with another universe).
related: Kerr black holes

Broad-absorption line quasar
broader: quasars
type: Concept

Magellanic irregular galaxies
type: Concept
broader: irregular-galaxies

Kerr metric
broader: spacetime-metric
type: Concept

Interstellar dust extinction
broader: interstellar-scattering
related: Interstellar dust
definition: The dimming of light traveling in the interstellar space due to the combined effects of absorption and scattering by interstellar dust particles. Interstellar extinction increases at shorter (bluer) wavelengths, resulting in interstellar reddening.
type: Concept
broader: interstellar-absorption

Stellar flares
related: Solar flares
broader: stellar-phenomena
type: Concept

High angular resolution
type: Concept
broader: astronomical-techniques

Focal ratio
broader: telescope-properties
type: Concept

Oxygen burning
definition: The stage when a star fuses oxygen into silicon and sulfur. It occurs only in massive stars, with a mass over eight solar masses.
broader: nucleosynthesis
type: Concept

Distributed computing
type: Concept
broader: computational-methods

Atomic data benchmarking
broader: atomic-physics
type: Concept

Ocean tides
broader: tides
type: Concept

Hydrosphere
type: Concept
definition: A term denoting the water portion of a planet's surface.
broader: planetary-structure

Local Group
type: Concept
definition: A small galaxy cluster of about 50 galaxies to which our Milky Way galaxy belongs. The Local Group occupies a volume of space nearly 10 million light-years across centered somewhere between the Andromeda Galaxy (M31) and the Milky Way, which are the dominant galaxies of the group; Andromeda being the principal member. Both of these galaxies exhibit spiral structures, and each is attended by a large family of satellite dwarf galaxies. The Local Group also includes a third spiral galaxy known as Triangulum (M33), which is bound to Andromeda. The remaining members span a range of Hubble classification types from dwarf spheroidal to Irr to Sb and Sc and cover a factor of 10 in metallicity. The total mass of the Local Group is estimated to be about 2 x 10^12 solar masses, although this value is still uncertain to within a factor of about 2. The velocities of the individual galaxies of the Local Group are not particularly high. Therefore no member is believed to be able to escape the group, which is thus considered to be gravitationally bound. Another remarkable member of the Group is IC 10.
broader: galaxy-groups

Stellar convection envelopes
type: Concept
broader: stellar-structures

Density wave model
type: Concept
broader: galaxy-formation

Emission line stars
broader: stellar-types
type: Concept

Sunyaev-Zeldovich effect
type: Concept
broader: radio-astronomy
broader: cosmic-microwave-background-radiation

Space observatories
type: Concept
broader: observatories

Optical pulsars
broader: pulsars
definition: A member of a rare class of pulsars, such as the Crab pulsar and Vela pulsar, which can be detected in the visible spectrum.
type: Concept

Multi-periodic variable stars
related: Multi-periodic pulsation
broader: periodic-variable-stars
type: Concept
broader: pulsating-variable-stars

Hz Herculis x-ray binary stars
type: Concept
broader: low-mass-x-ray-binary-stars

Sky surveys
definition: The observation and recording of large extents of the sky with a particular instrument using one or more wavelengths in the same spectral domain.
type: Concept
broader: catalogs
broader: surveys

Optical constants (Ice)
broader: ice-physics
type: Concept

Tidal tails
broader: galaxy-tails
definition: A long stream of stars and gas, often in the form of a spectacular tail, thrown off a galaxy when it collides with another galaxy. Two tidal tails form in each galaxy, and they are more spectacular when the masses of the two galaxies are comparable, and when their relative orbit is in the same sense as the rotation inside each spiral galaxy.
type: Concept
broader: interstellar-medium

Active solar corona
broader: solar-corona
type: Concept

Classical black holes
broader: black-holes
type: Concept

High-velocity stars
type: Concept
broader: galaxies

Dust formation
broader: dust-physics
type: Concept

Rapidly oscillating Ap star
definition: A chemically peculiar star characterized by the presence of high-frequency non-radial oscillations, with periods that range between about 4 and 16 min. These variations have periods from about 5 to 20 minutes and low amplitudes (B < 10 mmag). They are consistent with acoustic (p mode) pulsations of low degree and high radial overtone.
broader: ap-stars
broader: magnetic-variable-stars
type: Concept

Red noise
broader: time-series-analysis
type: Concept

Heterodyne receivers
type: Concept
broader: radio-telescopes

Huya
type: Concept
deprecated: :__

Cultural astronomy
type: Concept
broader: astronomical-research
broader: interdisciplinary-astronomy

Peculiar variable stars
broader: variable-stars
broader: chemically-peculiar-stars
type: Concept

Conical singularities
broader: gravitational-singularities
type: Concept

Stellar evolutionary types
broader: stellar-types
type: Concept
related: Hertzsprung Russell diagram

Selenology
broader: lunar-science
type: Concept

Enceladus
type: Concept
broader: saturnian-satellites

South ecliptic pole
type: Concept
broader: ecliptic-poles

Indigenous astronomy
broader: ethnoastronomy
type: Concept

Ultra-high-energy cosmic radiation
type: Concept
definition: A particle belonging to the most energetic population of cosmic rays with an energy above ~10^20 electron-volts. The UHECRs constitute a real challenge for theoretical models, because their acceleration requires extreme conditions hardly fulfilled by known astrophysical objects.
broader: cosmic-rays

Eccentric anomaly
definition: Of a planetary orbit, the angle measured from the perihelion position, to the center of the circumscribing auxiliary circle, to the projected position of the planet on the circle.
type: Concept
broader: orbital-anomalies

Eruptive variable stars
type: Concept
broader: intrinsic-variable-stars

Relativity
related: Gravitation
broader: astrophysical-processes
type: Concept

Dynamical parallax
definition: A method for deriving the distance to a binary star. The angular diameter of the orbit of the stars around each other and their apparent brightness are observed. By applying Kepler's laws and the mass-luminosity relation, the distance of the binary star can be calculated.
broader: parallax
type: Concept

Apparent magnitude
broader: magnitude
definition: A measure of a star's observed brightness (opposed to absolute magnitude); symbol m. It depends on the star's intrinsic brightness, its distance from the observer, and the amount of interstellar absorption. The brightest star Sirius has an apparent magnitude of -1.46, while the weakest stars visible with the naked eye in the most favorable observation conditions have magnitudes of about +6.5. The stars of magnitudes less than +23 are measured by professional observatories, whereas those of magnitudes less than +30 by a telescope such as the Hubble Space Telescope.
type: Concept

R associations
definition: A stellar association containing a number of reflection nebulae. The stars are of low or intermediate mass and young, less than a million years old. They are still surrounded by patches of dust that reflect and absorb light from the interstellar cloud in which they formed. This type of association was first suggested by Sidney van den Bergh.
type: Concept
related: Reflection nebulae
broader: stellar-associations

