Collision physics
Intermediate polar stars
DC stars
The Unified Astronomy Thesaurus (UAT) is an open, interoperable and community-supported thesaurus which unifies existing, divergent, and isolated controlled vocabularies in astronomy and astrophysics into a single high-quality, freely-available open thesaurus formalizing astronomical concepts and their inter-relationships. The UAT builds upon the IAU Thesaurus with major contributions from the Astronomy portions of the thesauri developed by the Institute of Physics Publishing and the American Institute of Physics. The Unified Astronomy Thesaurus will be further enhanced and updated through a collaborative effort involving broad community participation.
Astronomy
The Unified Astronomy Thesaurus
Absolute magnitude
Explosive astronomical burning
Explosive astrophysical burning
Astrophysical explosive burning
Main sequence stars
Major axis
Maksutov telescopes
Manganese stars
Mantle
Markarian galaxies
Mars
Mars trojans
Deimos
Martian moons
Moons of Mars
Phobos
Martian satellites
Astrophysical fluid dynamics
Mass concentration
Mascon
Mass-to-light ratio
Mass-ratio
Mass ratio
Mass spectrometers
Matter density
Prolonged sunspot minimum
Maunder minimum
Maximal Kerr black holes
Me stars
Me variable stars
Mean anomaly
Astrophysical magnetism
Mean density
Mean mass density
Mean matter density
Medium band photometry
Megalithic astronomy
Megamasers
Mercury manganese stars
Mercury-manganese stars
Meridian
Transit circles
Meridian circles
Mesogranulation
Mesopause
Super-radiant emission
Superradiant emission
Astrophysical masers
Mesosphere
Metal line absorbers
Meteor radiants
Meteor showers
Meteor streams
Trail of meteors
Trails of meteors
Meteor trails
Composition of meteorites
Meteorite composition
Meteorites
Meteoric clouds
Meteoric dust
Meteoroid dust clouds
Astrophysical processes
Meteoroids
Meteors
C h4
C h_4
Methane
Michelson interferometers
Michelson interferometry
Microgravity
Micrometeorites
Micrometeoroids
Astrophysicists
Disc of the Milky Way
Disk of the Milky Way
Milky Way Galaxy disc
Milky Way Galaxy disk
Milky Way disc
Milky Way galactic disc
Milky Way galactic disk
Milky Way disk
Dynamics of the Milky Way
Milky Way Galaxy dynamics
Milky Way dynamics
Evolution of the Milky Way
Milky Way Galaxy evolution
Milky Way evolution
Formation of the Milky Way
Milky Way Galaxy formation
Milky Way Galaxy origin
Milky Way origin
Origin of the Milky Way
Milky Way formation
Milky Way
Our galaxy
The Galaxy
Milky Way Galaxy
Milky Way Galaxy fountains
Milky Way Galaxy physics
Magnetic fields in the Milky Way
Milky Way Galaxy magnetic fields
Milky Way magnetic fields
Mass of the Milky Way
Milky Way Galaxy mass
Milky Way mass
Milky Way Galaxy rotation
Rotation of the Milky Way
Milky Way rotation
Astrosphere-interstellar medium interactions
Astrosphere/interstellar medium interactions
Astrosphere interstellar medium interactions
Galactic halo of the Milky Way
Halo of the Milky Way
Milky Way Galaxy halo
Milky Way galactic halo
Milky Way halo
Milky Way stellar halo
Millimeter astronomy
Millisecond pulsars
Mini Neptunes
Minor axis
Mira variables
Miras
Omicron ceti stars
Zeta aurigae stars
Mira variable stars
Missing mass
M o n d
Modified Newtonian dynamics
Astrospheres
Moldavites
H2 clouds
Molecular clouds
Molecular gas
Cluster parallax
Moving cluster parallax
Moving clusters
Multicolor photometry
Multi-color photometry
Multiperiodic pulsation
Multi-periodic pulsation
Multiperiodic variable stars
Multi-periodic variable stars
AGB
Asymptotic giant branch
Multiple stars
N body problem
N-body problem
N-body simulations
N galaxies
N stars
Nadir
Naked singularities
Narrow band photometry
Planet satellites
Planetary satellites
Secondary planets
Ataxites
Nautical almanacs
NFW
NFW profile
NFW-profile
Navarro-Frenk-White profile
NECs
NEOs
Near-Earth asteroids
Near-Earth comets
Near-Earth objects
Near infrared astronomical observations
Near infrared telescopes
Near ir astronomical observations
Near ir astronomy
Near-infrared astronomical observations
Near-infrared astronomy
Near-infrared telescopes
Near-ir astronomical observations
Near-ir astronomy
Near infrared astronomy
Near ultraviolet astronomical observations
Near ultraviolet telescopes
Near uv astronomical observations
Near uv astronomy
Near-ultraviolet astronomical observations
Near-ultraviolet astronomy
Near-ultraviolet telescopes
Near-uv astronomical observations
Near-uv astronomy
Near ultraviolet astronomy
Nebula
Nebulae
Neptune
Neptune trojans
Moons of Neptune
Neptunian moons
Nereid
Neptunian satellites
HI clouds
Neutral atomic hydrogen clouds
Neutral hydrogen clouds
Aten asteroids
Atens
Aten group
Neutrino astronomy
Neutrino decoupling
Neutrino masses
Neutrino oscillations
Neutrino telescopes
Neutron interferometry
Neutron star core
Neutron star cores
Neutron stars
Newtonian cosmology
Apohele asteroids
Apohele group
Atiras
Atira group
Newtonian gravitation
Newtonian telescopes
Night sky background
Night sky brightness
Night vision
Laughing gas
N2O
Sweet air
Nitrous oxide
Non-Gaussianity
Nonradial pulsation
Non-radial pulsations
Non-standard theories of gravity
Non-thermal sources
Nonthermal sources
Non-thermal radiation sources
Atmospheric circulation
Non-classical interferometry
Nonclassical interferometry
Non-rotating black holes
Nonrotating black holes
Nonstandard evolution
N.C.P.
NCP
North celestial pole
N.E.P.
NEP
North ecliptic pole
N.G.P.
NGP
North galactic pole
Nova-like variables
Nova-like variable stars
Novae
Nuclear abundances
Nuclear astrophysics
Atmospheric effects
Nucleated dwarf galaxies
Element formation
Formation of elements
Nucleosynthesis
O subgiants
O subgiant stars
O-association
O associations
O-clusters
O clusters
O dwarfs
O dwarf stars
O giants
O giant stars
Class O stars
O-type stars
O stars
O subdwarfs
O subdwarf stars
O supergiants
O supergiant stars
Atmospheric extinction
Refers to earth atmosphere
OB associations
OB stars
OB supergiants
OB supergiant stars
Ellipticity
Flattening
Oblateness
Oblique rotators
Imaging of stars
Stellar imaging
Observational astronomy
Observational cosmology
Observatories
Occultation
Occulting discs
Occulting disks
Atmospheric refraction
Ocean-atmosphere interactions
Water worlds
Ocean planets
Diurnal
Semi-diurnal
Ocean tides
Oe stars
Oef stars
Of type stars
Of-type stars
Of stars
OH-IR stars
OH/IR stars
Oort Cloud
Opik-Oort Cloud
Öpik–Oort cloud
Oort cloud
OCOs
Oort cloud objects
Oort constants
Atmospheric science
Open clusters
Open star clusters
Open universe
Opik's theory
Öpik theory
Öpik's theory
Opik theory
Opposition
Refers to planet position
Optical outbursts
Outbursts
Optical bursts
Optical double stars
Use only for pair in chance alignment; refer also to visual binary stars
Optical flares
Optical counterparts
Optical identification
Light interferometry
Stellar interferometry
Optical interferometry
Optical observation
Terrestrial scintillation
Atmospheric scintillation
Optical observatories
Optical phase-shifting interferometry
Phase shifting interferometry
Phase-shifting interferometry
Optical phase shifting interferometry
Optical position
Optical pulsars
Optical telescopes
Orbit determination
Apparent anomaly
Orbital anomalies
Keplerian elements
Orbital elements
Orbital evolution
Orbital motion
Atmospheric tides
Line of node
Orbital nodes
Orbital resonance gaps
Orbital resonances
Orbital theory
Orbiters
Orbital dynamics
Orbits
Formation of the universe
Origin of the universe
Orreries
Oscillating universe
Osculating orbit
Atomic interferometry techniques
Osculatory elements
Oxygen burning
Ozone layer
P-process
Parabolic orbit
Parallax
Parallel field
Parallax second
Parsec
Accelerating universe
Australites
Partial eclipses
Peculiar variable stars
Penrose limit
Penrose mechanism
Penrose process
Penumbras
Penumbra
Pericenter
Periapsis
Periastron
Perigee
Robotic telescope
Automated telescopes
Perihelion
Period determination
Periodic orbit
Periodic variable stars
Perseus Cluster
Perturbation theory
Perturbation methods
GW Virginis stars
Pg 1159-35 stars
PG 1159 stars
Phase angle
Phase defect
Phase effect
Automated patrol telescopes
Automatic patrol telescopes
Phase error
Phase switching
Philosophy of astronomy
PDRs
Photodissociation regions
Photoelectric magnitude
Photoelectric photometry
Stereophotogrammetry
Photogrammetry
Photographic astrometry
Photographic magnitude
Photographic photometry
Autumnal equinox
Photoheliographs
Photometric parallax
Photometric standard stars
Photometric systems
Photometry
Photon decoupling
Photon sphere
Stellar photospheres
Photovisual magnitude
Pick-up ions
Pickup ions
Bearing
Azimuth
Plages
Planet formation
Planet hosting stars
Planetary alignment
Planetary atmospheres
Planetary boundary layers
Planetary bow shocks
Planetary cores
Planetary interior
Planetary nebulae
B subgiants
B subgiant stars
Nuclei of planetary nebulae
Nucleus of planetary nebula
Planetary nebula nuclei
Planetary nebulae nuclei
Planetary polar regions
Planetary probes
Cassini ring division
Cassini's Division
Saturn ring division
Planetary ring divisions
Circumplanetary dust
Circumplanetary matter
Planetary rings
Planetology
Planetary science
Planetary structure
Planetary system formation
Planetary theory
Planetesimals
B dwarfs
B dwarf stars
Planets
Solar system planets
Plasma astrophysics
Plasma clouds
Plasma jets
Plasmapause
Continental drift
Plate tectonics
Plutinos
Pluto
Plutoids
Pogson's scale
Pogson scale
B giants
B giant stars
Pointing accuracy
Pointing error
Polar ice caps
Polar icecaps
Polar caps
Polar jets
Polar orbit
Antarctic barotroclinic tides
Antarctic barotropic tides
Antarctic tides
Arctic barotroclinic tides
Arctic barotropic tides
Arctic tides
Polar tides
Polarimetric instrumentation
Polarimetric instruments
Polarimeters
Polarimetry
B type stars
B-type stars
Class B stars
Type-B stars
B stars
Polycyclic aromatic hydrocarbons
Polytropic solutions
Polytropic stars
Polytropes
Population I stars
Pop II
Population II stars
First stars
Metal free stars
Population III stars
Position angle
Post-AGB
Post-asymptotic giant branch
pp-wave
pp-wave spacetimes
pp waves
Pre-main sequence
B subdwarfs
B subdwarf stars
PMS objects
PMS stars
Pre main sequence objects
Pre-main sequence objects
Pre-main sequence stars
Presolar nebula
Pre-solar nebulae
Primordial black holes
Primordial galaxies
Primordial magnetic fields
Stellar spatial motions
Proper motions
Proplyds
Protoclusters
Protogalaxies
Proton proton cycle
Proton proton reaction
Proton-proton cycle
Proton-proton reaction
B supergiants
B supergiant stars
Protoplanetary disks
Preplanetary nebulae
Protoplanetary nebula
Protoplanetary nebulae
Protostars
Ptolemy stones
Pulsar planets
Pulsar timing method
Pulsars
Pulsating variable stars
Pulsation frequency method
Pulsation modes
Baade Wesselink method
Expanding photosphere method
Expanding-photosphere method
Baade-Wesselink method
Purkinje shift
Purkinje effect
PV Telescopii
PV Telescopii variables
PV Telescopii variable stars
Quadrants
Quantum cosmology
Quantum gravity
Quasar-galaxy pairs
Quasar absorption line spectroscopy
Quasar microlensing
QSO
Quasi-stellar galaxies
Quasi-stellar object
Quasi-stellar radio sources
Quasars
Quasinormal modes
Polar crown filament
Quiet sun
Quintessence
R-process
R-association
R associations
R Coronae Borealis
R Coronae Borealis stars
R Coronae Borealis variables
R c b stars
R cor bor stars
R-Coronae Borealis variables
R Coronae Borealis variable stars
Class R stars
R-type stars
R stars
Radar astronomy
Bailys beads
Baily's beads
Radar telescopes
Radial pulsations
Astrometric radial velocity
Radial-velocity method
Spectroscopic radial velocity
Radial velocity
Radiative braking
Radiative transfer
Radiative transfer equation
Radio astrometry
Radio astronomy
Radio bursts
Baker-Nunn cameras
Radio continuum
Radio continuum emission
Radio cores
Radio galaxies
Radio hot spots
Radio interferometers
Radio interferometry
See radio interferometer
Radio jets
Radio lobes
Radio source feature
Radio-loud quasars
Radio loud quasars
Ba II stars
Barium stars
Radio observatories
Radio occultation
Radio plumes
Radio pulsars
Radio-quiet quasars
Radio quiet quasars
Radio receivers
MICA ICRF Radio Source Catalog
RADIO Master Catalog
Radio source catalogues
Radio source catalogs
Radio source counts
Radio lines
Radio sources
Radio spectroscopy
SB galaxies
Barred spiral galaxies
Radio telescopes
Radioheliographs
Rapid bursts
RoAp oscillations
Rapid stellar oscillations
Recombination (cosmology)
Recurrent novae
Red dwarf flare stars
Red giant branch
Red bump
Red giant bump
Matter-antimatter asymmetry
Baryogenesis
Red clump
Red giant clump
Red giant tip
Red giant branch stars
Red giants
Red giant stars
Red sequence galaxies
Red stragglers
Red-straggler stars
Red straggler stars
Red supergiants
Red supergiant stars
Reddened stars
Reddening law
Galaxy redshifts
Galaxy surveys
Redshift surveys
Red shift
Redshift
Redshifted
Baryon acoustic oscillations
Catoptric telescopes
Reflector telescopes
Reflecting telescopes
Refers to optical telescope
Reflection nebula
Reflection nebulae
Dioptrics
Refracting telescopes
Re-ionisation
Re-ionization
Reionisation
Reionization
Reissner Nordstrom black holes
Reissner Nordström black holes
Relativistic aberration
Relativistic binaries
Relativistic binary
Relativistic binary stars
Relativistic cosmology
Relativistic discs
Relativistic disks
Relativistic fluid dynamics
Baryon density
Relativistic jets
Relativistic mechanics
Relativistic stars
Relativistic astrophysics
Relativistic models
Relativity
Relaxation (physics)
Relaxation time
Remote telescope astrophotography
Resonant Kuiper-belt objects
Resonant Kuiper belt objects
Red-green-blue photometry
RGB photometry
RGU photometry
Celestial longitude
Right ascension
Particle accretion
Accretion
Baryonic dark matter
Ring galaxies
Ring nebula
Ring nebulae
Ring singularities
Roche radius
Roche limit
Roche lobe
Rotating black holes
Rotating shells
Rotation-powered pulsars
Rotation powered pulsars
Rovers
Bautz-Morgan class
Bautz-Morgan type
Cluster type variable stars
RR Lyrae
RR Lyrae type variable stars
RR Lyrae variables
RR Lyrae variable stars
RS Canum Venaticorum
RS Canum Venaticorum stars
RS Canum Venaticorum variables
RS-Canum Venaticorum variables
RS Canum Venaticorum variable stars
Runaway stars
RV Tau
RV Tauri
RV Tauri stars
RV Tauri variables
RV Tauri variable stars
S process
Slow-neutron-capture process
S-process
Be stars
P Cygni stars
S Doradus stars
S type stars
S-type stars
Se stars
Type-S stars
S stars
Saros series
Saros cycle
Saturn
Moons of Saturn
Ring shepherds
Saturnian moons
Saturnian satellites
STVG
Scalar-tensor-vector gravity
Scale height
SDOs
Scattered disk objects
Schmidt-Cassegrain telescopes
Schmidt telescopes
Schmidt cameras
Schwarzschild black holes
Schwarzschild solution
Schwarzschild vacuum
Schwarzschild metric
Gravitational radius
Schwarzschild radius
Limiting radius of black hole
Sculptor dwarf elliptical galaxy
Seasonal phenomena
Secondary cosmic rays
Secular parallax
Bediasites
Geology of the Moon
Lunar geology
Selenodesy
Selenology
Semibarium stars
Semi-barium stars
Semidetached binary stars
Semi-detached binary stars
Semiregular variable stars
Semi-regular variable stars
Semi-major axis
Semimajor axis
Semi-minor axis
Semiminor axis
Seyfert galaxies
Shadow bands
Shell burning
Nuclear fusion in shell
Shepherd moons
Shepherd satellites
Periodic comets
Short period comets
Short period variable stars
Siderostats
Sigma 8
Sigma-8
Sigma8
Silicate grains
Silicon burning
SiO masers
Silicon-monoxide masers
Silicon monoxide masers
Silicon stars
Single-dish antennas
Sky brightness
Sky noise
Sky surveys
S d ss photometry
Sloan photometry
Slow irregular variables
Slow irregulars
Slow irregular variable stars
Novoid
Slow novae
S m c
Small Magellanic Cloud
SSSBs
Bessel UB VRI photometry
Bessel photometry
Sociology of astronomy
Soft gamma repeaters
Soft gamma-ray repeaters
Solar planetary interactions
Solar planetary relation
Solar-planetary relation
Solar/planetary interactions
Solar-planetary interactions
Solar/terrestrial relations
Solar-terrestrial interactions
Solar abundances
Solar activity
Astrophysics of the sun
Solar astronomy
Specifically the study of the Sun.