Delta Delphini variable stars
type: Concept
broader: pulsating-variable-stars

Burkert profile
type: Concept
broader: astronomical-models

Solar gamma-ray emission
broader: solar-electromagnetic-emission
type: Concept

Population III stars
type: Concept
definition: A member of the first generation of stars, formed out of pristine gas, enriched by primordial nucleosynthesis alone. The material from which these stars formed consisted mostly of hydrogen and helium. Because neutral hydrogen clouds were free of dust, their cooling mechanism was drastically ineffective. As a result, these star forming clouds had a much higher temperature than in the present epoch, and their Jeans mass was much higher. Therefore, these first generation of stars were principally massive, with a typical mass scale of order of about 100 M_sun. Population III stars started forming about 300 million years after the Big Bang at redshifts between 50 and 6, when the Universe had between 1 and 5% of its present age. These stars were probably responsible for the reionization of the Universe. Given their high mass, they lived only a few million years ending with either a pair-instability supernova phase or a direct collapse to a black hole. Population III stars thus initiated the chemical enrichment of the Universe and opened the way to more normal modes of star formation, namely Population II. Some models predict a bimodal initial mass function for the first stars, allowing also for solar mass stars.
broader: stellar-populations

Astrophysicists
broader: astronomical-research
definition: A scientist who studies astrophysics.
type: Concept

Osculating orbit
type: Concept
broader: orbital-motion
definition: The Keplerian orbit that a satellite would follow after a specific time t if all forces other than central inverse-square forces ceased to act from time t on. An osculating orbit is tangent to the real, perturbed, orbit and has the same velocity at the point of tangency.

Horizon
broader: horizontal-coordinate-system
type: Concept

Extragalactic astronomy
definition: The branch of astronomy that deals with objects beyond the Milky Way, especially galaxies and quasars.
type: Concept
broader: galactic-and-extragalactic-astronomy

RRd variable stars
broader: double-mode-variable-stars
broader: rr-lyrae-variable-stars
type: Concept

Virgo Cluster
related: Virgo Supercluster
definition: The largest and nearest galaxy cluster to the Local Group. The Virgo cluster spans 120 square degrees on the sky and contains on the order of 2,000 galaxies. It is located at a distance of about 60 million light-years. It is an irregular cluster with no central concentration. The giant elliptical galaxy M87 is the most massive in the cluster.
type: Concept
broader: galaxy-clusters

Disk population
deprecated: :__
type: Concept

Compact binary components
deprecated: :__
type: Concept

Very Large Telescope
type: Concept
definition: An observing facility consisting of four 8.2 m telescopes, with the combined collecting area of a 16 m mirror, owned and operated by the European Southern Observatory at an altitude of 2635 m at the Paranal Observatory in Chile. The four reflecting unit telescopes are called Antu 'Sun' in the language of Chile's indigenous Mapuche people, Kueyen 'Moon,' Melipal 'Southern Cross,' and Yepun 'Venus.' Each unit is equipped with several sophisticated instruments. The light of the individual telescopes can be combined using interferometric techniques to achieve superior resolution. The wavelength range covered by the VLT is extremely wide, ranging from deep ultraviolet to mid-infrared.
broader: optical-observatories

Galaxy processes
broader: galaxy-physics
type: Concept

Ultraviolet extinction
broader: atmospheric-extinction
type: Concept

Astronomical detectors
type: Concept
definition: A device whose main function is detection. Any device or array of electronic sensors connected to a computer, used to detect the passage of a particle or photon; e.g. a charge-coupled device (CCD).
broader: astronomical-instrumentation

Virtual observatories
broader: astronomy-web-services
broader: observatories
type: Concept

M supergiant stars
broader: late-type-supergiant-stars
type: Concept
broader: m-stars
related: Red supergiant stars

Planetary magnetospheres
broader: planetary-science
type: Concept

Plate tectonics
related: Exoplanet plate tectonics
broader: tectonics
definition: The theory supported by a wide range of evidence in which the Earth's crust is composed of several large, thin, relatively rigid plates that move relative to one another. The interaction of the plates at their boundaries causes seismic and tectonic activity along these boundaries.
type: Concept

Zenith hourly rate
type: Concept
broader: meteor-showers
definition: The number of meteors expected to be seen under ideal conditions if the radiant of the meteoroid stream is at the zenith of the observer.

Nova-like variable stars
broader: stellar-remnants
type: Concept
broader: cataclysmic-variable-stars

PG 1159 stars
broader: do-stars
type: Concept
broader: multi-periodic-variable-stars

Space probes
broader: space-vehicles
definition: A spacecraft carrying instruments intended for use in exploration of the physical properties of outer space or celestial bodies other than Earth.
type: Concept

von Zeipel theorem
definition: A theorem that establishes a relation between the radiative flux at some colatitude on the surface of a rotating star and the local effective gravity (which is a function of the angular velocity and colatitude). For a rotating star in which centrifugal forces are not negligible, the equipotentials where gravity, centrifugal force, and pressure are balanced will no longer be spheres. The theorem states that the radiative flux is proportional to the local effective gravity at the considered colatitude, F(θ) ∝ g_eff (θ)α, where α is the gravity darkening coefficient. As a consequence, the stellar surface will not be uniformly bright, because there is a much larger flux and a higher effective temperature at the pole than at the equator (T_eff (θ) ∝ g_eff (θ)β, where β is the gravity darkening exponent. In massive stars this latitudinal dependence of the temperature leads to asymmetric mass loss and also to enhanced average mass loss rates.
broader: analytical-mathematics
type: Concept
related: Stellar rotation

Reaction rates
broader: astrochemistry
type: Concept

Optical filters
broader: astronomical-optics
type: Concept

Eccentricity
type: Concept
broader: orbital-elements
definition: The amount by which the orbit deviates from circularity: e = c/a, where c is the distance from the center to a focus and a the semi-major axis. If e = 0, the orbit is a circle. If e < 1, the orbit is an ellipse, if e > 1 it is a hyperbola, and if e = 1 it is a parabola. The eccentricity is one of the six orbital elements that define a Keplerian orbit.

Supergalactic longitude
broader: supergalactic-coordinate-system
type: Concept

Astrochemistry
related: H I regions
definition: The study of the chemical interactions between the gas and dust of the interstellar medium.
broader: interdisciplinary-astronomy
type: Concept

Fast solar wind
broader: solar-wind
type: Concept

Total eclipses
definition: An eclipse in which the whole of the disk of the Sun or Moon is obscured.
broader: eclipses
type: Concept

Evolution equation
broader: general-relativity
type: Concept

Stellar evolutionary tracks
broader: stellar-evolutionary-models
type: Concept

Slow novae
type: Concept
definition: A type of nova whose light curve exhibits a characteristically slow development, having a rise time of several days, maximum of several weeks, and slower decline.
broader: novae

Helium-poor stars
type: Concept
deprecated: :__

Solar macrospicules
broader: solar-spicules
type: Concept

Very Large Array
broader: radio-interferometers
definition: A radio interferometer consisting of 27 antennas, each 25 m in diameter, in a Y-shaped configuration. It is located about 100 km west of Socorro, New Mexico, and is operated by the United States National Radio Astronomy Observatory. The VLA has the resolution of a single antenna 36 km wide and the sensitivity of a dish 130 m across.
type: Concept

Radio interferometers
broader: radio-telescopes
definition: An interferometer designed for radio frequencies.
type: Concept

Gaia
type: Concept
broader: space-observatories

Orbital evolution
type: Concept
broader: cosmological-evolution

Radiative recombination
definition: The process by which an ionized atom binds a free electron in a plasma to produce a new atomic state with the subsequent radiation of photons.
broader: recombination
type: Concept

Caustic curve
broader: gravitational-lensing
type: Concept
definition: The intersection of a caustic surface with a plane passing through the beam of rays.

Structure determination
broader: quantum-chemical-calculations
type: Concept

Ice spectroscopy
type: Concept
broader: ice-physics
broader: spectroscopy

Solar flare spectra
type: Concept
broader: solar-flares

Extended radiation sources
broader: astronomical-radiation-sources
type: Concept

Cold dark matter
definition: Any hypothetical non-baryonic dark matter that is non-relativistic at the point of decoupling in the early Universe. CDM plays a key role in cosmic structure formation.
type: Concept
broader: dark-matter

Comet tails
type: Concept
broader: comets
definition: A formation of gas and/or dust that streams away from the coma of many comets under the influence of the Sun's radiation pressure and the solar wind.

Ascending node
definition: The point in an orbit where the orbiting body crosses a reference plane, such as the ecliptic or the celestial equator, going from south to north. The celestial longitude of the ascending node is one of the elements of the orbit.
broader: orbital-nodes
type: Concept

Galaxy mergers
broader: galaxy-interactions
type: Concept
broader: interacting-galaxies

Venus
broader: inner-planets
type: Concept
definition: The second planet from the Sun, at a mean distance of roughly 108.21 x 10^6 km (0.72 astronomical units). The greatest elongation of Venus is about 47°, making it visible at most about 3 hours after sunset or before sunrise. Venus has the most circular orbit of any planet in the solar system. Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass). Its chemical composition and density are comparable to those of the Earth. It takes Venus just under 224.401 days to orbit the Sun, compared to the 365 day orbital period of the Earth. Venus' rotation is retrograde, that is it actually rotates from east to west, as opposed to west to east (prograde) which is the common rotating direction of most other planets. Seen from Venus, the sun would rise in the west and set in the east. Moreover, it takes about 244 Earth days for Venus to rotate once (sidereal rotation). This is longer than its orbital period. The length of its solar day is about 117 Earth days. Venus rotation. Its axial tilt is only three degrees, so there are no seasons on Venus. The atmosphere on the surface of Venus consists mostly of carbon dioxide, with a small trace of nitrogen. Venus has a surface pressure about 90 times that of the Earth.

Cosmology
type: Concept
related: Dark matter
related: Protogalaxies
definition: The science of the origin, structure, and evolution of the Universe including the origin of galaxies, the chemical elements, and matter.

Exoplanet detection methods
broader: exoplanet-astronomy
type: Concept

Stellar distance
type: Concept
related: Stellar parallax
broader: astrometry

Zone of totality
broader: eclipses
type: Concept

Intergalactic gas
type: Concept
broader: intergalactic-medium

Apsidal motion
type: Concept
broader: orbital-motion
definition: Rotation of the line of apsides in the plane of the orbit in the same direction as the revolution of the secondary body. The major axis of the Earth's orbit rotates by 11.6 arcseconds per year.

Instability strip
definition: A narrow, almost vertical, band on the right hand side of the main sequence in the H-R diagram occupied by many different types of pulsating stars (RR Lyrae, Cepheids, W Virginis, ZZ Ceti). Stars traverse this region at least once after they leave the main sequence. The narrow temperature range of the instability strip corresponds to the stellar effective temperature that can sustain partial ionization zones, capable of maintaining stellar oscillations. The blue (hot) edge of the instability strip pertains to stars with surface temperatures hotter than ~7500 K. Because these stars have partial ionization zones too close to their surface, the pulsation mechanism is not active. The red (cooler) edge of the instability strip is determined by stars with a temperature lower than ~5500 K. In these stars convection prevents the build-up of heat pressure necessary to drive pulsations.
type: Concept
broader: hertzsprung-russell-diagram

O supergiant stars
broader: o-stars
broader: early-type-supergiant-stars
type: Concept

Manganese stars
type: Concept
broader: heavy-metal-stars
broader: ap-stars
broader: chemically-peculiar-stars

Non-standard theories of gravity
broader: gravitation
type: Concept

Seasonal phenomena
type: Concept
broader: atmospheric-effects
related: Earth's seasons

Exoplanet catalogs
broader: exoplanet-astronomy
type: Concept

Direct imaging
broader: exoplanet-detection-methods
type: Concept
broader: astronomical-techniques

Astronomy web services
type: Concept
broader: astronomy-software

CCD observation
type: Concept
broader: astronomical-techniques

Vega photometry
type: Concept
broader: broad-band-photometry

Interferometric binary stars
type: Concept
broader: binary-stars

Astrodynamics
broader: celestial-mechanics
type: Concept
definition: The science dealing with the motion of satellites, rockets, and spacecrafts. It uses the principles of celestial mechanics.