The Sun is not primarily thought of as a subclass of star, because it is discussed with different words in a different literature by a different group of scientists. - jegpeek
Solar atmosphere
Solar atmospheric motions
Solar chromosphere
Beta canis majoris stars
Solar constant
Solar core
Solar corona
Solar coronal holes
Solar coronal loops
Solar coronal streamers
Solar activity cycles
Solar cycle variations
Solar magnetic activity cycle
Sun cycle
Sun cycles
Solar cycle
Solar eclipses
Solar electromagnetic emission
Energetic solar particles
Suprathermal particles
Solar energetic particles
Evolution of the sun
Solar evolution
Solar EUV emission
Solar extreme ultraviolet emission
Solar facula
Solar faculae
Solar flocculi
Solar flocculus
Solar filaments
A solar prominence, but seen on the disk.
Solar flares
Solar gamma-ray emission
Solar γ-ray emission
Solar granulation
Solar instrumentation
Solar instruments
Achondrites
Bianchi cosmology
Solar interior
Solar macrospicules
Solar magnetic fields
Solar magnetic reconnection
Solar mass
Solar motion
With respect to nearby stars
Solar nebula
Solar nebulae
Solar neighbourhood
Solar neighborhood
Nucleogenesis
Big Bang nucleosynthesis
Solar neutrino problem
Solar neutrinos
Solar neutrons
Solar observatories
Solar optical telescopes
Solar oscillations
Solar parallax
Solar particle emission
Solar photosphere
Prominence
Solar eruptions
Solar prominence
Solar prominence eruptions
Solar prominences
A solar filament, but seen above the limb.
Big Bang theory
Solar radiation
Solar radio emission
Solar radio telescopes
Solar rotation
Solar spicules
Solar storm
Solar surface
Solar system
Planetary astronomy
Solar system astronomy
Referring specifically the study of our solar system, not exoplanets.
Binary pulsars
Solar system formation
Tower telescope
Solar telescopes
Solar transition region
Solar UV emission
Solar ultraviolet emission
Solar wind
Solar wind termination
Solstice
S c p
S. c. p.
South celestial pole
S e p
S. e. p.
South ecliptic pole
Binaries
Binary stars
S g p
S. g. p.
South galactic pole
Space astrometry
Debris
Space debris
Orbiting observatory
Space observatories
Astrophysical plasmas
Space plasmas
Probe
Space probes
Space science
Space research
Space based telescopes
Space-based telescopes
Space telescopes
Space vehicle instrumentation
Space vehicle instruments
Space vehicles
Bipolar nebula
Bipolar nebulae
Spacetime metric
Special theory of relativity
Special relativity
Speckle interferometry
Spectral index
Spectrometers
Spectrophotometric standards
Spectrophotometry
Spectroscopic binaries
Spectrum binary stars
Spectroscopic binary stars
Astronomical spectroscopy
Spectroscopy
Spiral arms
Bipolar sunspot groups
S galaxies
Spiral galaxies
Galactic pitch angle
Spiral pitch angle
Spokes
Standard candles
Standard stars
Star forming regions
Star atlases
Cluster of stars
Star clouds
Star clusters
Star distribution
Stellar statistics
Star counts
Star formation
Starburst galaxies
Star light polarization
Starlight polarisation
Starlight polarization
Star spots
Starspots
Statistical parallax
Steady state theory
Steady-state theory
Stellar-interstellar interactions
Stellar abundances
Stellar accretion
Stellar accretion discs
Stellar accretion disks
BL Lac objects
BL Lacertae objects
Stellar activity
Stellar ages
Star associations
Stellar associations
Stellar astrophysics
Stellar astronomy
Stellar atmospheres
Stellar atmospheric opacities
Stellar opacities
Stellar opacity
Stellar atmospheric opacity
Solar bow shock
Stellar bow shocks
Stellar bridges
Between galaxies
Spectra classification
Spectral classification
Spectral type
Spectrum classification
Stellar spectral classification
Stellar spectrum classification
Stellar classification
Black hole physics
Stellar colours
Stellar colors
Stellar cores
Stellar diffusion
Cosmic distance ladder
Cosmic distance scale
Extragalactic Distance Scale
Stellar distance
Stellar dynamics
Stellar effective temperatures
Evolution of stars
Star cycle
Stellar cycle
Stellar evolution
AGN
Active galactic nuclei
Spin-flip
Black hole spin-flip
Stellar evolutionary tracks
Stellar facula
Stellar faculae
Stellar feedback
Stellar flares
Stellar gas distribution
Stellar inner cores
Central region of stars
Stellar interiors
Bipolar outflows
Jet outflows
Stellar jets
Stellar kinematics
Salpeter IMF
Salpeter function
Stellar luminosity
Stellar luminosities
Thermodynamics of black holes
Black hole thermodynamics
Stellar magnetospheres
Stellar magnetic fields
Stellar black holes
Stellar mass black holes
IMF
Stellar mass functions
Stellar mass loss
Stellar masses
Stellar motion
Stellar nucleosynthesis
Stellar oscillations
Stellar parallax
Stellar phenomena
Black holes
Stellar photometry
Physics of stars
Stellar physics
Stellar populations
Stellar processes
Stellar characteristics
Stellar magnitude
Stellar properties
Stellar pulsations
Stellar radius
Stellar radii
Late stage stellar evolution
Late stages of stellar evolution
Late-stage stellar evolution
Stellar remnants
Stellar rings
Stellar rotation
Stellar spectral lines
Stellar structures
Stellar surfaces
Stellar tracking devices
Stars
Stellar types
Interstellar bubbles
Stellar-wind bubbles
Stellar wind bubbles
Stellar outflows
Stellar winds
Stellar x-ray flares
Stonehenge
Stone circles
Mesopeak
Stratopause
Blazars
Stratosphere
Four-color photometry
Stromgren uvby system
Uvby system
Stromgren spheres
Strömgren spheres
Strong gravitational lensing
Strontium stars
SU Ursae Majoris stars
Subgiants
Subgiant stars
Submillimeter astronomy
Sub-stellar companion stars
Sub-stellar companions
Substellar companions
Substellar companion stars
Summer solstice
Blue compact dwarf galaxies
Sunspot cycle
Sunspot groups
Relative sunspot number
SSN
Wolf number
Sunspot number
Sun spots
Sunspots
SZ effect
Sunyaev-Zeldovich effect
Super-Earths
Super Earths
Superbubbles
Superclusters
Supergalactic coordinate system
Supergalactic latitude
Blue cloud galaxies
Supergalactic longitude
Supergiant stars
Supergranulation
Supermassive black holes
Supernova dynamics
Supernova neutrinos
Supernova remnants
Supernovae
Acceleration of gravity
Surface gravity
Blue loop
Area photometry
Surface photometry
Surveys
Helium variable stars
SX Arietis stars
SX Arietis variables
SX Arietis variable stars
SX Phoenicis stars
SX Phoenicis variables
SX-Phoenicis variables
SX Phoenicis variable stars
Symbiotic binaries
Symbiotic stars
Symbiotic binary stars
Symbiotic novae
Synodic period
Syzygy
Conjunction or opposition of planets
T-association
T associations
T dwarf stars
T dwarfs
Blue stragglers
Blue straggler stars
T Tauri stars
Head tail galaxies
Tailed radio galaxies
Technetium stars
Observation techniques
Tektites
Telescope focal plane photography
Telescope mountings
Telescope mounts
Telescopes
Blue shift
Blueshift
Blueshifted
Termination shock
Tertiary stars
Earth's Moon
Luna
The Moon
The Sun
Thermosphere
Three body problem
Three-body problem
Tidal filament hypothesis
Tidal disruption
Tidal bulge
Tidal distortion
Tidal friction
Tidal interaction
Active galaxies
Bodes law
Titius-Bode law
Bode's law
Tidal radius
Tidal tails
Tides
Timing variation methods
Total eclipses
Kuiper belt objects
TNO
Trans Neptunian objects
Transneptunian objects
Trans-Neptunian objects
Trans-Planckian physics
Transit duration variation method
Transit instruments
Bok globules
Transit timing variation method
Astronomical transits
Transits
M33
Messier 33
NGC 598
Triangulum Galaxy
Trigonometric parallax
Ternary stars
Triple stars
Trinary stars
Trojan group
Trojans
Trojan asteroids
Trojan planets
Tropopause
Troposphere
True anomaly
Bolides
True position
Turnoff point
Twilight
Two body problem
Two-body problem
Color-color diagrams
Two color diagrams
Two-color diagrams
2D gravity
Two dimensional gravity
Two-dimensional gravity
Twotinos
Type Ia supernova
Type Ia supernovae
Type Ib supernovae
Bolometric correction
Type Ic supernovae
Type II supernovae
SS cygni stars
U Geminorum stars
Ultra high energy cosmic radiation
Ultra high energy cosmic rays
Ultra-high-energy-cosmic rays
Ultra-high-energy cosmic radiation
Ultracompact dwarf galaxies
ULIRG
Ultraluminous infrared galaxies
UV astronomy
Ultraviolet astronomy
Ultraviolet colour
Ultraviolet color
Ultraviolet extinction
In earth's atmosphere
Ultraviolet observatories
Bondi accretion
Ultraviolet photometry
UV sources
Ultraviolet objects
Ultraviolet sources
Ultraviolet surveys
UV telescopes
Ultraviolet telescopes
Umbra
SA galaxies
Unbarred spiral galaxies
Uncharged black holes
Upper atmosphere
Earth's atmosphere
Upsilon Sagittarii stars
Moons of Uranus
Oberon
Titania
Uranian moons
Uranian satellites
Uranus
Urca process
Ursa Minor dwarf spheroidal galaxy
UU Herculis stars
UV Ceti
UV-Ceti
UV Ceti stars
UV Persei stars
UX Ursae Majoris stars
Radiation belts
Variable radiation sources
Period variation
Period-change
Periodically variable
Periodically-variable
Variable star period change
Variables
Variable stars
Venus
Vernal equinox
Vertical circle
VLA
Very Large Array
VLT
Very Large Telescope
VLB arrays
Very long baseline interferometers
VLB interferometry
VLBI
Very long baseline interferometry
Bouwers telescope
Very small grains
Vilnius photometry
Virgo Cluster
Local Supercluster
Virgo SC
Virgo Supercluster
Virtual observatories
Visibility function
Visible astronomical observations
Visible telescopes
Visual binaries
Visual binary stars
Voids
Bp stars
Volcanoes
von Zeipel's theorem
von Zeipel theorem
VY Sculpturis stars
W Ursae Majoris variables
W Ursae Majoris variable stars
W Virginis stars
W Virginis variables
W-Virginis variables
W Virginis variable stars
V b l uw photometry
Walraven photometry
WHIM
Warm hot intergalactic medium
Warm-hot intergalactic matter
Warm-hot intergalactic medium
WDM
Warm dark matter
DIG
Defuse ionized gas
Reynolds' layer
WIM
Warm ionized medium
WNM
Warm neutral medium
Brane cosmology
H2O masers
Water masers
Aqueous vapor
Aqueous vapour
Water vapour
Water vapor
Waveband photometry
WC stars
WCN stars
WTTS
Weak-line T Tauri stars
Weak G band stars
Weak gravitational lensing
White dwarfs
White dwarf stars
Active sun
Bright nebula
Bright nebulae
Wide field telescopes
Wide-field telescopes
Wide binary stars
Wilson effect
Winter solstice
WN stars
WR stars
Wolf Rayet stars
Wolf-Rayet stars
Wolf diagram
Einstein-Rosen Bridge
Wormholes
WZ Sge stars
WZ Sagittae stars
Brightest cluster galaxies
X-ray galaxies
X-ray astronomy
X-ray binaries
X-ray binary stars
X-ray bright point
X-ray bursters
X-ray bursts
X-ray detectors
X-ray identification
X-ray novae
X-ray observatories
Brightness temperature
X-ray photometry
X-ray quasars
X-ray sources
X-ray stars
X-ray surveys
X-ray telescopes
Xenobiology
Y dwarf stars
Y dwarfs
Rho Cassiopeiae
Rho cas stars
Yellow hypergiant stars
Yellow-straggler stars
BALQSO
Broad-absorption line quasar
Young disc cepheid
Young disk Cepheid stars
Young disk Cepheid variables
Young disk Cepheids
Young disk Cepheid variable stars
Young star clusters
YSO
Young stellar objects
Z Andromedae stars
Z Camelopardalis stars
ZDI
Zeeman Doppler imaging
Zenith
ZHR
Zenith hourly rate
Broadband photometry
Wide-band photometry
Broad band photometry
Zenith sectors
Zenith telescopes
PZT
Photographic zenith tube
Zenithal tubes
Zenith tubes
ZAHB
ZAHB stars
Zero-age horizontal branch stars
ZAMS
Zero-age main sequence
Zero-age main sequence stars
Zirconium stars
Zodiacal dust
Zodiacal dust cloud
Zodiacal light
Zodiacal cloud
Zone of totality
ZZ Ceti stars
DZ stars
DQ stars
Brown dwarf stars
Brown dwarfs
Shell stars
Transient sources
X-ray transient sources
Gamma-ray transient sources
Burkert profile
Burst astrophysics
BV photometry
BW stars
Adaptive interferometry
BY Draconis stars
Calendars
UCBVc BV photometry
Cape photometry
Cape RI photometry
CNO cycle
Carbon cycle
Carbon cycling
Carbon nitrogen cycle
Carbon-nitrogen cycle
Carbon burning
CO2
CO_2
Carbon dioxide
Carbon detonation
Carbon flashes
Carbon planets
Carbon stars
53 Persei stars
Ae stars
Carbonaceous chondrites
Carbonaceous grains
Cassegrain telescopes
Cataclysmic binaries
Cataclysmic binary stars
Cataclysmic variables
Cataclysmic variable stars
Catadioptric telescopes
Catalogues
Catalogs
Caustic-crossing
Caustic crossing
CCD observation
Charge-coupled device photometry
CCD photometry
Type-cD galaxies
cD-type galaxies
cD galaxies
Aerolites
Celestial equator
Celestial mechanics
Celestial objects catalogs
Celestial poles
Celestial sphere
Centre of mass
Center of mass
Cepheid distance
Bump cepheid
Cepheid stars
Cepheid variables
Cepheids
Classical Cepheid
Cepheid variable stars
Aeronomy
CH stars
Charged black holes
Chemical abundance
Element abundance
Chemical abundances
Chemical enrichment
CP stars
Metal-rich stars
Chemically peculiar stars
Chlorofluorocarbons
Chondrites
Chondrules
Alfven surface waves
Alfvén surface waves
Alfvén waves
Alfven waves
Stellar chromospheres
Cthonian planets
Chthonian planets
CII region
Circumstellar clouds
Circumstellar discs
Circumstellar disks
Circumstellar dust
Expanding circumstellar envelope
Stellar envelope
Circumstellar envelopes
Circumstellar gas
Circumstellar grains
Beta persei stars
Algol variable stars
Circumstellar masers
Circumstellar matter
Circumstellar shells
Classical black holes
All sky camera
All-sky cameras
Classical Kuiper-belt objects
Cubewanos
cubewanos
Classical Kuiper belt objects
Classical nova
Classical novae
C tt s
Classical T Tauri stars
Classification systems
Close binaries
Close binary stars
Close encounters
Closed universe
Cloud monitors
Cosmic microwave background radiation detectors
CMBR detectors
Almanachs
Almanacs
Carbon-nitrogen stars
CN stars
CNO anomaly
CO line emission
Cocoon stars
Coelostats
Cold dark matter
CNM
Cold neutral medium
Cloud collapse
Collapsing clouds
Collimated light
Collimation
Coma Cluster
Needs disambiguation or creation of at least two separate terms. Wikipedia, Coma (cometary): "In astronomy, a coma (from the Greek κόμη, "hair") is the nebulous envelope around the nucleus of a comet." WIkipedia, Coma (optics): "In optics (especially telescopes), the coma (aka comatic aberration) in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components which results in off-axis point sources such as stars appearing distorted, appearing to have a tail (coma) like a comet. Specifically, coma is defined as a variation in magnification over the entrance pupil. In refractive or diffractive optical systems, especially those imaging a wide spectral range, coma can be a function of wavelength, in which case it is a form of chromatic aberration." -BG
Comet tail streamers
Comet tails
Cometary globules
Cometary nebulas
Cometary nebulae
Alt-azimuth mounting
Altazimuth
Altazimuth mountings
Altazimuth mounts
Altitude-azimuth
Altitude-elevation mounts
Azimuth mounting
Alt-azimuth mounts
Comets
Compact dwarf galaxies
Compact object binaries
Compact binary stars
Compact galaxies
Compact H II region
Compact nebula
Compact nebulae
Compact stars
Compact objects
Compact radiation sources
Alternatives to inflation
Alternatives to cosmic inflation
Companion galaxies
Secondary stars
Companion stars
Comparison stars
Computational astronomy
Conical singularities
Conjunction
Constellations
Contact binary stars
A dwarfs
A dwarf stars
Elevation angle
Altitude angle
Cool cores
Cool intergalactic medium
Core collapse supernova
Core collapse supernovae
Core-collapse supernova
Core-collapse supernovae
Coronas
Stellar coronae
AM Canum Venaticorum stars
CME
Coronographic detection
Interplanetary co-rotating streams
Interplanetary corotating streams
Corotating streams
Cosmic anisotropy
Cosmical electrodynamics
Cosmic electrodynamics
Cosmological inflation
Inflation (cosmology)
Cosmic inflation
Polars
AM Herculis stars
Cosmic isotropy
Cosmic magnetic fields theory
CMB
CMBR
Cosmic microwave background
MBR
Relic radiation
Cosmic microwave background radiation
Cosmic noise
Cosmic ray astronomy
Cosmic ray detectors
Cosmic ray nucleosynthesis
Air shower
Auger shower
Cosmic ray showers
Cosmic ray objects
Cosmic ray sources
Cosmic radiation
Primary cosmic ray
Cosmic rays
Metallic line stars
Metallic-line stars
Am stars
Cosmic web
Chemical cosmology
Cosmic chemistry
Cosmochemistry
Nucleocosmochronology
Cosmochronology
Cosmogeny
Cosmogony
Lambda
Cosmological constant
Cosmological constant experiments
Astronomical evolution
Cosmological evolution
Classical tests of cosmology
Cosmological models
Cosmological neutrinos
Cosmological parameters
Amateur astronomers
Cosmological parameters from LSS
Cosmological parameters from large scale structure
Cosmological parameters from large-scale structure
Cosmological perturbation theory
Cosmological phase transitions
Physical cosmology
Cosmology
Curvature singularities
Curved space
D galaxies
D region
D layer
DA stars
Damped Lyman-alpha systems
Backyard astronomy
Stargazing
Amateur astronomy
Dark adaptation
Retinal adaption
Dark adaption
Dark energy
Absorption nebula
Absorption nebulae
Dark clouds
Dark nebula
Dark nebulae
Dark interstellar clouds
Dark matter
Dark matter density
Dark matter detectors
Dark matter distribution
DB stars
Van den Bergh classification
Van den Bergh classification of galaxies
DDO classification
Amor asteroids
Amor group
DDO photometry
de Sitter universe
De Vaucouleurs system
De Vaucouleurs-Sandage classification
De Vaucouleurs Sandage classification
Debris discs
Debris disks
Q-parameter
Deceleration parameter
Celestial latitude
Declination
Deep-space probes
Deep space probes
Degenerate matter
Type II irregular galaxies
Irregular II galaxies
Amorphous irregular galaxies
Al Velorum stars
Delta Scuti variables
Dwarf Cepheids
Delta Scuti variable stars
Dense interstellar clouds
Density parameters
Density wave theory
Density wave model
South node
Descending node
Detached binary stars
Detached objects
Diamond ring effect
Diffuse interstellar bands
Analytical mathematics
Translucent interstellar clouds
Diffuse interstellar clouds
Diffuse molecular clouds
Diffuse nebula
Diffuse nebulae
Diffuse sky radiation
Diffuse radiation
Diffuse x-ray background
Dirac cosmology
Direct-detection interferometry
Direct detection interferometry
Discrete radio sources
M31
Messier 31
NGC 224
Andromeda Galaxy
Disc flaring
Disk flaring
Disc galaxies
Disk galaxies
Distance indicators
Distance measure
Diurnal parallax
DO stars
Dobsonian telescopes
Telescope dome
Domes
Excluding lunar dome
A giants
A giant stars
Angular distance
Apparent distance
Apparent separation
Angular separation
Doppler-imaging
Doppler imaging
Doppler effect
Doppler shift
Double-mode Cepheid stars
Double-mode Cepheid variables
Double-mode Cepheids
Double-mode Cepheid variable stars
Double mode pulsation
Double-mode pulsation
Double mode variable stars
Double-mode variable stars
Binary galaxies
Double galaxies
Double quasars
Draco dwarf galaxy
Dredge up
Dredge-up
Annual equation
Drift scans
Drift scan imaging
Dumbbell galaxies
Infrared cirrus
Dust continuum emission
Dust nebula
Dust nebulae
Dust shells
Dwarf ellipticals
Dwarf elliptical galaxies
Dwarf galaxies
Dwarf irregular galaxy
Dwarf irregulars
Irregular dwarf galaxies
Dwarf irregular galaxies
Dwarf nova
U Geminorum-type variable stars
Dwarf novae
Solar system dwarf planets
Dwarf planets
Heliocentric parallax
Annual parallax
Dwarf spheroidals
Dwarf spheroidal galaxies
Dynamical evolution
Gravitational drag
Dynamical friction
Dynamical parallax
E+A galaxies
E-folding time
E components
Kennelly–Heaviside layer
E layer
Early type emission stars
Early-type emission stars
Early type galaxies
Early-type galaxies
Annular eclipses
Early type stars
Early-type stars
Early type supergiant stars
Early-type supergiants
Early-type supergiant stars
Early type variable stars
Early-type variable stars
Early astronomical instrumentation
Early astronomical instruments
Early stellar evolution
Early universe
Earth moon system
Earth-moon system
Terrestrial atmosphere
Earth atmosphere
Earth trojans
Planet Earth
Earth (planet)
Anomalistic month
Eccentric anomaly
Eccentricity
Eclipses
Eclipsing binary minima timing method
Eclipsing binaries
Eclipsing stars
Eclipsing variable stars
Photometric binary stars
Eclipsing binary stars
Ecliptic coordinate system
Ecliptic latitude
Ecliptic longitude
Ecliptic poles
Effective temperature
Ansa
Ansae
Einstein field equations
Einstein rings
Einstein universe
Ejecta
Ellipsoidal binary stars
Ellipsoidal variable stars
E galaxies
Elliptical galaxies
Eccentric orbit
Emerging flux tubes
Emission line galaxies
Antapex
Emission line stars
Emission nebulae
Ephemeral active regions
Ephemerides
Equation of the centre
Equation of the center
Equator position
Celestial coordinate
Equatorial system
Equatorial coordinate system
Equatorial mountings
Equatorial mounts
Equatorial zone
Antarctic observatories
Equinox
Ergosphere
Eruptive phenomena
Eruptive variables
Eruptive variable stars
Ethnoastronomy
Evaporating gaseous globules
Event horizons
Anthropic principle
Evolution equation
Post-main sequence stars
Evolved stars
Exoplanet systems
Exoplanet astronomy
Exoplanet atmospheres
Exoplanet catalogs
Exoplanet detection
Extrasolar planet detection
Extrasolar planetary system detection
Exoplanet detection methods
Antumbra
Exoplanet dynamics
Exoplanet evolution
Exoplanet formation
Exoplanet plate tectonics
Exoplanet rings
Exoplanet structure
Exoplanet surface characteristics
Exoplanet tides
Extrasolar planets
Exoplanets
Specifically planets and related bodies found orbiting stars that are not the Sun.
Exosphere
A-type stars
Class A stars
Type A stars
A stars
Peculiar A-type stars
Ap stars
Exo-zodiacal dust
Exozodiacal dust
Expanding stellar envelopes
Expanding universe
Explosive nucleosynthesis
Extended radiation sources
Extinction
Extragalactic astronomy
Extragalactic magnetic fields
Extragalactic radio sources
Apastron
Unidentified sources of radiation outside the Solar System
Extrasolar radiation
Rocky planets
Extrasolar rocky planets
Extreme carbon stars
Extreme horizontal branch stars
Extrinsic variable stars
F components
F dwarfs
F dwarf stars
F giants
F giant stars
Appleton layer
F layer
Earth upper atmosphere
Class F stars
F-type stars
F stars
F subdwarfs
F subdwarf stars
F subgiants
F subgiant stars
F supergiants
F supergiant stars
Faber Jackson distance
Used to measure distances to galaxies, not stars. This term should be moved.
Faber-Jackson distance
Etalons
FPI
Fabry-Perot interferometers
Faint blue stars
Fanaroff Riley class I
Fanaroff Riley class II
Fanaroff-Riley class I
Fanaroff-Riley class II
Fanaroff-Riley radio galaxies
FIR astronomy
Far infrared astronomical observations
Far ir astronomical observations
Far ir astronomy
Far-infrared astronomical observations
Far-ir astronomical observations
Far-ir astronomy
Far infrared astronomy
Synthesis imaging
Synthetic aperture
Aperture synthesis
Fast novae
Few-body systems
Fibrills
Field galaxies
FOV
Field of vision
Fields of view
Fields of vision
Field of view
At telescope' foci
Filamentary nebula
Filamentary nebulae
Finding charts
Finding lists
Fireballs
Fundamental Katalog
FK catalog
Apex
Flare stars
Flash spectrum
Flash spectra
Flash stars
Flat space
Flux calibration
Flyby missions
Forbush decrease
Forbush effect
Fornax dwarf spheroidal galaxy
Free-floating planets
Interstellar planets
Nomad planets
Orphan planets
Rogue planets
Free floating planets
Aphelion
Friedmann-Robertson-Walker metric
Friedmann universe
Frozen-in elements
Frozen in elements
FU Orionis
FU-Orionis
FU Orionis stars
Fundamental properties of stars
Fundamental parameters of stars
G dwarfs
G dwarf stars
G giants
G giant stars
Class G stars
G-type stars
G stars
G subdwarfs
G subdwarf stars
Apapsis
Apoapsis
Apocentre
Apocenter
G subgiants
G subgiant stars
G supergiants
G supergiant stars
Galaxy accretion discs
Galactic and extragalactic astronomy
Galactic anticenter
Center of the Milky Way
Galactic Centre
Milky Way Galaxy nucleus
Milky Way center
Galactic center
Galactic coordinate system
Galactic cosmic rays
Galactic latitude
Galactic longitude
Apogee
Galactic poles
Galactic radio sources
Radio sources which pertain to the Milky Way galaxy
Galactic winds in the Milky Way
Milky Way Galaxy winds
Milky Way galactic winds
Galactic winds
Includes galactic winds in the Milky Way.
Galaxies
Galaxy abundances
Galaxy accretion
Ages of galaxies
Galactic age
Galactic ages
Galaxy ages
Anticenter
Galaxy anticenters
Galaxy bulges
Apollo asteroids
Apollo group
Galaxy chemical evolution
Circumnuclear matter
Galactic circumnuclear matter
Galaxy circumnuclear disk
Classification of galaxies
Galactic morphology
Galaxy classification
Galaxy morphological classification
Galaxy morphology
Galaxy classification systems
Galaxy cluster counting
Galaxy cluster counts
Cluster of galaxies
Galaxy clustering
Galaxy clusters
Galactic collisions
Galaxy collisions
Galactic colors
Galactic colours
Galaxy colors
Galaxy counts
Galaxy disks
Visual magnitude
Apparent magnitude
Galaxy distances
Galaxy dynamics
Galaxy encounters
Galactic equators
Galaxy equators
Evolution of galaxies
Galaxial evolution
Galaxy evolution
Galaxy formation
Galactic fountains are considered as both structural features and dynamic processes.
Galaxy fountains
Galaxy groups
Galaxy infall
A subdwarfs
A subdwarf stars
Apparent places
Apparent position
Galactic interactions
Galaxy interactions
Galactic jets are considered as both structural features and dynamic processes.