Heliopause
type: Concept
broader: heliosphere
definition: The edge of the solar system where the pressure of the solar wind balances that of the interstellar medium plasma. In other words, the surface boundary that separates the heliosphere from interstellar space. It is estimated to be situated 100 astronomical units or more from the Sun. A bow shock likely forms as the interstellar medium wind approaches the heliosphere and is deflected around the heliosphere, forcing it into a teardrop-shaped structure with a long, comet-like tail.
broader: solar-wind

Teharonhiawako
type: Concept
deprecated: :__

Electronic spectroscopy
type: Concept
broader: spectroscopy

Credible region
broader: uncertainty-bounds
type: Concept

Reaction rate equations
type: Concept
broader: reaction-models

Lunar eclipses
definition: The darkening of the Moon which occurs when the Moon enters the umbra of the Earth's shadow. This phenomenon can occur only when the full Moon is near one of the lunar nodes of its orbit around the Earth. There will be a total eclipse if the entire Moon enters the umbra, otherwise the eclipse will be partial when the Moon is somewhat to the north or south of the node and does not cross the shadow entirely. During the eclipse the Moon looks more or less dark, depending especially on the transparency of the Earth's atmosphere. The refraction of Sun's light through the atmosphere sometimes gives a red color to the eclipsed Moon. Colored fringes can be seen around the shadow edge during partial eclipses. Because an eclipse of the Moon is due to the cutting off of the Sun's light, it is visible from the entire hemisphere where the Moon is above the horizon. The maximum duration of a total lunar eclipse, when the Moon passes through the shadow centrally, is 1h 47m.
broader: eclipses
type: Concept

Bayes' Theorem
broader: bayesian-statistics
definition: A theorem in probability theory concerned with determining the conditional probability of an event when another event has occurred. Bayes' theorem allows revision of the original probability with new information. Its simplest form is: P(A|B) = P(B|A) P(A)/P(B), where P(A): independent probability of A, also called prior probability; P(B): independent probability of B; P(B|A): conditional probability of B given A has occurred; P(A|B): conditional probability of A given B has occurred, also called posterior probability. Same as Bayes' rule.
type: Concept

Meteor streams
type: Concept
definition: The meteoroids distributed all along an orbit and diffused somewhat around it.
broader: meteors

OB associations
definition: A loosely bound grouping of O and B stars that typically stretches up to several hundred light-years and may contain between a dozen and several hundred O stars and B stars. The members of an OB association are young and of roughly the same age. OB associations dissipate in a few tens of millions of years.
broader: stellar-associations
type: Concept

Maximum likelihood estimation
type: Concept
broader: astrostatistics-strategies

Cirrus spissatus
type: Concept
deprecated: :__

Lunar atmosphere
broader: lunar-science
type: Concept

Solar particle emission
related: Solar neutrinos
type: Concept
broader: solar-physics

F subgiant stars
type: Concept
broader: f-stars
broader: subgiant-stars

Radiation interactions (Ice)
type: Concept
broader: ice-physics

Extinction
definition: Dimming of light by an intervening medium (the Earth's atmosphere or the interstellar medium). It is usually due to both scattering and absorption.
type: Concept
broader: astronomical-optics

Gamma distribution
type: Concept
broader: astrostatistics-distributions

Near-sun comets
definition: Encompasses all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU).
type: Concept
broader: comets

Transit instruments
broader: astronomical-instrumentation
definition: An instrument mounted so as to allow it to be pointed only at objects in the sky crossing the local meridian. Also known as transit telescopes.
type: Concept

Gaseous nebulae
definition: An H II region, a planetary nebula, or a supernova remnant.
related: Emission nebulae
type: Concept
broader: nebulae

Stellar wind bubbles
type: Concept
broader: interstellar-medium

Trinary stars
broader: multiple-stars
type: Concept

Intergalactic clouds
definition: Intergalactic matter in the form of clouds.
broader: intergalactic-medium
type: Concept

Michelson interferometers
type: Concept
broader: interferometers
related: Michelson interferometry

Annual microlensing parallax
type: Concept
broader: microlensing-parallax

Zenith distance
type: Concept
broader: horizontal-coordinate-system

Steady-state theory
broader: origin-of-the-universe
type: Concept

Infrared excess galaxies
type: Concept
broader: galaxies

Exoplanets
broader: exoplanet-astronomy
type: Concept
related: Natural satellites (Extrasolar)
definition: A planet which belongs to a star other than Sun, and therefore does not belong to our solar system; same as exoplanet.
related: Solar system planets

Nitrous oxide
broader: greenhouse-gases
type: Concept

Secondary mirror
broader: reflecting-telescopes
type: Concept

Photodissociation regions
related: Photodissociation reactions
broader: molecular-clouds
type: Concept

Cosmic ray nucleosynthesis
type: Concept
broader: nucleosynthesis

Deprecated: Radiative Processes
type: Concept
deprecated: :__
use instead: Radiative processes

Surface processes
type: Concept
broader: planetary-surfaces

Radio hot spots
broader: active-galactic-nuclei
type: Concept

Cosmological phase transitions
type: Concept
broader: cosmology

Relativistic stars
broader: neutron-stars
type: Concept

Neptune
type: Concept
definition: The eighth planet from the Sun and the fourth largest by size in the solar system. The equatorial radius of Neptune is 24,764 km (3.883 Earths), its semi-major axis is 30.11 astronomical units (4.50 x 10^9 km), and its orbital period is 164.8 yr. Neptune has at least 14 moons, the largest ones are Triton, Proteus, and Nereid, whereas its smaller moons are: Naiad, Thalassa, Despina, Galatea, Larissa, Halimede, Sao, Laomedeia, Psamathe, and Neso. Neptune has an incredibly thick atmosphere comprised of 74% hydrogen, 25% helium, and approximately 1% methane. Particles of icy methane in its upper atmosphere give Neptune its deep blue color. Large storms whirl through Neptune's upper atmosphere, and high-speed winds track around the planet at up 600 m/s, fastest recorded in the solar system. One of the largest storms ever seen was recorded in 1989. Called the Great Dark Spot, it lasted about five years. Neptune has a very thin collection of rings. They are likely made up of ice particles mixed with dust grains and possibly coated with a carbon-based substance.
related: Neptunian satellites
broader: outer-planets

Planet hosting stars
broader: exoplanet-astronomy
type: Concept

Late-type galaxies
definition: In the Hubble classification, a galaxy on the left part of the Hubble sequence.
broader: galaxies
type: Concept

Curvature singularities
type: Concept
broader: gravitational-singularities

Open source software
type: Concept
broader: software-licensing

Solar parallax
type: Concept
broader: stellar-parallax
definition: The angle subtended (8''.79) by the equatorial radius of the Earth at a distance of 1 astronomical unit.