Galaxy jets
Galaxy kinematics
Galaxy luminosity
Galaxy luminosities
Galaxy magnetic fields
Galactic magnitudes and colours
Galaxy magnitudes
Galaxy mass distribution
Galaxy masses
Galaxy mergers
Galaxy nuclei
Apparent FTL
Effective FTL
Fast-than-light
Faster than light
Superluminal motion
Apparent superluminal motion
Galactic pairs
Pair of galaxies
Galaxy pairs
Galaxy photometry
Galactic physics
Galaxy physics
Galaxy planes
Galactic motion
Galactic processes
Galaxy processes
Galactic properties
Galaxy properties
Galaxy radial velocities
Galactic radius
Galaxy radii
Galaxy rotation
Rotation curves
Galaxy rotation curves
Apsidal motion
Galaxy spurs
Galactic stellar populations
Galaxy stellar content
Galactic structure
Galaxy structure
Galactic tidal interactions
Galactic tides
Galaxial tidal interactions
Galaxial tides
Galaxy tidal interactions
Galaxy tides
Galactic triplets
Galaxy triples
Galaxy triplets
Galaxy winds
Galilean moons
Galilean satellites
Gamma astronomy
Gamma ray astronomical observations
Gamma ray astronomy
Gamma-ray astronomical observations
γ-ray astronomy
Gamma-ray astronomy
γ-ray bursts
Cosmic gamma ray burst
Cosmic gamma-ray burst
GRB
Gamma ray burst
Gamma ray bursts
Gamma-ray bursts
Apse
Gamma-ray detectors
Gamma ray line
Gamma ray observatories
Gamma-ray observatories
Gamma objects
Gamma sources
Gamma-ray objects
Gamma telescopes
Gamma-ray telescopes
Gamma Cassiopeiae stars
Gamma radiation
Dust to gas ratio
Dust-to-gas ratio
Gas to dust ratio
Gas-to-dust ratio
Gaseous nebula
Gaseous nebulae
Ancient astronomy
Archeoastronomy
Astroarchaeology
Astroarcheology
Archaeoastronomy
Counterglow
Gegenschein
General theory of relativity
General relativity
Ubvgri photometry
Geneva photometry
Geocentric orbit
Geocorona
Geodesics
Geomagnetic fields
Geostationary satellites
Geosynchronous satellites
Argument of perigee
Giant ellipticals
Giant elliptical galaxies
Giant galaxies
Giant molecular clouds
Giant radio galaxies
CS stars
Giant stars
Globular clusters
Globular star clusters
Constructible universe
Godel universe
Goedel universe
Gödel universe
Gould's Belt
Goulds belt
Gould Belt
Stellar granule
Argument of perihelion
Gravitational condensate stars
Gravastars
Gravitational force
Gravity
Gravitation
Gravitational collapse
Gravitational deflection
Gravitational disruption
Gravitational energy
Gravitational equilibrium
Gravity field
Gravitational fields
Gravitational instability
Gravitational interaction
Armil
Armilla
Spherical astrolabes
Armillary spheres
Gravitational lens
Gravitational lensing
Gravitational lensing shear
Microlensing
Gravitational microlensing
Cosmic singularities
Singularity
Space-time singularities
Spacetime singularities
Gravitational singularities
Gravitational wave astronomy
Gravitational wave detectors
Gravitational wave sources
Gravitational waves
Gravity anomaly
Artificial satellites
Gravity brightening
Gravity darkening
Grazing-incidence telescopes
Grazing incidence telescopes
Great Attractor
Green valley galaxies
Greenhouse gases
Gregorian telescopes
Ground based astronomy
Ground-based astronomy
Ground based telescopes
Ground-based telescopes
Ground telescopes
Gunn g,r,i,z photometry
Gunn photometry
H-alpha stars
North node
Ascending node
21 cm lines
21-cm line emissions
21-cm lines
H I lines
HI line
Hydrogen lines
H alpha photometry
H beta photometry
HI regions
H I regions
H I I clouds
H I I regions
H II clouds
HII clouds
HII regions
H II regions
Habitable planets
Habitable zone
Hadron epoch
Hadron barrier
Hall current thrusters
Hall effect thrusters
Hall thrusters
Halo stars
A subgiants
A subgiant stars
Asteroid belt
Hardness ratio
Haro galaxies
Black hole evaporation
Hawking radiation
Hayashi track
Heavy-metal stars
Heavy metal stars
Heliocentric orbit
Heliopause
Helioseismic pulsations
Helioseismology
Asteroid occultation
Heliosheath
Heliosphere
Heliostats
Helium rich stars
Helium-rich stars
Helium burning
CP4
He-weak stars
Helmholtz Kelvin contraction
Helmholtz-Kelvin mechanism
Kelvin-Helmholtz contraction
Helmholtz-Kelvin contraction
HD catalog
Henry Draper catalog
Asteroids
Henyey track
HH objects
HH-object
Herbig Haro objects
Herbig–Haro object
Herbig-Haro objects
HAeBe stars
Herbig Ae-Be stars
Herbig stars
Herbig Ae/Be stars
Hertzsprung gap
Color-color diagrams of stars
Color-magnitude diagrams of stars
H-R diagrams
HR diagram
HRD
Hertzsprung-Russell diagram
Hertzsprung/Russell diagrams
Hertzsprung–Russell diagram
Hertzsprung Russell diagrams
Heterodyne interferometry
Superhet
Superheterodyne receivers
Heterodyne receivers
H I shells
HI shells
HCG
Hickson compact group
Astroseismology
Stellar seismology
Asteroseismology
Hierarchical cosmology
High energy cosmic radiation
High energy cosmic rays
High-energy cosmic rays
High-energy cosmic radiation
HMXBs
High-mass X-ray binary stars
High-redshift galaxies
HVC
High velocity clouds
High-velocity clouds
High velocity stars
High-velocity stars
High latitude field
High-altitude balloons
High altitude balloons
High energy astronomy
High energy process
High-energy astronomy
High-energy astrophysics
High-energy process
High energy astrophysics
Extraterrestrial life
Space biology
Astrobiology
HTRA
High time resolution astrophysics
Hilda group
Hirayama families
Holmberg radius
Holmium stars
Holographic interferometry
HB stars
Horizontal-branch stars
Horizontal branch stars
Alt/Az coordinate system
Horizon (horizon coordinate system)
Horizon coordinate system
az/el coordinate system
Horizontal coordinate system
Horizontal parallax
Astrochemistry
HDM
Hot dark matter
Hot intergalactic medium
HIM
Hot ionized medium
Epistellar jovians
Pegasean planets
Pegasids
Roaster planets
Hot Jupiters
Hot Neptunes
Hour angle
Hubble photometry
HST photometry
Hubble classification
Hubble galaxy classification
Hubble sequence
Hubble's classification
Hubble classification scheme
Hubble constant
Hubble diagram
Orbital mechanics
Astrodynamics
Hubble radius
HST
Hubble Space Telescope
Expansion timescale
Hubble time
Hubble law
Humidity
Hungaria group
Magneto-hydrodynamic simulations
Hydrodynamical simulations
Hydrogen burning
Hydrogen deficient stars
Astrographic catalogues
Astrographic catalogs
Hydrosphere
OH masers
Hydroxyl masers
OH sources
Hydroxyl sources
Hyperbolic orbit
Hypergiant stars
Hypernova
Hypernovae
Hypervelocity stars
Impact phenomena
Astroinformatics
Inclination
Indochinites
Inflationary universe
IRAS
Infrared Astronomical Satellite
Infrared astronomy
Infrared dark clouds
IR excess
IR excess stars
Infrared excess
IR excess galaxies
IR excess galaxy
IR-excess galaxies
IR-excess galaxy
Infrared excess galaxy
Infrared excess galaxies
Astrometric binaries
Astrometric binary stars
Infrared galaxies
Infrared observatories
Infrared photometry
IR sources
Infrared sources
IR telescopes
Infrared telescopes
Initial conditions of the universe
Initial mass function
Terrestrial planet
Instability strip
Equipment and apparatus
Instrumentation
A supergiants
A supergiant stars
Astrometry
Total magnitude
Integrated magnitude
Interacting binary stars
Colliding galaxies
Interacting galaxies
Intercloud gas
Intercloud medium
Interdisciplinary astronomy
Interferometric instrumentation
Interferometric instruments
Interferometers
Interferometric binaries
Interferometric binary stars
Interferometric correlation
Interferometry
Intergalactic clouds
Astronomers
Intergalactic dust clouds
Intergalactic filaments
Intergalactic gas
IGM
Intergalactic matter
Intergalactic medium
Intergalactic medium phases
Intermediate-mass black holes
Intermediate stars
Internal waves
Celestial coordinate systems
Celestial reference system
Astronomical coordinate systems
Interplanetary discontinuities
Interplanetary dust clouds
Interplanetary dust
Interplanetary gas
Interplanetary grains
Interplanetary magnetic fields
Interplanetary material
Interplanetary matter
Interplanetary medium
Interplanetary particle acceleration
Interplanetary physics
Interplanetary scintillation
Interplanetary shocks
Astronomical databases
Interplanetary turbulence
Interstellar absorption
Interstellar abundances
Interstellar atoms
Interstellar atomic gas
Interstellar globules
Interstellar clouds
Interstellar cyclotron emission
Cosmic dust
Dust grain
Dust grains
Interstellar dust clouds
Interstellar dust grains
Interstellar grains
Interstellar dust
Interstellar dust extinction
Interstellar dust grain processes
Interstellar grain processes
Interstellar dust processes
Interstellar dynamics
Astronomical detectors
Interstellar electromagnetic emissions
Interstellar infrared emissions
Interstellar optical emissions
Interstellar emissions
Covers all interstellar emissions of electromagnetic radiation.
Interstellar extinction
Filaments
Interstellar filaments
Interstellar line absorption
Interstellar line emission
For interstellar emission lines, besides H1 and CO, use this concept.
Interstellar magnetic fields
Interstellar masers
ISM
Interstellar material
Interstellar matter
Interstellar media
Interstellar medium
Interstellar wind
Interstellar medium wind
Interstellar molecules
Astronomical location
Interstellar phases
Highly ionised interstellar gas
Highly ionized interstellar gas
Interstellar ionized gas
Ionised interstellar gas
Ionised interstellar gases
Ionized interstellar gas
Ionized interstellar gases
Interstellar plasma
Interstellar radiation field
Interstellar reddening
Interstellar scattering
Interstellar scintillation
Synchrotron emission
Interstellar synchrotron emission
Interstellar thermal emission
ICM
Intracluster matter
Intracluster medium
Unstable stars
Intrinsic variable stars
Astronomical models
Ionospheric scintillation
Iron meteorites
Irregular galaxies
Irregular variable stars
Isothermal sphere profile
Jeans mass
Astronomical object identification
Jets
Johnson-Morgan filter system
Three-color photometry
UBV RI jkl photometry
UBV RI photometry
UBV photometry
Johnson photometry
Jovian moons
Moons of Jupiter
Jovian satellites
Jupiter
Jupiter trojans
K dwarfs
K dwarf stars
K giants
K giant stars
Class K stars
K-type stars
K stars
K subdwarfs
K subdwarf stars
Astronomical optics
K subgiants
K subgiant stars
K supergiants
K supergiant stars
K mechanism
K-mechanism
Kappa mechanism
Kapteyn's Selected Areas
Kapteyn Selected Areas
Keplerian orbit
Kerr-Newman black holes
Kerr black holes
Kerr metric
Kinematic parallax
Cosmic radiation sources
Astronomical radiation sources
Kron cousins photometry
Kron-Cousins RI photometry
Kron-Cousins photometry
Kruskal-Szekeres diagram
Kruskal diagram
Edgeworth-Kuiper belt
Edgeworth–Kuiper belt
Kuiper belt
L dwarf stars
L dwarfs
L-galaxies
L galaxies
L subdwarf stars
L subdwarfs
L-points
Lagrange points
Libration points
Lagrangian points
Lambda density
Lambda Eridani stars
Abell clusters
Astronomical reference materials
Land atmosphere interactions
Land-atmosphere interactions
Landers
Large scale structure of the cosmos
Large scale structure of the universe
Large-scale structure of the cosmos
Large-scale structure of the universe
L m c
Large Magellanic Cloud
Laser guide stars
Laser interferometry
Cool dwarf stars
Late-type dwarfs
Late-type dwarf stars
Late type galaxies
Late-type galaxies
Cool giant stars
Late type giant stars
Late-type giants
Late-type giant stars
Late type stars
Late-type stars
Astronomical research
Cool supergiant stars
Late-type supergiants
Late-type supergiant stars
Late stellar evolution
Lemaître universe
Lemaitre universe
S0 galaxies
Lenticular galaxies
Leptogenesis
Physical libration
Libration
Lightcurves
Light curves
Light-year
Lightyear
Light year
Dome seeing
Astronomical seeing
Laser Interferometer Gravitational-Wave Observatory
LIGO
Limb brightening
Limb darkening
Limiting magnitude
Line of sight
LINER galaxies
Li stars
Lithium rich stars
Lithium-rich stars
Lithium stars
Lithosphere
Local Group
Astronomer's sextants
Framed sextants
Mural sextants
Astronomical sextants
Long baseline interferometers
Long baseline interferometry
Long period comets
Long period variables
Long-period variables
Long period variable stars
Longitude of the ascending node
Longitude of the descending node
Low-density fields
LMXB
LMXBs
Low-mass X-ray binaries
Astronomical site protection
LSBG
Low surface brightness galaxies
Luminosity class
Luminosity classification
Luminosity function
Luminous arcs
Luminous blue variables
Luminous blue variable stars
LIRG
Luminous infrared galaxies
Lunar atmosphere
Composition of the moon
Lunar composition
Lunar craters
Astronomical unit
Lunar domes
Lunar eclipses
Lunar evolution
Lunar landmarks
Lunar features
Lunar geochronology
Lunar geodetics
Lunar geodesy
Lunar gravity
Lunar gravitational field
Lunar highlands
Suggested to delete. -KF
Lunar impacts
Lunar interior
Lunar magnetic fields
Lunar mare
Lunar seas
Moon mare
Moon maria
Moon seas
Lunar maria
Lunar mineralogy
Lunar mountains
Lunar node regression
Lunar occultation
Lunar origin
Lunar petrology
Phase moon
Lunar phase
Lunar probes
Astrophotographers
Astrophotography
Lunar rays
Refers to lunar feature
Lunar rills
Lunar rilles
Lunar physics
Lunar science
Lunar seismology
Lunar surface
Lunar theory
Related to the orbit of the Moon around the Earth.
Lunar transient phenomena
Lyman-alpha forest clouds
LAE
Lyman-alpha emitters
Lyman-alpha galaxies
Lyman-break galaxies
Astrophysical black holes
Ly-α forest
Lyman forest
Lyman-alpha forest
Lyα forest
Lyman limit systems
M dwarfs
M dwarf stars
M giants
M giant stars
Class M stars
M-type stars
M stars
M subdwarfs
M subdwarf stars
M subgiants
M subgiant stars
M supergiants
M supergiant stars
Mach Zehnder modulators
Mach zehnder interferometers
Mach–Zehnder modulators
Mach-Zehnder interferometers
Astrophysical dust processes
Nubeculae Magellani
Magellanic Clouds
Magellanic Stream
Magnetars
Magnetic anomalies
Magnetic fields
Cobalt stars
Magnetic stars
Magnetic variable stars
Magnitude
Active solar regions
Ultraviolet transient sources
Population 1 stars
Population II Cepheid stars
Used for the subset of Me stars that are variable stars. Not all Me stars are variable stars.
Solar astrophysics
World Wide Telescope
Bailey type stars
Polyaromatic hydrocarbons
Intermediate polars
Galaxy accretion disks
Elliptical orbit
AGB stars
Solar physics
Astroparticle physics
Intermediate type stars
Massive stars
Astronomy software
Astronomy web services
Astronomy data acquisition
Astronomy data analysis
Software licensing
Public domain software
Publicly available software
Software available on request
Astronomy data reduction
Open source software
Private software
Astronomy data modeling
Software documentation
Educational software
Software tutorials
Brown dwarf
L-type brown dwarfs
L brown dwarfs
L-type brown subdwarfs
L brown subdwarfs
T-type brown dwarfs
T brown dwarfs
T-type brown subdwarfs
T brown subdwarfs
Y-type brown dwarfs
Y brown dwarfs
A type stars
A-type dwarf stars
B-type dwarf stars
Cool stars
Hot stars
A-type dwarfs
Ae-type stars
A-type giants
A-type giant stars
Am-type stars
Peculiar A stars
Ap-type stars
A-type subdwarfs
A-type subdwarf stars
A-type subgiants
A-type subgiant stars
A-type supergiants
B-type dwarfs
Be-type stars
B-type giant stars
B-type giants
Bp-type stars
B-type subdwarf stars
B-type subdwarfs
B-type subgiant stars
B-type subgiants
B-type supergiant stars
B-type supergiants
BW-type stars
Oef-type stars
Oe-type stars
WCN-type stars
WC-type stars
WN-type stars
OB-type stars
OB-type supergiant stars
OB-type supergiants
O-type dwarfs
O-type dwarf stars
O-type main sequence stars
O-type giant stars
O-type giants
O-type subdwarf stars
O-type subdwarfs
O-type subgiant stars
O-type subgiants
O-type supergiant stars
O-type supergiants
F-type dwarf stars
F-type dwarfs
F-type giant stars
F-type giants
F-type subdwarf stars
F-type subdwarfs
F-type subgiant stars
F-type subgiants
F-type supergiants
F-type supergiant stars
G-type dwarfs
G-type main sequence stars
G-type dwarf stars
G-type giant stars
G-type giants
G-type subdwarf stars
G-type subdwarfs
G-type subgiant stars
G-type subgiants
G-type supergiant stars
G-type supergiants
K-type dwarfs
K-type main sequence stars
K-type dwarf stars
K-type giant stars
K-type giants
K-type subdwarf stars
K-type subdwarfs
K-type subgiant stars
K-type subgiants
K-type supergiant stars
K-type supergiants
Red dwarfs
M-type main sequence stars
M-type dwarf stars
M-type dwarfs
Me-type stars
M-type giants
M-type giant stars
Me-type variables
Me-type variable stars
Me variables
M-type subdwarf stars
M-type subdwarfs
M-type subgiant stars
M-type subgiants
M-type supergiant stars
M-type supergiants
N-type stars
Zero age main sequence
Beta Cephei stars
Delta Cepheid variable stars
Delta Cephei stars
Delta Cepheids
Fast solar wind
Slow solar wind
Solar meridional circulation
Solar coronal mass ejections
Yellow stragglers
Yellow straggler stars
Blue-straggler stars
Gamma rays
Gamma-rays
Gamma ray lines
Gamma-ray lines
Gamma-ray radiation sources
Gamma ray sources
Gamma-ray sources
Soft gamma ray repeaters
Solar gamma ray emission
Gamma-ray burst
Gamma ray transient sources
Gamma-ray transient source
Solar granule
Granule
Stellar granules
Solar granules
Astronomical instrumentation
γ-ray telescopes
Gamma ray telescopes
RRab stars
RRa stars
RRab
RRab variable stars
RRb stars
Bailey type ab stars
Bailey stars
RR(B) stars
Magellanic irregular galaxies
Type I irregular galaxies
Irregular I galaxies
Magellanic irregular is a classification of galaxies. Though the SMC is an example of this classification, it does not specifically describe ONLY the Magellanic Clouds.
Bailey type d stars
RRc stars
RRc variable stars
Bailey type c stars
Gamma-ray bursters
Circumplanetary medium
Refers to the specific shape and form of a particular classification of galaxies. Does not refer to the disk part of a galaxy (see "Galaxy disk").
Refers to the disk part of a galaxy. Does not refer to a specific shape and form of a particular classification of galaxies (see "Disk galaxies").
Stellar disks
Refers to the disk of stars that make up part of the disk of the galaxy. This concept would specific exclude the dust and gas components of the galaxy's disk.
Galaxy stellar disks
High mass stars
High-mass stars
High-mass X-ray binaries
Harvard classification
Henry Draper catalogue
Multiple mirror telescopes
Galaxy halos
Circumgalactic medium
Gaseous galaxy halo
Galaxy stellar halos
Galaxy dark matter halos
The region of the Sun's atmosphere between the chromosphere and corona.
Stellar coronal mass ejections
Stellar CME
Galactic accretion disks
Pop I
Astrostatistics
Algorithms
Astrostatistics distributions
Astrostatistics strategies
Astrostatistics techniques
Astrostatistics tools
Expectation-Maximization Algorithm
Markov chain Monte Carlo
Affine invariant
Gibbs Sampler
Importance sampling
Metropolis-Hastings
Nested sampling
Gamma distribution
Normal distribution
Pareto distribution
Poisson distribution
Sampling distribution
Bayesian statistics
Maximum likelihood estimation
Nonparametric hypothesis tests
Nonparametric inference
Parametric hypothesis tests
Survival analysis
Bootstrap
Classification
Clustering
Cross-validation
Deconvolution
Detection
Model selection
Multivariate analysis
Regression
Spatial point processes
Time series analysis
Uncertainty bounds
Wavelet analysis
Bayes factor
Bayesian information criterion
Chi-squared statistic
Fisher's Information
Kullback-Leibler distance
Bayes' Theorem
Hierarchical models
Posterior distribution
Prior distribution
Kolmogorov-Smirnov test
Smoothing
Gaussian Processes regression
F test
Mixture model
Neural networks
Outlier detection
Random Forests
Support vector machine
Gaussian mixture model
Convolutional neural networks
Single-linkage hierarchical clustering
Akaike information criterion
Likelihood ratio test
Dimensionality reduction
Principal component analysis
Linear regression
Measurement error model
Minimum chi-squared
Nonlinear regression
Robust regression
Minimum spanning tree
Two-point correlation function
Voronoi tessellation
Irregular cadence
Light curve classification
Period search
Red noise
Transient detection
Fast Fourier transform
Lomb-Scargle periodogram
Phase dispersion minimization
Confidence interval
Credible region
Fitting
friend-of-friends algorithm
friend-of-a-friend algorithm
Deep learning
MLE
Power law distribution
Schechter function
Elemental abundances
Hydrodynamics
Magnetohydrodynamics
Astronomy databases
Computational methods
MHD
Magnetohydrodynamical simulations
Astronomy simulations
Radiative transfer simulations
Astronomy data visualization
GPU computing
Cloud computing
Distributed computing
Center to limb observations
Spectropolarimetry
Spectro-polarimetry
Solar active regions
Solar active region magnetic fields
Solar active region velocity fields
Solar active region filaments
Sunspot flow
Delta sunspots
Active solar chromosphere
Solar filament eruptions
Solar radio flares
Solar x-ray flares
Solar flare spectra
Solar white-light flares
Solar magnetic bright points
Quiescent solar prominence
Ellerman Bombs
Quiet solar chromosphere
Solar chromospheric heating
Solar fibrils
Solar mottles
Active solar corona
Solar coronal heating
Coronae
Quiet solar corona
Solar coronal radio emission
Solar coronal seismology
Solar coronal waves
Solar differential rotation
Stellar coronal holes
Stellar coronal dimming
Stellar coronal lines
Stellar coronal loops
Stellar coronal plumes
Solar coronal transients
F region
E region
Solar radiative zone
Solar CME shocks
Solar coronal mass ejection shocks
Solar radius
Solar irradiance
Solar magnetic flux emergence
Solar dynamo
Convective envelopes
Stellar convection envelopes
Convective zones
Stellar convective zones
Convection zones
Convective shells
Stellar convective shells
Underabundances
Cosmic abundances
Isotopic abundances
Metal abundances
Metallicity
Overabundances
Abundance ratios
Galactic abundances
Intergalactic abundances
Laboratory astrophysics
Photospheres
Chromospheres
E corona
Rich galaxy clusters
Poor galaxy clusters
High-redshift galaxy clusters
High redshift clusters
High-redshift clusters
Poor clusters
Rich clusters
Mercury
Mercury (planet)
Observation methods
Techniques
Methods
Astronomical methods
Astronomical techniques
Radio transient sources
Fast radio burst
Fast radio bursts
Lorimer Burst
FRB
FRBs
Elemental composition
UV transient sources
X-ray point sources
Radiative magnetohydrodynamics
Radiative MHD
PAHs
Population 2 stars
Population 3 stars
Pop III
Population one stars
Population two stars
Young, metal-rich stars commonly found in the spiral arms of the Milky Way galaxy
Population three stars
Hypothetical population of extremely massive and hot stars with virtually no metals.
Stars on the main sequence
Hertzsprung-Russell diagrams
SRa variable stars
SRb variable stars
SRc variable stars
SRd variable stars
SRS variable stars
SRA stars
SRa stars
SRb stars
SRB stars
Lb variable stars
TZ Cassiopeia stars
TZ Cassiopeia
FK Comae stars
Lc variable stars
Quenched galaxies
Intermediate-type stars
Astronomical simulations
AGN host galaxies
Milky Way dark matter halo
Dark matter halos
Dark halos
Dark halo
Reverberation mapping
Planets (Solar system)
Planets (Extrasolar)
Refers to planets in our Solar system.
Exoplanet atmospheric variability
Exoplanet atmospheric composition
Exoplanet surface composition
Exoplanet surface variability
Extrasolar ice giants
Classical satellites
Exomoons
Natural satellites (Extrasolar)
Irregular exomoons
Irregular extrasolar natural satellites
Irregular natural satellites (Extrasolar)
Irregular natural extrasolar satellites
Irregular moons (Extrasolar)
Moons (Solar system)
Natural satellites (Solar system)
M-sigma relation
Irregular satellites
Irregular moons (Solar system)
Irregular natural satellites
Photo-evaporating protoplanetary disks
Galaxy environments
Polynuclear aromatic hydrocarbons
Peculiar giants
Chemically peculiar giant stars
Peculiar giant stars
Photometer
Cooling flows
Scaling relations
Galaxy spheroids
Low-luminosity active galactic nuclei
Low-luminosity AGN
High-luminosity active galactic nuclei
High-luminosity AGN
X-ray active galactic nuclei
X-ray AGN
Main belt asteroids
Space weather
Horizontal branch star
Chemical composition
Apsis
Apsides
Chandrasekhar limit
Optical doubles
Not to be confused with the Schonberg-Chandrasekhar limit.
The nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun.
Solar coronal lines
Solar coronal plumes
Dense clouds
Galaxy quenching
Galactic bulge
Milky Way bulge
Referring to the bulge in the Milky Way Galaxy. For other galaxies use "Galaxy bulges."
Cosmic background radiation
CBR
Use for cosmic background radiations other than microwave, i.e. Cosmic gamma-ray background radiation or Cosmic ultraviolet background radiation, etc.
Groups of galaxies
H1 line emission
H I line emission
SSI
Solar spectral irradiance
K region
Morgan-Keenan classification
Morgan-Keenan spectral types
MK classification
Chandrasekhar-Schonberg limit
Population II Cepheid variable stars
Schonberg-Chandrasekhar limit
Not to be confused with the Chandrasekhar limit.
Schoenberg-Chandrasekhar limit
Elliptical orbits
RRd stars
RRd variable stars
Delta Delphini variable stars
Galaxy chemical composition
Stellar evolutionary models
Hertzsprung Russell diagram
Giant branches
Giant branch
Main sequence
Horizontal branch
Young massive clusters
DQ Herculis stars
High time resolution astronomy
Weak-emission T Tauri stars
Weak Emission T Tauri stars
Low mass stars
Stellar evolutionary types
Dwarf stars
Subdwarf stars
Stellar spectral types
Interstellar cyclotron radiation
Blank field
Blank fields
Circular orbit
Circular orbits
Colour equation
Color equation
Solar convective zone
High resolution spectroscopy
Lyman alpha forest
Charge exchange ionization
Molecular physics
Electron impact ionization
Photoionization
Charge exchange recombination
Mass spectrometry
Wavelength identification
Radiative recombination
Pre-biotic astrochemistry
Reaction catalysts
Reaction rates
Spectral line lists
Line lifetimes
Nuclear reaction cross sections
Particle physics
Ice destruction
Beta Lyrae variable stars
Beta Lyrae stars
Asymptotic giant branch stars
Degenerate dwarf stars
Degenerate dwarf
Direct imaging
Planetary microlensing
Magnetospheric radio emissions
Radial velocity methods
Transit photometry
Far-infrared interferometry
Dwarf planets (Solar system)
Hubble Space Telescope photometry
HST Fnnn(LMNW) photometry
Lyman alpha forest clouds
Far infrared interferometry
Stellar granulation
Zero age horizontal branch stars
Field stars
B(e) stars
B[e] stars
Not to be confused with Be stars. The concept should be written as B[e] star, the substitution or omission of square brackets is due to technical restrictions.
B[e]-type stars
B(e)-type stars
CEMP stars
Carbon enhanced metal poor stars
roAp stars
Alpha2 Canum Venaticorum stars
Rapidly oscillating Ap star
Gamma Dor
Gamma Doradus variable stars
γ Doradus
γ Dor
History of astronomy
Solar-type stars
Sun-like stars
Solar analogs
Line intensities
Exobiology
Anomalous Cepheid variable stars
BL Boötis stars
BL Boo stars
BL Bootis stars
Atomic physics
Atom interferometry
Atomic data benchmarking
De-excitation rates
Excitation rates
Ionization
Isotope shifts
Recombination
Spectral line identification
Transition probabilities
Nuclear physics
Radical-radical recombination
Particle astrophysics
Plasma physics
Theoretical techniques
Solid matter physics
Dust and ices astrophysics
Ice formation
Radiative processes
Experimental techniques
Dust and ices physics
Atomic spectroscopy
Collisional broadening
Dielectronic recombination
Line positions
Molecular spectroscopy
Photoelectron spectroscopy
Atomic interferometry
Shocks
Theoretical models
Zeeman-Doppler imaging
Reissner-Nordstrom black holes
Reissner-Nordström black holes
T subdwarf stars
T subdwarfs
Visible sources
Visible radiation sources
Bekenstein-Hawking entropy
Coronagraphic imaging
Coronographic imaging
Coronagraphic detection
LIRGs
Alpha2 CVn stars
Compact group of galaxies
Optical sources
Time domain astronomy
Studying how astronomical objects change over time.
Solar K corona
Solar K coronal region
Stellar K corona
Stellar K coronal regions
Solar F corona
Solar F coronal region
Stellar F corona
Stellar F coronal regions
F corona
Solar E corona
Solar E coronal region
Stellar E coronal regions
K corona
Stellar M coronal regions
Solar M coronal region
M regions
M region
Stellar E corona
Blazhko effect
Double periodic variable stars
DPV
Double periodic variables
Blue large-amplitude pulsators
BLAP
BLAPs
Blue large-amplitude pulsating stars
Planetary surfaces
SSSB
Surface variability
Surface composition
Surface ices
Surface ice
Planetary surface ices
Planetary surface ice
Planetary surface
Surface processes
Planetary surface composition
Planetary surface variability
Exoplanet surfaces
Atmospheric variability
Planetary atmospheric variability
Atmospheric composition
Post-asymptotic giant branch stars
Post-AGB stars
Alpha Cygni variable stars
Alpha Cygni variables
A Cyg variables
α cyngi variable stars
Biomarkers
OGLE small amplitude red giant stars
OSARG
OSARGs
OGLE Small Amplitude Red Giants
Optical gravitational lensing experiment small amplitude red giants
Optical gravitational lensing experiment small amplitude red giant stars
Not to be confused with "Double-mode variable stars."
Not to be confused with "Double periodic variable stars."
Beta Cephei variable stars
Beta Cepheid variable stars
Delta Cephei variables
Population I Cepheids
Type I Cepheids
Type II Cepheid variable stars
BL Herculis variable stars
BL Herculis stars
Galaxy tails
Ram pressure stripped tails
Search for extraterrestrial intelligence
Technosignatures
Biosignatures
Spectral energy distribution
SED
Used for both the Solar systems historic protoplanetary disk and for protoplanetary disks external to the Solar system.