Charge transfer
broader: collision-processes
type: Concept

Exoplanet formation
broader: exoplanet-astronomy
type: Concept

Jeans mass
type: Concept
definition: The minimum mass for an interstellar cloud below which the thermal pressure of the gas prevents its collapse under the force of its own gravity. High density favors collapse, while high temperature favors larger Jeans mass.
broader: gravitational-collapse

Multi-periodic pulsation
type: Concept
related: Multi-periodic variable stars
broader: stellar-pulsations

Asteroids
type: Concept
broader: small-solar-system-bodies
related: Asteroid belt
definition: A small rocky object orbiting the Sun. There are millions of asteroids moving in orbits in the main asteroid belt, between Mars and Jupiter and in the Kuiper belt. The largest and the first discovered, Ceres, about 1,000 km in size, is now classified as dwarf planet (2006 IAU General Assembly). The largest asteroid in the solar system (Ceres apart), is Pallas, with a size of 582 x 556 x 500 km. On the other hand, the smallest asteroid ever studied is the 2 meters space rock 2015 TC25, which was observed when it made a close flyby of Earth in October 2015.
related: Interplanetary medium
related: Asteroid dynamics
related: Near-Earth objects

Evolved stars
definition: A star that has left the main sequence.
type: Concept
related: Hertzsprung Russell diagram
broader: stellar-evolutionary-types

Geneva photometry
type: Concept
broader: medium-band-photometry

Catadioptric telescopes
type: Concept
broader: optical-telescopes

Strontium stars
type: Concept
broader: heavy-metal-stars
broader: chemically-peculiar-stars

Standard candles
type: Concept
broader: stellar-distance

Interstellar line emission
broader: interstellar-emissions
type: Concept

Infrared excess
broader: infrared-astronomy
type: Concept
definition: Infrared emission from a star in excess of that expected from a blackbody with the temperature corresponding to the temperature of the star.

Astronomy data sonification
broader: astronomy-data-analysis
type: Concept

Interplanetary dust
type: Concept
definition: Particles of dust in the interplanetary medium. They are left-overs from the beginning of the solar system or from other sources such as sublimating comets. Their existence was first deduced from observations of zodiacal light.
broader: interplanetary-medium

Cool intergalactic medium
broader: intergalactic-medium-phases
type: Concept

Retrograde orbit
type: Concept
broader: orbital-motion

Internal waves
type: Concept
broader: astrophysical-fluid-dynamics

Starlight polarization
type: Concept
broader: stellar-properties

Apex
definition: The point on the celestial sphere toward which the Sun is apparently moving relative to the local standard of rest. Its position, in the constellation Hercules, is approximately R.A. 18h, Dec. +30°, close to the star Vega. The velocity of this motion is estimated to be about 19.4 km/sec (about 4 AU/year). As a result of this motion, stars seem to be converging toward a point in the opposite direction, the solar antapex.
type: Concept
broader: solar-motion

Solar mottles
type: Concept
broader: solar-spicules

Apogee
definition: The point in the orbit of the moon or an artificial satellite that is farthest from the terrestrial center and at which the body's velocity is at a minimum.
broader: apsides
type: Concept

Private software
type: Concept
broader: software-licensing

Bediasites
type: Concept
broader: tektites

Free floating planets
definition: A population of substellar objects which are not bound to stars; they are detected in young star clusters. Their masses, estimated from their fluxes, is several Jupiter masses, lower than those of brown dwarfs. Their formation is not yet explained. Among the envisaged possibilities: 1) These objects form like stars, from protostellar core collapse and subsequent accretion; 2) they form as low-mass members of small groups, and are ejected from the group; 3) they form like planets within circumstellar disks of higher-mass objects, but are ejected either due to internal dynamics or external interactions.
related: Gravitational microlensing
broader: exoplanets
related: Gravitational microlensing exoplanet detection
type: Concept

Elliptical orbits
broader: orbital-elements
type: Concept
broader: orbital-motion

DO stars
type: Concept
definition: A white dwarf whose spectrum shows strong lines of singly ionized helium He II; He I or H may be present. As a DO star cools, the He II will recombine with free electrons to form He I, eventually changing the DO type into a DB white dwarf.
broader: white-dwarf-stars

Nonclassical interferometry
type: Concept
broader: interferometry

Radiative transfer
definition: The process by which the electromagnetic radiation passes through a medium that may contain any combination of scatterers, absorbers, and emitters.
type: Concept
related: Radiative transfer equation
broader: cosmology

Galaxy disks
type: Concept
related: Milky Way disk
broader: galaxy-structure

Solar cycle
broader: helioseismology
definition: The periodic variation in frequency or number of solar active events (sunspots, prominences, flares, and other solar activity) occurring with an interval of about 11 years. The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe (1789-1875), a German apothecary and amateur astronomer, who after 17 years of observations noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk. Solar cycle numbering goes back to the 18th century, when the Cycle 1 peak occurred in 1760. Cycle 23 peaked in 2000, and the following Cycle 24 will reach its maximum in 2013.
type: Concept

Space telescopes
type: Concept
definition: A telescope which is placed in an orbit around the Earth and operates through commands from sent from the control center on Earth, such as Hubble space telescope, Herschel satellite, Infrared Astronomical Satellite (IRAS), Infrared Space Observatory (ISO), International Ultraviolet Explorer (IUE), Planck Satellite, Spitzer Space Telescope.
broader: space-vehicle-instruments
broader: telescopes

Reissner-Nordström black holes
broader: nonrotating-black-holes
definition: A type of electrically charged black hole that is spherically symmetric and non-rotating. Another type of such objects is the Kerr-Newman black hole.
type: Concept
broader: charged-black-holes

Stephan's quintet
deprecated: :__
type: Concept

S Doradus stars
broader: luminous-blue-variable-stars
broader: emission-line-stars
definition: A type of massive, blue supergiant, variable star, also known as a Hubble-Sandage variable or a Luminous Blue Variable (LBV). S Doradus stars are the most luminous stars in the Galaxy and are easily identified in other nearby galaxies. They are named after the prototype, S Doradus, in the Large Magellanic Cloud.
type: Concept

Space plasmas
type: Concept
broader: plasma-astrophysics

Lithium stars
type: Concept
broader: chemically-peculiar-stars

Stellar gas distribution
type: Concept
broader: stellar-structures

Cosmological constant
type: Concept
definition: A term introduced by Einstein into his gravitational field equations in order to allow a solution corresponding to a static Universe. The cosmological constant is physically interpreted as due to the vacuum energy of quantized fields.
broader: observational-cosmology
broader: dark-energy

Logos
type: Concept
deprecated: :__

Micrometeorites
definition: A small grain sized meteorite which can only be positively identified under the microscope.
type: Concept
broader: meteorites

Charged black holes
type: Concept
broader: classical-black-holes

Post-asymptotic giant branch
broader: hertzsprung-russell-diagram
type: Concept
related: Post-asymptotic giant branch stars

Cosmic background radiation
type: Concept
broader: recombination-cosmology

Optical interferometry
type: Concept
broader: interferometry

Luminous arcs
type: Concept
broader: einstein-rings

HI shells
broader: h-i-regions
type: Concept

Stellar mass loss
type: Concept
related: Solar coronal mass ejections
broader: stellar-phenomena
related: Stellar jets
related: Stellar coronal mass ejections

Planetary interior
type: Concept
broader: planetary-structure

DZ stars
broader: white-dwarf-stars
type: Concept
definition: A white dwarf whose spectrum shows metal lines only; no H or He.