Chwolson ring
Einstein-Chwolson ring
Astrometric exoplanet detection
Astrometric planet detection
Astrometric planet detection (extrasolar)
Protoplanetary disks (Extrasolar)
This concept does not refer specifically to the Filamentary Nebula, a named nebula that is part of the Western Veil (also known as Caldwell 34), consisting of NGC 6960.
Galaxy voids
Intergalactic voids
Dirty snowball model
Dirty snowball theory
Comae
Comas
Main-belt comets
Comet interiors
Cometary interiors
Cold Neptunes
Transmission spectroscopy
Atmospheric transmission
Radio AGNs
AGNs
X-ray AGNs
Radio active galactic nuclei
Radio AGN
Stellar occultation
Skylight
Sky radiation
Diffuse skylight
Binary lens microlensing
Triple lens microlensing
High-resolution microlensing event imaging
Astrometric microlensing effect
Binary source microlensing
Finite-source photometric effect
Lens orbital motion
Microlensing parallax
Xallarap effect
Microlensing optical depth
Microlensing event rate
Satellite microlensing parallax
Annual microlensing parallax
Topocentric microlensing parallax
Geocentric microlensing parallax
Orbital microlensing parallax
Caustic curve
Critical curve
Gravitational microlens
Gravitational microlensing exoplanet detection
Gravitational microlensing planet detection
Ionized coma gases
Cometary ionospheres
Multiple star evolution
Roche lobe overflow
Common envelope binary stars
Stellar mergers
Merging stars
Common envelope binaries
Common envelope episodes
High mass x-ray binary stars
Low-mass x-ray binary stars
Hz Herculis x-ray binary stars
Hz Herculis x-ray binaries
Single x-ray stars
Solar x-ray emission
Neutral coma gases
Coma dust
Comet nuclei
Comet surfaces
Comet volatiles
Common envelope evolution
Planetary surface processes
Terrestrial microlensing parallax
Solar apex
Solar antapex
Cometary atmospheres
Comet atmospheres
Abell 1656
Geocentric parallax
Hubble-Lemaitre constant
A term denoting the water portion of a planet's surface.
MKK classification
Yerkes classification
Yerkes spectral classification
Morgan-Keenan-Kellman classification
Helium poor stars
Helium-poor stars
Helium-weak stars
Helium weak stars
Flat-spectrum radio quasars
FSRQ
OVV quasars
Optically violent variable quasars
Optically violent variable quasar
OVV quasar
HPQ
Highly polarized quasars
Highly polarized quasar
CDQ
Core-dominated quasars
Core-dominated quasar
Ultraluminous x-ray sources
ULX
Ultraluminous supersoft sources
ULS
Pulsating ultraluminous X-ray sources
PULX
Luminous supersoft x-ray source
SSXS
Supersoft sources
SSS
Astronomy education
Refers to both methods currently used to teach the science of astronomy and to an area of pedagogical research that seeks to improve those methods.
Stellar streams
Magnitude systems
High angular resolution
This is phenomenon observed in RR Lyr type stars and demonstrates as changes of the light curve on timescales longer than pulsation period. The effect was first found more than 100 years ago and its origin is still not well understood and subject of active research. - https://github.com/astrothesaurus/UAT/issues/286
Center of gravity
κ-mechanism
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.
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.
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.
Vilnius photometric system
Geneva photometric system
DDO photometric system
Strömgren photometry
Strömgren photometric system
Walraven photometric system
uvbyβ photometric system
AB photometry
AB magnitude
AB photometric system
Vega photometry
Vega photometric system
Extreme ultraviolet astronomy
EUV
EUV astronomy
Helium strong stars
Galaxy spectroscopy
A star that burns its hydrogen content to produce its energy and therefore belongs to the main-sequence luminosity class.
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.
AB magnitude system
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.
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.
AM CVn stars
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.
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.
A person who engages in astronomy as a pastime rather than as a profession.
The astronomical activities carried out by amateur astronomers.
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.
The 'handles,' or extremities, of planetary rings, especially Saturn's, as viewed from Earth.
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.
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.
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.
A man-made equipment that orbits around Earth or a solar system body.
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.
A person who practices astronomy as a profession rather than as a hobby, in contrast to an amateur astronomer.
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.
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).
The design and use of scientific instruments for detection, measurement, control, computation, and so on to observe and study astronomical objects.
A combined and coordinated set of data that supplies astronomy information for a specific purpose in a variety of forms.
A general term used to describe a collection of computer programs, procedures, and documentation that perform some astronomy related tasks on a computer system.
The act or process of educating in astronomy. The knowledge or abilities gained through being educated in astronomy.
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.
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.
A collective designation for massive O and B stars.
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.
A spiral galaxy that exhibits a bar-shaped structure in its nucleus.
BAO
Two stars gravitationally bound to each other, so that they revolve around their common center of gravity.
A term specifying BL Lac objects or quasars when the continuum radiation emitted from the active nucleus is highly polarized and very variable.
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.
A meteor which is extremely bright, particularly one that breaks up during its passage through the atmosphere.
A class of red giant stars whose spectra show strong molecular bands of carbon compounds.
A list or record of items systematically arranged with descriptive details.
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.
Not to be confused with Centaurus meteor shower.
Centaur group
Centaur asteroids
Centaurs
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.
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.
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.
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.
Maser emission from molecules in the circumstellar envelopes of red giants, and also from regions around protostars.
Any concentration of material in the form of a disk orbiting around a star.
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.
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.
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.
Systems for the grouping of astronomical objects into categories on the basis of physical, morphological, or evolutionary characteristics.
The systematic grouping of stars and stellar objects into categories on the basis of physical, morphological, or evolutionary characteristics.
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.
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.
A visible mass of water droplets and/or ice particles in the atmosphere above the Earth's surface.
A star hidden in a dense envelope of gas and dust which is a strong source of infrared emission.
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.
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.
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.
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.
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.
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.
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.
The fainter of the two stars in a binary star system.
A non-variable star used to monitor the variations of another nearby star.
Planetary magnetospheres
Planetary magnetosphere
Near Earth objects
Prebiotic astrochemistry
Pre-biotic chemistry
Prebiotic chemistry
Terrestrial planets
A grouping of conspicuous stars that, when seen from Earth, form an apparent pattern. The sky is divided into 88 constellations.
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.
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.
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.
A binary star system whose components are not in contact and in which no significant mass exchange is occurring.
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.
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.
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.
An irregularly shaped and low density interstellar cloud visible in the optical wavelengths.
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).
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.
dEs
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.
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.
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.
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).
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.
Einstein static Universe
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.
Ecliptic system
Coordinate system with the ecliptic as the fundamental plane.
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.
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.
An ionized nebula whose spectrum consists of emission lines.
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.
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.
A star that has left the main sequence.
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.
A sketch or image used to recognize objects in the field of view of a telescope.
A member of a class of dwarf stars that undergoes sudden, intense outbursts of light (mean amplitude about 0.5-0.6 mag).
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.
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.
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.
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.
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.
The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum.
Not to be confused with a gravity wave.
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).
The region of space in which gravitational attraction exists.
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.
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.
A type of galaxies characterized by strong emission in the blue and violet regions of the spectrum. They are often elliptical or lenticular.
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.
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.
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.
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.
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.
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.
The object or phenomenon at the origin of a gamma-ray burst.
An astronomical object that emits gamma rays.
An H II region, a planetary nebula, or a supernova remnant.
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.
In contrast to a dark nebula, a bright cloud of interstellar gas and dust. The term designates both emission nebulae and reflection nebulae.
A cloud of gas and dust in the interstellar space.
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.
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.
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.
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.
HMXB
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.
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.
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.
Hilda asteroids
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.
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.
A star lying on the horizontal branch.
The coordinate system based on the position of the observer. The horizontal plane is the fundamental plane and the coordinates are altitude and azimuth.
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.
Hubble-Lemaitre law
Hubble's law
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.
The relative amount of a given chemical element with respect to other elements.
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.
A highly energetic supernova explosion. This phenomenon, which is more violent than a typical supernova event, is accompanied by a gamma-ray burst.
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.
IRDC
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.
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.
A telescope capable of observing infrared radiation from astronomical objects.
Intergalactic matter in the form of clouds.
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.
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.
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.
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.
A maser phenomenon created by young stars and protostars in the surrounding dense molecular clouds of gas and dust.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Fountain-like formations of gas and/or dust gushing out from compact regions of some astronomical objects.
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.
A black hole that possesses only mass (not electric charge) and rotates about a central axis. It has an ergosphere and a stationary limit.
A rotating charged black hole. Compare with the Kerr black hole and the Reissner-Nordstrom black hole.
Kirkwood gaps
Kirkwood gap
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.
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.
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.
A space probe designed to land on a planet or other solid celestial body.
LMC
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.
In the Hubble classification, a galaxy on the left part of the Hubble sequence.
A star of spectral type K, M, S, or C, with a surface temperature lower than that of the Sun.
A comet with orbital period of more than 200 years.
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.
Low-mass star
A star whose mass is around that of the Sun.
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.
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.
LBV
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.
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.
A probe for exploring and reporting on conditions on or about the Moon.
Transient lunar phenomenon
TLP
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.
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.
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).
The study of the upper atmosphere, especially of regions of ionized gas.
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.
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).
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.
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.
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.
A telescope mounting which has its two axes of movement aligned with the horizon and the zenith.
A stony meteorite consisting of silicate minerals. This alternative name for a stony meteorite, is now largely obsolete.
A step-by-step problem-solving procedure, especially an established, recursive computational procedure for solving a problem in a finite number of steps.
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.
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.
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.
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.
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.
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.
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.
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.
The point at which a binary star is furthest from its companion.
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.
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.
The point in the orbit of a planet, or other object in the solar system, which is furthest from the Sun.
The point on an elliptic orbit at the greatest distance from the principal focus or center of attraction.
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.
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.
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.
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.
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.
The angular distance between the ascending node of an object orbiting the Earth and its perigee, measured from the Earth.
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.
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.
The region of the solar system located between Mars and Jupiter where over a million objects bigger than 1 km across orbit the Sun.
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.
The study of life throughout the Universe, also known as exobiology.
The study of the chemical interactions between the gas and dust of the interstellar medium.
The science dealing with the motion of satellites, rockets, and spacecrafts. It uses the principles of celestial mechanics.
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.
The precise measurement of the positions and motions of astronomical objects.
A field of force that is generated by electric currents, or, equivalently, a region in which magnetic forces can be observed.
A star whose spectral lines show the Zeeman effect.
The photography of stars, other celestial bodies, and stellar fields.
Asteroids in our solar system that are found in the area between Mars and Jupiter.
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.
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.
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.
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.
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.
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.
A Be star with forbidden lines in emission in its spectrum. B[e] stars show large infrared excess due to circumstellar dust emission.
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).
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'.
The hypothetical mechanism of creating the baryon asymmetry in the Universe. Explaining the observed matter asymmetry is an important open question in physical cosmology.
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.
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.
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.
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.
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.
The meteoroids distributed all along an orbit and diffused somewhat around it.
Meteoroid stream
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.
The difference between the visual magnitude and bolometric magnitude.
A small grain sized meteorite which can only be positively identified under the microscope.
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.
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.
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.
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.
The intersection of a caustic surface with a plane passing through the beam of rays.
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.
The branch of astronomy that deals with the calculation of motions of celestial bodies under the action of their mutual gravitational attractions.
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.
A fixed point in a body through which the resultant force of gravitational attraction acts.
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.
The relative amount of a given chemical element or chemical compound with respect to another element or compound in a given sample.
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).
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.
Dust, gas and plasma around stars, generally present in the form of stellar winds or nebulae ejected by the stars.
The systematic grouping of astronomical objects into categories on the basis of physical, morphological, or evolutionary characteristics.
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.
Grouping of a number of similar astronomical objects.
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.
To pack and align photons or atomic particles parallel to a particular direction.
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.
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.
A Galactic H II region with an electron density ≥ 10^3 cm^-3 and of a linear dimension ≤ 1 pc.
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.
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.
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.
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.
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.
A philosophical, religious, or mythical story of the creation or origin of the Universe, usually referring to the solar system.
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.
The science of the origin, structure, and evolution of the Universe including the origin of galaxies, the chemical elements, and matter.
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.
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.
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.
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.
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.
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°.
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.
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.
A member of a rare class of pulsars, such as the Crab pulsar and Vela pulsar, which can be detected in the visible spectrum.
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.
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.
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).
One of the two points of intersection of the orbit of a secondary body with the plane of reference through the primary.
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.
A spacecraft or satellite designed to orbit a planet or other solar system body.
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.
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.
Any operation on an electrical or electromagnetic signal to recover information from it.
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.
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.
A vault, having a circular plan and usually in the form of a portion of a sphere, that shelters an optical telescope.
Orrery
A mechanical device that illustrates the relative positions and motions of the planets and moons in the solar system in heliocentric model.
A planet that revolves around the Sun beyond the asteroid belt, namely Jupiter, Saturn, Uranus, and Neptune.
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.
An eclipse that is not total.
A type of cataclysmic variable binary star system with a white dwarf and a cool main-sequence secondary star.
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.
A process by which the nucleosynthesis products inside a star are drawn to upper layers of the star.
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.
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.
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).
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.
A member of a class of relatively young stars, containing a large fraction of metals, found mainly in the disk of the Galaxy.
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.
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.
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.
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.
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.
Mini black hole
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.
A high redshift, metal-deficient galaxy that formed very early in the history of the Universe.
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.
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.
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.
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.
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.
One of the two coordinates in the ecliptic system; the angle measured from the ecliptic, positive toward the north.
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.
A short-lived transition object between the asymptotic giant branch (AGB) and planetary nebula phases.
PPN
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.
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.
A planet orbiting a pulsar. The first such planet to be discovered was around a millisecond pulsar known as PSR 1257+12.
A measure of the surface temperature of a star derived from the total emitted energy, assuming that the star is a blackbody emitter.
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.
Pulsating stars
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.
The time within which the amplitude of an oscillation increases or decreases by a factor e (= 2.71828...).
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.
Material, in solid, liquid, or gaseous form, thrown out by a stellar body, especially as a result of supernova explosion.
A table of computed positions occupied by a celestial body over successive intervals of time such as daily; plural ephemerides.
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.
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.
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.
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.
A planetary system consisting of exoplanets orbiting a star other than Sun.
A quasar that has the same characteristics as a radio-quiet quasar with the addition of having strong radio emissions.
A planet which belongs to a star other than Sun, and therefore does not belong to our solar system; same as exoplanet.
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.
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.
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.
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.
A burst of emission in the radio frequencies of the electromagnetic spectrum.
Dimming of light by an intervening medium (the Earth's atmosphere or the interstellar medium). It is usually due to both scattering and absorption.
The branch of astronomy that deals with objects beyond the Milky Way, especially galaxies and quasars.
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.
An interferometer designed for radio frequencies.
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.
An astrophysical jet appearing in the radio wavelengths of the electromagnetic spectrum.
An extended region of diffuse radio emission, often dumbbell shaped, that surrounds a radio galaxy.
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.
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.
The entire angular expanse of the sky viewed by an optical instrument.
A point or small portion of the sky giving stronger radio emission than the sky in its vicinity.
A telescope whose receiver is sensitive to radio waves.
A star that does not belong to a stellar cluster, but happens to be adjacent to it.
A cataclysmic variable star that undergoes outbursts similar to those found in novae.
The calibration of the flux received by a detector in terms of absolute units.
The process of converting crude observational data into usable information for scientific interpretation, by correcting, rearranging, ordering, and simplifying.
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'.
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.
Schönberg-Chandrasekhar limit
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.
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.
A star whose light has undergone reddening.
A spectral line whose wavelength does not coincide with its theoretical value and is shifted toward longer wavelengths.
A telescope in which the image is produced by reflection of light by a concave mirror.
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.
A telescope in which an image is formed by the refraction of light through a lens or lens system.
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.
A galaxy cluster with a particularly large number of galaxies.
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.
Ritchey-Chrétien telescopes
Ritchey-Chretien telescopes
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.
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.
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.
A black hole that possesses angular momentum, as first postulated by Roy C. Kerr in 1963. Opposite of a stationary black hole.
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.
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.
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.
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.
Sgr dSph
Sgr dE
Sag DEG
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.
Sagittarius dwarf elliptical galaxy
Sagittarius dwarf spheroidal galaxy
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.
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.
SETI
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.
A type of star, usually G or K giants, whose spectra show unusually strong absorption lines of barium, strontium, and other s-process elements.
A binary system whose secondary member fills its Roche lobe but whose primary member does not.
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.
Half the length of the major axis of an ellipse; a standard element used to describe an elliptical orbit.
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.
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.
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.
A sharp change in the properties of a gas (density, pressure, temperature).
A comet with a period less than 200 years.
Short-period comet
Short-period variable stars
Short-period variable
Short-period variables
Short period variable
Short period variables
A variable star that has a relatively short period with respect to stars of similar types.
A mirror arrangement with clock drive that reflects light from a celestial body to a fixed position.
The observation and recording of large extents of the sky with a particular instrument using one or more wavelengths in the same spectral domain.
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.
SMC
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.
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.
Solar-like star
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.
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.
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.
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.
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.
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.
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.
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.
Polar plume
Solar polar plume
A coronal feature of the Sun, which appears as long, thin streamers that project outward from the Sun's north and south poles.
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.
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.
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.
One of the convective cells constituting the solar granulation.
An instrument especially designed for solar observations.
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.
The cloud of interstellar gas and dust from which the Sun and the rest of the solar system initially formed.
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.
A telescope designed so that heating effects produced by the Sun do not distort the images.
A spacecraft carrying instruments intended for use in exploration of the physical properties of outer space or celestial bodies other than Earth.
The region in a spiral galaxy that contains concentrations of gas, dust, and massive stars. Spiral arms are created by density waves.
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.
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.
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.
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).
A group of stars dynamically associated so that they have a common motion with respect to the local standard of rest.
The number of stars that appear in a given region of sky, usually counted on a photographic plate or CCD image.
Star formation regions
A region in the interstellar medium where processes of star formation are going on or have occurred in the past.
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.
Starburst galaxy
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.
Stromgren photometry
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Head-tail galaxies
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.
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.
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.
An eclipse in which the whole of the disk of the Sun or Moon is obscured.
The apparent shift of the wavelength towards the shorter wavelength region of the radiation spectrum of an approaching object due to the Doppler effect.
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.
A star that has arrived on the horizontal branch after leaving the red giant branch. It begins helium burning in its core.
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.
Zenith hour rate
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.
YSOs
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.
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.
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.
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.
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.
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.
X-ray transient
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.
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.
A subclass of dwarf novae with an extremely long outburst period, comparable with that of a recurrent nova.
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).
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.
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.
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.
A binary system with semi-major axis as large as 10,000 astronomical units.
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.
DA white dwarf
DA white dwarf stars
A white dwarf whose spectrum shows the Balmer lines of hydrogen only, with no helium or metals.
A white dwarf whose spectrum shows strong He I in the absence of hydrogen or metal lines.
DB white dwarf
DB white dwarf stars
A white dwarf showing a continuous spectrum with no readily apparent lines.
DC white dwarf
DC white dwarf stars
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.
DO white dwarf
DO white dwarf stars
A white dwarf whose spectrum shows carbon features of any kind.
DQ white dwarf
DQ white dwarf stars
A white dwarf whose spectrum shows metal lines only; no H or He.
DZ white dwarf
DZ white dwarf stars
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.
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.
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.
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.
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.
An opening in the Earth's crust from which lava, ash, and hot gases flow or are ejected during an eruption.
Volcano
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.
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.
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.
Van Allen belts
Van Allen radiation belts
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.
The abundance of a chemical element being lower than a reference value, in particular compared to that of the Sun.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The probability that a quantum-mechanical system will make a transition from a given initial state to a given final state.
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.
Transit telescopes
Bondi-Hoyle accretion
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).
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.
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.
A scientist who studies astrophysics.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
A galaxy consisting of a thin disk of stars and interstellar matter which may include spiral arms and bar.
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.
Dirty iceball model
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.
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.
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.
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.
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.
GMC
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.
A meteorite which is composed mainly of iron mixed with smaller amounts of nickel. Iron meteorites make up about 4.4% of all meteorites.
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.
Algol-type variables
Either of the regions around the poles of a planet.
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.
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.
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.
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.
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.
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.
A supernova arising from the core collapse of a massive star.
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.
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.
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.
The central galaxy bulge of the Milky Way.
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.
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.
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.
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.
The study dealing with the processes that gave rise to galaxies in a remarkably early Universe.
The four largest and brightest satellites of Jupiter, that is: Io, Europa, Ganymede, and Callisto.
The study of gamma rays from extraterrestrial sources, especially gamma-ray bursts.
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.
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.
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.
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.
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.
A conspicuous family of stars in the Hertzsprung-Russell diagram composed of red, evolved stars with large sizes.
The universal phenomenon of attraction between material bodies.
Collapse of a mass of material as a result of the mutual gravitational attraction of all its constituents.
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.
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.
The process by which fluctuations in an infinite medium of size greater than a certain length scale (the Jeans length) grow by self-gravitation.
Mutual attraction between any two bodies that have mass.
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.
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.
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.
Astronomical study carried out using a telescope on Earth, as opposed to that from an orbiting satellite.
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].
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.
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.
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.
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.
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.
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.
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.
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.
Gases responsible for the greenhouse effect. These gases include: water vapor; carbon dioxide; methane; nitrous oxide; chlorofluorocarbons; and tropospheric ozone.
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.
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.
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.
A cosmology characterized by clustering of galaxy clusters in increasingly larger systems.
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.
Any form of dark matter which had a significant velocity dispersion (comparable to the velocity of light), when the Universe first became matter-dominated.
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.
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.
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.
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.
A plot of the redshift of galaxies against their distance or against their apparent magnitude.
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.
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.
Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.
The branch of physics dealing with the motion, energy, and pressure of neutral fluids.
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.
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.
The study of infrared properties of astronomical objects.
Infrared emission from a star in excess of that expected from a blackbody with the temperature corresponding to the temperature of the star.
Ultra-diffuse galaxies
Ultradiffuse galaxies
Ultradiffuse galaxy
Ultra-diffuse galaxy
Ultra diffuse galaxies
Ultra diffuse galaxy
UDG
Solar system terrestrial planets
Inner planets
The planets Mercury, Venus, Earth and Mars collectively. They are also known as the terrestrial planets.
Solar system gas giant planets
Outer planets
Gas giants
Extrasolar gaseous planets
Extrasolar gas planets
Extrasolar gaseous giant planets
Extrasolar gas giants
Jovian planets
Optical astronomy
Visible astronomy
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.
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.
The magnitude an extended object (nebula, galaxy, etc.) would have if all of its light were concentrated into a point source.
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.
A medium in which several molecular clouds are situated.
The matter or environment between the galaxies of a cluster.
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.
Electrically charged particles of the solar wind and gas liberated from comets within the solar system.
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.
The absorption of light from stars and other objects by intervening gas and dust in interstellar space.
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).
A global ionizing radiation in the interstellar medium provided by various sources all together.
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).
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.
The process by which ions are produced, typically occurring by interaction with electromagnetic radiation ('photoionization'), or by collisions with atoms or electrons ('collisional ionization').
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.
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.
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.
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.
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.
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.
In Morgan's classification, an elongated galaxy of low surface brightness.
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.
A curve showing the behavior of the light from a variable star over a period of time.
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.
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.
An observed increase in the intensity of radio, extreme ultraviolet, or X-radiation from the Sun from its center to its limb.
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.
The faintest magnitude reachable by an instrument.
The imaginary straight line connecting the object and the objective lens of the viewing device.
In statistics, a regression method that establishes a linear relationship between two random variables.
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.
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.
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.
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.
A bowl-like depression on the rigid surface of the Moon, usually caused by the high-speed impact of a colliding object.
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.
A light color area on the Moon, as contrasted with lunar maria.
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.
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.
A long, narrow, straight or sinuous trench or valley with steep walls and roughly parallel sides on the surface of the Moon.
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.
Two irregular satellite galaxies of our own Galaxy which are visible from the Southern Hemisphere as misty patches in the night sky.
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.
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.
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.
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).
The greatest diameter of an ellipse; it passes through the two foci.
A major subdivision of a planet's internal structure, located beneath the planet's crust and above the central core.
The family of asteroids located at either of the stable Lagrangian points (L4 or L5) of the orbit of Mars.
A region on the surface of the Moon where the gravitational attraction is slightly higher than normal due to the presence of dense rock.
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.
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.
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.
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.
An imaginary great circle on the celestial sphere that passes through its poles and the observer's zenith.
A telescope with a graduated vertical scale, used to measure the declinations of heavenly bodies and sometimes to determine the time of meridian transits.
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.
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.
The state or condition where the force of gravity is very weak, e.g. the weightlessness experienced inside an orbiting spacecraft.
The axis of an ellipse that is perpendicular to the major axis at a point equidistant from the foci.
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.
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.
The point on the celestial sphere blocked from view by Earth and diametrically opposite to the zenith.
A natural satellite orbiting an extrasolar planet.
A solar system object that revolves around a primary body and is not man made.
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.
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.
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).
The branch of physics which is concerned with the study of atomic nuclei, subatomic particles, and their exploitation.
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.
Planetary dynamics
Volcanism
Tectonics
Post-starburst galaxies
Post-starburst galaxy
Post starburst galaxies
Post starburst galaxy
PSB
Star-planet interactions
Interactions between exoplanets and their stars. The similar concept "Solar-planetary interactions" is specific to interactions between the Sun and Solar system planets.
Stellar-planetary interactions
Specific to interactions between the Sun and Solar system planets. The similar concept "Star-planet interactions" is about exoplanets and their stars.
Specific to interactions between the Sun and Earth. The similar concept "Solar-planetary interactions" is about interactions between the Sun and other Solar system planets, or all of them more generally.
Star-exoplanet interactions
The property possessed by a round body that is flattened at the poles.
The application of observational data to the study of the Universe as a whole.
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.
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.
A Freidmann-Lemaitre cosmological model in which space is infinite and of negative curvature or Euclidean, and which expands forever.
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.
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.
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.
The stage when a star fuses oxygen into silicon and sulfur. It occurs only in massive stars, with a mass over eight solar masses.
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.
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.
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.
The point in an orbit of a body where it is closest to its primary; opposite of apoapsis.
In a binary system, the point in the orbit where a component is nearest the other component; opposite of apastron.
The point in the orbit of a body revolving around the Earth at which it is nearest to the Earth; opposite of apogee.
The nearest point to the Sun in an orbit around the Sun; opposite of aphelion.
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).
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.
The magnitude of an object as measured with a photoelectric photometer.
A photometry in which the magnitudes are obtained using a photoelectric photometer.
The apparent magnitude of a star as determined by measuring its brightness on a photographic plate. The photographic magnitude scale is now considered obsolete.
The physical process in which an incident high-energy photon ejects one or more electrons from an atom, ion, or molecule.
An instrument for measuring the amount of light.
A method of deriving the distance of a star using its apparent magnitude and the absolute magnitude inferred from its spectral type.
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.
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.
Magnitude defined for the combination of a photographic plate and a yellow filter, approximating the spectral sensitivity of the eye.
The branch of astronomy that deals with the science of planets, or planetary systems, and the solar system.
The study of the physical properties of the various forms of plasmas and their processing.
The sharp outer boundary of the plasmasphere, at which the plasma density decreases by a factor of 100 or more.
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.
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.
A spacecraft orbit that passes over, or close to, the geographic poles of the Earth or some other solar system object.
The measurement of the polarization state of light, usually through the use of a polarimeter.
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.
A star that evolves in the Hayashi phase and has not yet reached the zero-age main sequence.
A thermonuclear reaction in which two protons collide at very high velocities and combine to form a deuterium.
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.
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.
The Sun when the 11-year cycle of solar activity is at a minimum.
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.
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.
The slowing down of a star's rotation due to radiative momentum transfer caused by emission of electromagnetic radiation.
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.
The process by which the electromagnetic radiation passes through a medium that may contain any combination of scatterers, absorbers, and emitters.
The branch of astronomy that deals with the study of the Universe by means of radio waves.
A continuum emission with frequencies in the radio range of the electromagnetic spectrum.
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.
The capture of an electron by a positive ion. It is the inverse process to ionization.
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.
The ionization of neutral hydrogen atoms formed during recombination epoch in the early Universe.
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.
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.
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.
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.
In general relativity, the metric that describes the space-time outside a static mass with spherically symmetric distribution.
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.
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.
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).
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.
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.
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.
Atmospheric (airglow, auroral emission, artificial light) or extraterrestrial (scattered sunlight from Moon, scattered starlight, interplanetary dust) foreground light that interferes with observations.
The mathematical process that makes a curve smooth.
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.
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.
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.
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).
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.
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.
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).
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.
The angle subtended (8''.79) by the equatorial radius of the Earth at a distance of 1 astronomical unit.
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.
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.
All the constituents making up the Sun's emission: photons, electrons, protons, neutrinos, and atomic nuclei.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Stars for which accurate color indices and/or magnitudes exist, defining a standard system.
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.
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.
The branch of astronomy that deals with the study of stars, their physical properties, formation, and evolution.
The gradual changes in physical state (spectrum, luminosity, temperature) and chemical composition that occurs during the life of a star.
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.
The nuclear reaction process taking place inside stars, whereby chemical elements are produced from pre-existing nuclei heavier than hydrogen and helium.
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).
The precise measurement of a star's brightness, usually through several specific wavelength bands.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The disruption of an extended astronomical object under the action of the tidal forces exerted by another nearby object.
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.
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.
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.
The lower part of the Earth's atmosphere in which temperature decreases with height, except for local areas of temperature inversion.
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.
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.
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.
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.
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.
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.
A dark area, especially the blackest part of a shadow from which all light is cut off.
The general term applied to the atmosphere above the troposphere.
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.
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.
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.
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.
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.
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.
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.
The Fourier transform of a source's brightness distribution, weighted by the characteristics of the interferometer's antennas.
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.
Water in the gaseous state. Atmospheric water in vapor form; one of the most important components of the atmosphere.
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.
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.
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.
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.
The study of synthesizing and manipulating biological devices and systems.
The point on the celestial sphere directly above the observer's head, opposite to the direction in which gravity acts. Opposite of nadir.
Wind that a star perceives from the ISM.
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.
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.
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.
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.
Occultation of a star by another object.
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.
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.
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.
A class of pulsating variable stars with short periods. These stars are hot (hence "blue" in name) and show relatively large amplitudes. The evolutionary state of these stars is not known. -- https://github.com/astrothesaurus/UAT/issues/286
Some of the binary stars show additional period that this ~33 times longer than the orbital period and this group is called DPVs. Most of these systems are also showing eclipses, but that's not required. DPVs studied in detail show circumprimary discs. Note that the name seems general, but refers to only a specific group of binary stars. -- https://github.com/astrothesaurus/UAT/issues/286
These stars are usually hot blue-white stars of spectral class B and should not be confused with Cepheid variables, which are named after Delta Cephei and are luminous supergiant stars. -- Wikipedia, https://en.wikipedia.org/wiki/Beta_Cephei_variable
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.
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.
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.
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.
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.
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.
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.
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.