Stellar associations
related: Stellar kinematics
type: Concept
broader: star-clusters
definition: A large, loose grouping of 10 to 1000 stars that are of similar spectral type and share a common origin. The members move together through space, but have become gravitationally unbound. Stellar associations are primarily identified by their common movement vectors and ages.

Surveys
broader: astronomical-reference-materials
type: Concept

Meteor trails
type: Concept
broader: meteors

Dust nebulae
type: Concept
broader: nebulae

Gravity darkening
broader: stellar-rotation
definition: The darkening, or brightening, of a region on a star due to localized decrease, or increase, in the effective gravity. Gravity darkening is explained by the von Zeipel theorem, whereby on stellar surface the radiative flux is proportional to the effective gravity. This means that in rotating stars regions close to the pole are brighter (and have higher temperature) than regions close to the equator. Gravity darkening occurs also in corotating binary systems, where the tidal force leads to both gravity darkening and gravity brightening. The effects are often seen in binary star light curves. Recent theoretical work has shown that gravity darkening is not well represented by the von Zeipel theorem. This is supported by new interferometric observations of some rapidly rotating stars indicating that the von Zeipel theorem seems to overestimate the temperature difference between the poles and equator.
type: Concept

Mercury (planet)
type: Concept
broader: inner-planets
definition: The closest planet to the Sun and one of five planets visible with the naked eye. The greatest elongation of Mercury is about 28°, making it visible at most about 112 minutes after sunset or before sunrise. It lies at a mean distance of about 0.39 astronomical units from the Sun. Mercury is just 4,879 km in diameter, about 2.6 times smaller than the Earth. Its orbital period is 87.97 Earth days. Mercury has a high density, 5.4 g cm^-3, with only the Earth having a higher density among the planets. This is largely due to Mercury being composed mainly of heavy metals and rock. One solar day on Mercury lasts the equivalent of 176 Earth days while the sidereal day (the time for 1 rotation in relation to a fixed point) lasts 59 Earth days. Mercury is nearly tidally locked to the Sun and over time this has slowed the rotation of the planet to almost match its orbit around the Sun. Mercury also has the highest orbital eccentricity of all the planets with its distance from the Sun ranging from 46 to 70 million km. Mercury has just 38% the gravity of Earth, this is too little to maintain an atmosphere against solar winds, which blow it away. The surface of Mercury which faces the Sun has temperatures of up to 427°C, whilst on the alternate side this can be as low as -173°C. Mercury's core has more iron than any other planet in the solar system. This has to do with its formation and early life. If the planet formed quickly, increasing temperatures of the evolving Sun could have vaporized much of the existing surface, leaving only a thin shell.

T associations
definition: A stellar association containing many T Tauri stars. Examples include the Taurus-Auriga T association, the nearby TW Hydrae association, and Vela T1 and T2.
type: Concept
broader: stellar-associations

Polarimetry
type: Concept
broader: exoplanet-detection-methods
broader: astronomical-techniques
definition: The measurement of the polarization state of light, usually through the use of a polarimeter.

Lunation
type: Concept
broader: lunar-theory

Lens orbital motion
broader: gravitational-microlensing
type: Concept

Hubble time
broader: cosmological-parameters
type: Concept
definition: An estimate for the age of the Universe by presuming that the Universe has always expanded at the same rate as it is expanding today. It is the inverse of the Hubble-Lemaitre constant.

Stellar astronomy
definition: The branch of astronomy that deals with the study of stars, their physical properties, formation, and evolution.
type: Concept

Neutrino masses
type: Concept
broader: neutrino-astronomy

W Virginis variable stars
definition: A member of a class of pulsating stars with a period of 1 to 35 days located in the instability strip of the H-R diagram. Also known as type II Cepheid variables, W Virginis stars are typically 1.5 mag fainter than classical Type I Cepheids and have a mass less than that of the Sun. They also exhibit a period-luminosity relation which is distinct, but works in a similar way to the relation for Type I Cepheids. Hence W Virginis stars can also be used to measure Galactic and extragalactic distances.
type: Concept
broader: type-ii-cepheid-variable-stars

RS Canum Venaticorum variable stars
type: Concept
broader: flare-stars

Ice physics
type: Concept
broader: solid-matter-physics

Amateur astronomy
broader: astronomical-methods
definition: The astronomical activities carried out by amateur astronomers.
type: Concept
related: Amateur astronomers

Classification
type: Concept
definition: The systematic grouping of astronomical objects into categories on the basis of physical, morphological, or evolutionary characteristics.
broader: astrostatistics-techniques

Production techniques
broader: experimental-techniques
type: Concept

Galaxy radii
broader: galaxy-properties
type: Concept

Cosmic isotropy
type: Concept
broader: cosmic-microwave-background-radiation

Stellar occultation
broader: occultation
type: Concept

Ixion
deprecated: :__
type: Concept

Red sequence galaxies
broader: galaxy-colors
type: Concept

Y dwarfs
definition: A type of ultra-cool brown dwarf with an effective temperature lower than 500 K. Near infrared spectra of these objects show deep absorption bands of H_{2}O and CH_4. So far only seven brown dwarf candidates belonging to this class have been found, all Y0 subtypes. These objects are very dim, with H magnitudes 19-23. The precise definition of the Y class requires new findings in the future about these objects.
type: Concept
broader: brown-dwarfs

Binoculars
broader: astronomical-instrumentation
type: Concept

Rotating black holes
type: Concept
broader: classical-black-holes
definition: A black hole that possesses angular momentum, as first postulated by Roy C. Kerr in 1963. Opposite of a stationary black hole.

Carbon planets
broader: exoplanets
type: Concept

Forbush effect
type: Concept
broader: cosmic-rays

Be type stars
type: Concept
deprecated: :__

Cosmological principle
type: Concept
broader: cosmology

Proplyds
broader: protoplanetary-disks
definition: A circumstellar disk of dense gas and dust surrounding a young newly formed star, a T Tauri star or Herbig star. In particular, an externally ionized protoplanetary disk seen in emission in the Orion Nebula.
type: Concept

Stellar dynamics
broader: stellar-physics
type: Concept
definition: The field of astrophysics that describes systems of many point mass particles whose mutual gravitational interactions determine their orbits. These systems include star clusters, globular clusters, and galaxies. Stellar dynamics deals with systems in which each member contributes importantly to the overall gravitational field and is usually concerned with the statistical properties of many orbits. It can be compared to the kinetic theory of gases developed in the late 19th century. In contrast, celestial mechanics deals with systems where the gravitational force of a massive planet or star determines the orbits of its satellites.