Asteroid
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
An observational technique that uses the interference phenomenon to substantially increase the resolving power of telescopes.
Between two points A and B, the angle subtended by lines drawn from an observing point O to A and B.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
A colorless, odorless, inflammable gas of formula CH4; the simplest hydrocarbon.
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.
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.
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).
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.
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.
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 strong source of radio emission, called Sagittarius A, that is less than 15 astronomical units in diameter. Observations of the speeds at which clouds of ionized gas are revolving round the Galactic center imply that several million solar masses of material are concentrated within a region of about one light-year in radius. Since only about half of this mass can be accounted for by stars, it seems likely that the balance (about 2.5 million solar masses) is contained in a central black hole and that accretion onto this black hole is the underlying source of the energy radiated by Sagittarius A. The Milky Way also has a dark matter component. The Galactic rotation curve indicates that there is a large amount of invisible non-baryonic surrounding the whole Galaxy.
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.
Measurement of the absolute fluxes of the components of different frequencies in the spectrum of a light source.
Galactic archaeology
Study of galactic evolution through study of stellar populations.
Calibration
Earth's clouds
Clouds
Atmospheric clouds
Comet Halley
(1) Ceres
Asteroid Ceres
(134340) Pluto
134340 Pluto
Ceres
1 Ceres
Vesta
(4) Vesta
4 Vesta
Asteroid Vesta
Planetary ionospheres
Planetary climates
(433) Eros
433 Eros
Asteroid Eros
Eros
Titan
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.
Triton
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.
Jupiter's satellites
Jupiter's moons
Ganymede
Europa
Jupiter II
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.
Jupiter III
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.
Jupiter I
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.
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.
1P/Halley
Halley's comet
Henry Draper Catalogue
Astronomy using the (human) eye as the primary detector; this includes both naked-eye observations and observations through telescopes without further instrumentation (like cameras, spectrographs, etc).
Visual observation
Remote sensing
Technique that utilizes electromagnetic waves to detect, measure, and obtain information about an object that is not in contact with the sensing apparatus.
Intended to cover planetary remote sensing, a "catch-all term for any planetary work using remote observations. It would include radar, but also VNIR, TIR, etc. wavelengths."
Aurorae
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.
Lightning
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.
Zodiacal dust bands
Dust bands
109P/Swift-Tuttle
P/Shoemaker-Levy 9
Enceladus
Mimas
Near-sun comets
Encompasses all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU).
Sundivers
Sungrazers
Sunskirters
Comets with orbits that intersect the solar photosphere.
Objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit.
Kreutz group
Kreutz sungrazers
Kracht group
Marsden group
Marsden sunskirters
Meyer group
Meyer sunskirters
Kracht sunskirters
Sun-plunging
Sun-plunger
Sun-plunging comet
Sun-plunging comets
Aperiodic comets
IRAS dust bands
Interstellar objects
Interstellar comets
2I/Borisov
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.
IRAS galaxies
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.
Plutonian satellites
Charon
Nix
Styx
Kerberos
Hydra
Moons of Pluto
Pluto's moons
Ariel
Umbriel
Tethys
Dione
Naiad
Thalassa
Leda
Thebe
Amalthea
Eris
Makemake
Minor planets
Earth ionosphere
Earth's ionosphere
Comet origins
Sun-Earth interactions
Planetary-disk interactions
Exoplanet migration
Planetary migration
Asteroid surfaces
Asteroid dynamics
Asteroid rotation
Small Solar System bodies
Small Solar System body
Natural satellite dynamics
Natural satellite formation
Satellite formation
Comet dynamics
Asteroid satellites
Asteroid moons
Binary asteroids
Binary systems
Multiple systems
Double asteroids
Satellites of asteroids
Minor planet moons
Binary small Solar System body systems
Multiple small Solar System body systems
Binary small Solar System bodies
Multiple small Solar System bodies
Multi-body asteroids
Binary SSSB systems
Binary SSSBs
Multiple SSSB systems
Multiple SSSBs
Natural satellite surfaces
Natural satellite atmospheres
1I/'Oumuamua
Multiple asteroids
Asteroid multiples
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.
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).
Objects that pass within 33 solar radii of the Sun's center, equal to half of Mercury's perihelion distance.
Meteoroid dust
Cambridge Catalogue of Radio Sources
Modified gravity
Solar terrestrial relation
Gamma ray transient source
γ-ray transient source
γ-ray transient sources
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.
A-type supergiant stars
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.
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.
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.
Galaxy colours
Multi-colour photometry
Two-colour diagrams
Solar analogues
Centre to limb observations
Equation of centre
Galactic anticentre
Galactic centre
Galaxy anticentres
Astronomy data modelling
Millimetre astronomy
Submillimetre astronomy
Celestial objects catalogues
Exoplanet catalogues
FK catalogue
Astronomy data visualisation
Brown dwarves
L dwarves
T dwarves
Y dwarves
Charge exchange ionisation
Electron impact ionisation
Ionisation
Photoionisation
Ionised coma gases
Hot ionised medium
Warm ionised medium
Expectation-Maximisation Algorithm
Phase dispersion minimisation
Pulsar wind nebulae
Plerion
PWN
PWNe
Crab Nebula
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.
A bubble of shocked relativistic particles, produced when a pulsar's relativistic wind interacts with its environment.
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.
Atomic data
Atomic reactions
Charge transfer
Electron impact excitation
Mutual neutralization
Electron-impact excitation
Electron-impact ionization
Time-of-flight mass spectrometry
TOFMS
Molecular beams
Ion trapping
Ion-storage rings
Molecular reactions
Nuclear decay
Condensed matter physics
Ice physics
Dust physics
Mineral physics
Reaction models
Quantum-chemical calculations
Chemical kinetics
Potential energy surfaces
Structure determination
Chemical thermodynamics
Chemical reaction network models
Monte Carlo methods
Reaction rate equations
Crossed-beams
Merged-beams
Supersonic expansion
Production techniques
Electric discharge
Laser ablation
Pyrolysis
Electronic spectroscopy
Rotational spectroscopy
Vibrational spectroscopy
Ice spectroscopy
Microwave spectroscopy
Millimeter-wave spectroscopy
Cavity-enhanced Fourier-transform
Chirped-pulse Fourier-transform
Direct absorption
CP-FTMW
Complex organic molecules
COMs
Fullerenes
Metal-containing molecules
Molecular data
Molecular structural parameters
Molecular spectroscopic constants
Electron recombination reactions
Ion-neutral reactions
Isomerization reactions
Neutral-neutral reactions
Photodissociation reactions
Small molecules
Dust destruction
Dust formation
Optical constants (Dust)
Dust composition
Ice composition
Ice phases
Ice porosity
Optical constants (Ice)
Physical properties (Ice)
Radiation interactions (Ice)
Thermal properties (Ice)
Callisto
Adaptive optics
Craters
Cratering
Lunar cratering
Gamma-ray spectroscopy
Infrared spectroscopy
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.
Collisional processes
Collisional processes within planetary systems.
Radar observations
Planetary geology
Geological processes
Planetary thermal histories
James Webb Space Telescope
Ultraviolet spectroscopy
UV spectroscopy
Planetary system evolution
Solar system evolution
Regolith
Ring resonance
Spin-orbit resonances
Solid body tides
Impact gardening
Atmospheric dynamics
Atmospheric evolution
Glaciation
Glaciers
Planetary mineralogy
Astronomy data sonification
Astronomy image processing
Gamma ray spectroscopy
Exoplanet atmospheric dynamics
Exoplanet atmosphere dynamics
Exoplanet atmospheric evolution
Exoplanet atmosphere evolution
Exoplanet atmosphere composition
Exoplanet atmosphere variability
Atmosphere evolution
Atmosphere dynamics
Atmospheric structure
Atmosphere structure
Exoplanet atmospheric structure
Exoplanet atmosphere structure
Earth's seasons
Comet dust tails
Comet ion tails
Greenhouse effect
Lunar regolith
Lunation
Pallas
Light pollution
Magnetic poles
Magnetic storms
Albedo
Kepler's laws
Nuclear fission
Nuclear fusion
Analemma
Prograde orbit
Retrograde orbit
Binoculars
Stellar aberration
Optical aberration
Optical filters
Asterisms
Gnomons
Planispheres
Sundials
Ecliptic
Zodiac
Circumpolar
Horizon
Cardinal directions
Zenith distance
Chromatic aberration
Spherical aberration
Eye pieces
Primary mirror
Secondary mirror
Lenses
Space stations
International Space Station
Telescope properties
Aperture
Focal length
Focal ratio
Prime focus
Rockets
Keplerian telescopes
Galilean telescopes
Star lore
Starlore
Magnetogram
Gaia
Geocentric model
Heliocentric model
Cosmological principle
Galaxy bars
Galactic bar
Cultural astronomy
Indigenous astronomy
Astrosociology
Exocomets
Comets (Extrasolar)
Not to be confused with Exocomets.
Not to be confused with Interstellar objects.
Aldebaran
Arcturus
Betelgeuse
Polaris
Proxima Centauri
Sirius
Quadrupole mass spectrometry
Natural satellite evolution
High contrast techniques
High contrast imaging
High contrast spectroscopy
Experimental data
Theoretical data
Stephan's quintet
Natural decay
Molecule destruction
Molecule formation
Experimental models
Darkrooms
Huya
Far-infrared astronomy
Neutrino oscillation
I galaxies
RRA stars
Continuum radio emission
Galaxy components
Elemental abundance
Solar composition
Solar magnetism
Stellar chemical composition
Stellar composition
Stellar elemental abundances
Seyfert's sextet
Fork mounts
Yoke mounts
English mounts
German equatorial mounts
Horseshoe mounts
Two-spectrum binary stars
Compact binary components
Compact binary systems
Equinox correction
Eruptive binary stars
Accreting binary stars
Disk population
Disk stars
RC Aurigae stars
RR Telescopii stars
Supernova evolution
White dwarf evolution
S Vulpeculae stars
Intermediate population stars
Gravitational microlensing [Exoplanets]
Polarimetry [Exoplanets]
Radial velocity methods [Exoplanets]
Individual planetary nebulae
Individual quasars
Be type stars
Exoplanet astrometry
Lithium abundance
Helium abundance
Beryllium abundance
Boron abundance
Cirrus vertebratus
Cirrus uncinus
Cirrus spissatus
Cirrus floccus
Cirrus castellanus
Noctilucent clouds
Cirrus fibratus
19521 Chaos (Kuiper Belt object)
Borasisi
Deucalion
Logos
Teharonhiawako
Quaoar
Varuna
Haumea
Sedna
Ixion
Orcus
Penumbral filaments
Cometary studies
Wilson-Bappu Effect
Solar properties
This concept should possibly be deprecated? At the very least "L" and "T" type dwarf stars should be removed as child concepts, as those are both actually brown dwarfs, which are not stars.
Instead, concept could possibly be renamed "late type dwarfs" as not all child concepts are stars.
"Diffuse radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or particulates in the atmosphere. Also called sky radiation, diffuse skylight, or just skylight, it is the determinative process for changing the colors of the sky." https://en.wikipedia.org/wiki/Diffuse_sky_radiation
An Etymological Dictionary of Astronomy and Astrophysics, by M. Heydari-Malayeri
Modified from An Etymological Dictionary of Astronomy and Astrophysics, by M. Heydari-Malayeri
The International Astronomical Union (IAU) at its 30th Meeting approved the Resolution B4 proposed by the IAU Executive Committee recommending the use of Hubble-Lemaitre law instead of Hubble's law, after Edwin Hubble (1889-1953), the American astronomer who published his results in 1929 and Georges Lemaître, Belgian priest and astronomer, who published a paper on the expansion of the Universe in 1927. From An Etymological Dictionary of Astronomy and Astrophysics, by M. Heydari-Malayeri.
Adapted from An Etymological Dictionary of Astronomy and Astrophysics, by M. Heydari-Malayeri
Glossary of Astronomy and Astrophysics, J. Hopkins, University of Chicago Press (1976)
Astrobites Guide to Classification of Galaxies and AGNs
See instead http://astrothesaurus.org/uat/1266 (Plutinos)
See instead http://astrothesaurus.org/uat/529 (Far infrared astronomy)
See instead http://astrothesaurus.org/uat/935 (Long period variable stars)
See instead http://astrothesaurus.org/uat/944 (Luminous blue variable stars)
See instead http://astrothesaurus.org/uat/1066 (Mira variable stars)
See instead http://astrothesaurus.org/uat/1104 (Neutrino oscillations)
See instead http://astrothesaurus.org/uat/553 (FU Orionis stars)
See instead http://astrothesaurus.org/uat/1327 (R Coronae Borealis variable stars)
See instead http://astrothesaurus.org/uat/864 (Irregular galaxies)
See instead http://astrothesaurus.org/uat/1413 (RRab stars)
See instead http://astrothesaurus.org/uat/634 (Gamma-ray telescopes)
See instead http://astrothesaurus.org/uat/1340 (Radio continuum emission)
See instead http://astrothesaurus.org/uat/1410 (RR Lyrae variable stars)
See instead http://astrothesaurus.org/uat/719 (Henry Draper Catalog)
See instead http://astrothesaurus.org/uat/622 (Galaxy structure)
See instead http://astrothesaurus.org/uat/224 (Chemical abundances)
See instead http://astrothesaurus.org/uat/1474 (Solar abundances)
See instead http://astrothesaurus.org/uat/1503 (Solar magnetic fields)
Extranous grouping concept, low usage in literatuere.
See instead http://astrothesaurus.org/uat/1577 (Stellar abundances)
See instead http://astrothesaurus.org/uat/468 (Equatorial mount)
See instead http://astrothesaurus.org/uat/1557 (Spectroscopic binary stars)
See instead http://astrothesaurus.org/uat/574 (Galaxy abundances)
See instead http://astrothesaurus.org/uat/283 (Compact binary stars)
See instead http://astrothesaurus.org/uat/672 (Gravitational microlensing)
See instead http://astrothesaurus.org/uat/1278 (Polarimetry)
See instead http://astrothesaurus.org/uat/1332 (Radial velocity)
See instead http://astrothesaurus.org/uat/80 (Astrometry)
See instead http://astrothesaurus.org/uat/142 (Be stars)
UAT does not provide a general concept for specific objects.
See instead http://astrothesaurus.org/uat/74 (Astrobiology)
See instead http://astrothesaurus.org/uat/1484 (Solar coronal holes)
See instead http://astrothesaurus.org/uat/307 (Stellar coronal holes)
See instead http://astrothesaurus.org/uat/2055 (Radiative processes)
See instead http://astrothesaurus.org/uat/1779 (Voids)
See instead http://astrothesaurus.org/uat/2044 (Morgan-Keenan Classification)
See instead http://astrothesaurus.org/uat/250 (Classical Kuiper belt objects)
See instead http://astrothesaurus.org/uat/258 (Earth's clouds)
See instead http://astrothesaurus.org/uat/717 (Helium-weak stars)
See instead http://astrothesaurus.org/uat/419 (Dwarf planets)
See instead http://astrothesaurus.org/uat/1268 (Plutoids)
See instead http://astrothesaurus.org/uat/1430 (Scattered disk objects)
See instead http://astrothesaurus.org/uat/271 (Comae)
See instead http://astrothesaurus.org/uat/280 (Comets)
See instead http://astrothesaurus.org/uat/1169 (Optical observation)
See instead http://astrothesaurus.org/uat/946 (Luminous infrared galaxies)
See instead http://astrothesaurus.org/uat/72 (Asteroids)
See instead http://astrothesaurus.org/uat/1469 (Small solar system bodies)
Admittedly clunky phrasing, suggestions welcome.
Adapted from Nature Subjects, https://www.nature.com/subjects/atomic-and-molecular-collision-processes