Circular orbits
type: Concept
broader: orbital-motion

Strömgren photometry
definition: A four-color stellar photometric system devised by B. Strömgren. It is based on measurements in the ultraviolet (3500 Å), violet (4100 Å), blue (4670 Å), and yellow (5470 Å) regions of the spectrum. The filters bandwidths are 340, 200, 160, and 240 Å respectively. Also known as Strömgren four-color photometry.
type: Concept
broader: medium-band-photometry

Aphelion
definition: The point in the orbit of a planet, or other object in the solar system, which is furthest from the Sun.
broader: apsides
type: Concept

Spectrophotometry
definition: Measurement of the absolute fluxes of the components of different frequencies in the spectrum of a light source.
type: Concept
broader: photometry

Bolides
definition: A meteor which is extremely bright, particularly one that breaks up during its passage through the atmosphere.
type: Concept
broader: fireballs

Accretion
definition: The process by which an object increases its mass under the influence of its gravitational attraction. Accretion plays a key role in a wide range of astrophysical phenomena. In particular stars result from the accretion of material by a protostar from a surrounding molecular cloud. The accumulation of mass on the protostar involves the formation of an accretion disk. Theoretical and observational investigations of protostars and newborn stars indicate the important role of magnetic fields in this process. They favor the magnetospheric accretion model for mass transfer from the circumstellar disk onto the newborn star. In this model, the stellar magnetosphere truncates the disk at a few stellar radii. Gas from the disk accretes onto the star along the magnetic field lines and hits the stellar surface at approximately the free fall velocity, causing a strong accretion shock. Various emission lines are formed in the infalling magnetospheric flow. Moreover, optical/ultraviolet excess continuum emission is produced in the accretion shocks. The accretion is accompanied by mass ejection through collimated bipolar jets.
type: Concept
broader: newtonian-gravitation

Population I stars
definition: A member of a class of relatively young stars, containing a large fraction of metals, found mainly in the disk of the Galaxy.
type: Concept
broader: stellar-populations

Geocentric model
broader: cosmological-models
type: Concept

Radio plumes
type: Concept
broader: active-galactic-nuclei

Gravitational energy
definition: The energy that an object possesses because of its position in a gravitational field, especially an object near the surface of the Earth where the gravitational acceleration can be assumed to be constant, at about 9.8 m s^-2.
type: Concept
broader: newtonian-gravitation

Quasar microlensing
broader: gravitational-microlensing
type: Concept

Analytical mathematics
related: Computational astronomy
type: Concept
broader: astronomical-techniques

Astronomical models
broader: analytical-mathematics
type: Concept

Reaction catalysts
broader: astrochemistry
type: Concept

Earth atmosphere
type: Concept
related: Atmospheric extinction
broader: planetary-atmospheres

Johnson photometry
type: Concept
broader: broad-band-photometry

Bipolar sunspot groups
broader: sunspot-groups
type: Concept

Rapid stellar oscillations
type: Concept
broader: stellar-oscillations

W Ursae Majoris variable stars
broader: eclipsing-binary-stars
type: Concept
broader: contact-binary-stars

Observational astronomy
type: Concept

Dark matter detectors
type: Concept
broader: astronomical-detectors

Space weather
related: Solar-planetary interactions
definition: The varying conditions in space and specifically in the near-Earth environment. Space weather is chiefly solar driven, resulting from solar activities such as solar flares, solar wind, and coronal mass ejections that affect magnetosphere, ionosphere, and thermosphere. Non-solar sources such as Galactic cosmic rays, meteoroids, and space debris can all be considered as altering space weather conditions at the Earth. Space weather may affect the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health. The research in this field aims at monitoring and diagnosis of space weather conditions and constructing reliable numerical prediction models.
type: Concept
broader: solar-system

Low-density fields
broader: large-scale-structure-of-the-universe
type: Concept

Ice porosity
type: Concept
broader: ice-physics

P-process
broader: nucleosynthesis
type: Concept

Likelihood ratio test
broader: model-selection
type: Concept

Orcus
deprecated: :__
type: Concept

Isomerization reactions
broader: molecular-reactions
type: Concept

Scalar-tensor-vector gravity
broader: non-standard-theories-of-gravity
type: Concept

Right ascension
definition: A coordinate in the equatorial system measured from the vernal equinox eastward to the point where the object hour circle intersects the celestial equator. Right ascension (symbol α) is expressed in hours, minutes, and seconds.
broader: equatorial-coordinate-system
type: Concept

Planispheres
type: Concept
broader: early-astronomical-instruments

Solar chromospheric heating
type: Concept
broader: solar-chromosphere

Diamond ring effect
type: Concept
definition: An intense flash of light that happens a few seconds before and after totality during a solar eclipse. The effect is caused by the last rays of sunlight before totality (or the first rays of sunlight after totality) shining through valleys on the edge of the Moon.
broader: solar-eclipses

Anomalistic month
type: Concept
definition: The time interval of 27.554551 days (27d 13h 18m 33.2s), on average, between two successive passages of the Moon through the perigee of its orbit.
broader: lunar-theory

High-velocity clouds
definition: A population of neutral or partly ionized gas clouds in the Galactic halo which are seen as high-altitude structures in the atomic hydrogen 21 cm emission at high radial velocities (v_LSR > 100 km/sec). They have substantial neutral column densities (> 10^19 cm^-2) and their metallicities range from 0.1 to about 1.0 times solar. The distances to the majority of them remain unknown. They may represent the continuing infall of matter onto the Local Group.
type: Concept
broader: interstellar-clouds

Stellar populations
broader: stellar-astronomy
type: Concept

Radio telescopes
definition: A telescope whose receiver is sensitive to radio waves.
type: Concept
broader: telescopes

Diffuse interstellar clouds
broader: interstellar-clouds
type: Concept
definition: An interstellar cloud in which hydrogen is completely dissociated and which is less dense and dusty than molecular clouds. In diffuse interstellar clouds photoabsorption of the background ultraviolet (UV) radiation field is an important dissociating and ionizing process. Typical densities and temperatures of diffuse clouds are 10^2 to 10^3 cm^-3 and 20 to 100 K respectively. Because of modest extinctions (≤ 1 mag), photodissociation processes are important in diffuse clouds preventing the formation of larger molecules.

Anthropic principle
type: Concept
definition: The idea that the existence of life and, in particular, our presence as intelligent observers, constrains the nature of the Universe. It is an attempt to explain the observed fact that the fundamental constants of nature are just right or fine-tuned to allow the Universe and life to exist.
broader: cosmology

Lc variable stars
type: Concept
broader: irregular-variable-stars

Cold neutral medium
type: Concept
broader: interstellar-phases

Hot Jupiters
type: Concept
definition: A giant, gaseous, Jupiter-like planet lying too close to its parent star and having an orbital period from a few days to a few weeks. The existence of hot Jupiters is usually interpreted in terms of planetary migration. These planets can, in principle, be formed at larger distances from their stars and migrate to the inner regions due to dynamical interaction with the proto-planetary disk.
broader: exoplanets

Ultraviolet surveys
type: Concept
broader: surveys

Trans-Planckian physics
type: Concept
broader: black-hole-physics

Armillary spheres
broader: early-astronomical-instruments
type: Concept
definition: An ancient instrument, used since ancient times until the Middle ages and later, to determine positions of celestial bodies. It consisted of an assemblage of rings, all circles of the same sphere, designed to represent the positions of the important circles of the celestial sphere.

Magnetospheric radio emissions
broader: exoplanet-detection-methods
type: Concept
broader: astronomical-techniques
broader: planetary-magnetosphere

Aerolites
broader: meteorites
type: Concept
definition: A stony meteorite consisting of silicate minerals. This alternative name for a stony meteorite, is now largely obsolete.

Polar orbit
type: Concept
broader: orbital-motion
definition: A spacecraft orbit that passes over, or close to, the geographic poles of the Earth or some other solar system object.

Cassegrain telescopes
type: Concept
definition: A reflecting telescope whose primary mirror has a hole bored through the center to allow the reflected light from the convex secondary mirror be focused beyond the back end of the tube.
broader: reflecting-telescopes

Magellanic Clouds
type: Concept
definition: Two irregular satellite galaxies of our own Galaxy which are visible from the Southern Hemisphere as misty patches in the night sky.
related: Milky Way Galaxy
broader: local-group

Modified Newtonian dynamics
type: Concept
broader: non-standard-theories-of-gravity

Neural networks
type: Concept
broader: classification

Expanding universe
broader: cosmological-models
type: Concept
definition: The deduction based on the observational fact that the greater the distance to a galaxy, the greater the redshift in its spectral lines (Doppler effect). The observations strongly indicate that galaxies appear to be moving away from us with speeds proportional to their distance. This is in agreement with the overall expansion of the Universe.

Photographic astrometry
type: Concept
broader: astrometry

Robust regression
broader: regression
type: Concept

Volcanism
broader: planetary-dynamics
type: Concept

Solar system astronomy
type: Concept

E-folding time
definition: The time within which the amplitude of an oscillation increases or decreases by a factor e (= 2.71828...).
broader: cosmic-inflation
type: Concept

Population II Cepheid variable stars
broader: population-ii-stars
type: Concept

Interstellar medium wind
broader: stellar-interstellar-interactions
type: Concept

Main-belt comets
type: Concept
broader: comets

Solar magnetic reconnection
type: Concept
broader: solar-magnetic-fields

Lunar geodesy
type: Concept
broader: selenology

Methane
type: Concept
definition: A colorless, odorless, inflammable gas of formula CH4; the simplest hydrocarbon.
broader: greenhouse-gases

Barred spiral galaxies
broader: spiral-galaxies
type: Concept
definition: A spiral galaxy that exhibits a bar-shaped structure in its nucleus.

Quasar absorption line spectroscopy
broader: intergalactic-medium
type: Concept

Mean anomaly
broader: orbital-anomalies
type: Concept
definition: The angle between the periapsis of an orbit and the position of a hypothetical body that orbits in the same period as the real one but at a constant mean angular velocity.

Comet origins
related: Comets
type: Concept
broader: planetary-science

Type Ib supernovae
definition: A Type I supernova that has neutral helium line (He I) at 5876 Å, and no strong silicon (Si II) absorption feature at 6150 Å. Type Ib supernovae are believed to result from the evolution of massive stars.
broader: core-collapse-supernovae
type: Concept

Boron abundance
deprecated: :__
type: Concept

Survival analysis
type: Concept
broader: astrostatistics-strategies

Classical T Tauri stars
definition: A T Tauri star in which accretion from a circumstellar disk is responsible for ultraviolet and infrared excess emission and for a moderate to strong emission line spectrum superimposed on the photospheric spectrum. Classical T Tauri stars probably evolve into weak-line T Tauri stars when their disks are fully accreted by the stars.
broader: t-tauri-stars
type: Concept

Yoke mounts
type: Concept
deprecated: :__

Galaxy fountains
broader: galaxy-processes
broader: galaxy-properties
type: Concept
related: Milky Way Galaxy fountains

Giant radio galaxies
type: Concept
broader: radio-galaxies

Reverberation mapping
broader: astronomical-techniques
type: Concept

Natural satellite evolution
broader: planetary-science
type: Concept

Coma Cluster
definition: The nearest rich cluster of galaxies which contains more than a thousand known galaxies, is about 20 million light-years in diameter, and lies about 280 million light-years away in the constellation Coma Berenices.
broader: galaxy-clusters
type: Concept

Fast Fourier transform
broader: period-search
type: Concept

Near infrared astronomy
broader: infrared-astronomy
type: Concept

Cirrus floccus
type: Concept
deprecated: :__

Pallas
type: Concept
broader: main-belt-asteroids

Finding charts
broader: astronomical-reference-materials
type: Concept
definition: A sketch or image used to recognize objects in the field of view of a telescope.

Bode's law
type: Concept
definition: The empirical rule relating the approximate distances of the solar system planets from the Sun. The original formulation was: a = (n + 4) / 10, where a is the mean distance of a planet from the Sun in astronomical units and n = 0, 3, 6, 12, 24, 48, 96, 192 (doubling for each successive planet). The planets were seen to fit this sequence quite well, provided the asteroids between Mars and Jupiter are counted as one planet, as did Uranus discovered in 1781. However, Neptune and the ex-planet Pluto do not conform to the rule. The question of whether there is any physical significance to the 'law,' i.e. some dynamical reason that will explain planetary orbit spacing has led to much discussion during the past two centuries. Today, many astronomers are very skeptical and consider this 'law' to be numerical coincidence.
broader: orbital-theory

Astrophysical explosive burning
type: Concept
broader: nuclear-astrophysics

Neutrino telescopes
type: Concept
broader: telescopes

Orbital resonances
type: Concept
broader: orbits
broader: gravitational-interaction

Geomagnetic fields
broader: magnetic-fields
type: Concept

North galactic pole
type: Concept
broader: galactic-poles

Ansae
type: Concept
broader: planetary-rings
definition: The 'handles,' or extremities, of planetary rings, especially Saturn's, as viewed from Earth.

Fullerenes
type: Concept
broader: molecular-physics

Phase effect
type: Concept
broader: astrometry

Semi-barium stars
broader: barium-stars
type: Concept

Wide-field telescopes
type: Concept
broader: telescopes

Solar coronal seismology
broader: solar-corona
type: Concept