Astrophysics (Index)About

galaxy

(extragalactic nebula)
(gravitationally-bound group of stars)

A galaxy is a group of millions or billions of stars gravitationally bound, such as the Milky Way. Other galaxies are observed as relatively-fuzzy objects, some showing a spiral pattern. In past terminology, galaxy meant what we now call the Milky Way, and other galaxies observed were considered a type of nebula. The question of whether these nebulae were beyond the Milky Way was of research interest for some time and even after that was established, galaxies were still often referred to as extragalactic nebulae.

Galaxy classification:

Elliptical, lenticular, and possibly irregular galaxies are termed early-type galaxies (ETGs) and disk galaxies are termed late-type galaxies (LTGs), with the most fully-formed spirals being "latest": this is somewhat opposite of the use of the terms early/late for stars: the galaxies labeled "early-type" generally have fewer early-type stars; Edwin Hubble introduced the early/late galaxy terms because folks were used to the terms for stars, but their implication regarding stellar evolution was being (properly) dismissed at the time, and Hubble explicitly noted he was not implying any galaxy-evolution temporal relationship.

Dwarf galaxies are small galaxies, as small as 100 parsecs across, which may be elliptical or irregular or some types specific to dwarfs, e.g., dwarf spheroidal galaxies. Some dwarf galaxies are satellite galaxies.

Star formation is most seen in spiral/disk galaxies. Elliptical galaxies have been theorized to be the results of mergers that have lost their gas, e.g., by going through a starburst galaxy stage. Molecular gas depletion time is the time it takes for the gas in a star-forming galaxy to be used. The term blue galaxy designates a galaxy with early-type (hot, blue) stars, indicating recent stellar formation, and red galaxy indicates the opposite, with late-type, red stars, whose age-range suggests they are older. This represents a galaxy bimodality of galaxies forming stars and not.

The movement of a galaxy's stars as compared to the mass revealed by the stars indicates there is hidden mass in galaxies, which has been termed dark matter. In some galaxies, including the Milky Way, a central supermassive black hole has been detected, and it is suspected this is normal or universal for substantial galaxies, but they do not provide enough mass to explain the stars' motion.

For the purposes of modeling, such as processes within galaxies, galaxies are often modeled with a basic/simplified shape, such as spherical (for the simplest models), axisymmetric, or triaxial.


(object type)
Further reading:
http://en.wikipedia.org/wiki/Galaxy

Referenced by pages:
Abell Catalog
active galaxy
AEGIS
Arecibo General Catalog (AGC)
active galactic nucleus (AGN)
Alcock-Paczyński effect (AP effect)
ALFALFA
alpha CO (αCO)
Arp-Madore Catalog of Southern Peculiar Galaxies (AM)
adaptive mesh refinement (AMR)
Atlas of Peculiar Galaxies (APG)
plate measuring machine (APM)
APM Galaxy Survey (APMUKS)
ARCADE
Arakelian Catalog (Ark)
asterism
astronomical catalog
astronomical object
astronomical quantities
astronomical survey
Athena
Atlas 3D
axisymmetric
AzTEC-3
Balmer series (H)
band shifting
baryon acoustic oscillations (BAO)
bar
baryonic matter
Balmer-break galaxy (BBG)
brightest cluster galaxy (BCG)
BDF
black hole accretion rate (BHAR)
binary star
black hole (BH)
blind survey
Butcher-Oemler effect (BOE)
Baryon Oscillation Spectroscopic Survey (BOSS)
binary SMBH (BSMBH)
Asiago Supernova Catalog (B/SN)
Boltzmann Transport Equation (BTE)
bulge
Caldwell Catalog
CALIFA
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS)
Canis Major Dwarf Galaxy
CARMA EDGE
cold dark matter (CDM)
celestial mechanics
Cepheid variable (CEP)
CfA Redshift Survey
CFHTLS
Canada-France Redshift Survey (CFRS)
cosmic gamma ray background (CGB)
Catalog of Galaxies and Clusters of Galaxies (CGCG)
circumgalactic medium (CGM)
chemodynamics
cosmic infrared background (CIB)
ionized carbon fine structure line ([CII])
Circinus Galaxy (ESO 97-G13)
conditional luminosity function (CLF)
cloud
cluster radius
carbon monoxide (CO)
cold gas
COLD GASS
color-color diagram (CCD)
color-magnitude diagram (CMD)
Comet Galaxy
computational astrophysics
conservation law
constellation
cooling function
COPSS
Coriolis force
cosmic distance ladder
cosmic string
cosmic web
cosmological simulation
cosmological zoom simulation
Cosmic Evolution Survey (COSMOS)
Council of Giants
conditional stellar mass function (CSMF)
Cerro Tololo Inter-American Observatory (CTIO)
Cygnus A (3C 405)
dark flow
dark galaxy
dark matter
dark matter halo
data cube
DECaLS
DEEP2
deep field (DF)
deep survey
Dark Energy Survey (DES)
de Vaucouleurs' profile
DEVILS
disk galaxy
velocity dispersion (σ)
dispersion measure (DM)
Deep Lens Survey (DLS)
dark matter filament
Dwingeloo Obscured Galaxy Survey (DOGS)
Drake equation
dropout
dwarf galaxy
dwarf galaxy problem
dwarf spheroidal galaxy (dSph)
dynamical friction
dynamical instability
dynamo
EAGLE
effective radius (Re)
Extended Groth Strip (EGS)
Event Horizon Telescope (EHT)
ejection
emission line galaxy (ELG)
elliptical galaxy
EMPIRE Survey
extreme mass ratio inspiral (EMRI)
ESO Nearby Abell Cluster Survey (ENACS)
epoch of reionization (EOR)
epicycle
epicyclic frequency
epoch of galaxy formation
extremely red object (ERO)
escape fraction
escape velocity (Ve)
ESO/Uppsala Survey of the ESO (B) Atlas
evaporation
extended source
extinction
extragalactic astronomy
Faber-Jackson relation (FJR)
faint blue galaxy (FBG)
star formation feedback
field galaxy
Faint Infrared Grism Survey (FIGS)
final parsec problem
FIRE Simulations
first galaxies
FORNAX
fossil group
frame of reference
fast radio burst (FRB)
Fanaroff-Riley classification
Fairall Galaxy List (Frl)
fundamental plane
GADGET-2
galactic astronomy
galactic disk
galactic electron density
galactic halo
galactic plane
galactic tide
galaxy age determination
galaxy bias
galaxy classification
galaxy cloud
galaxy cluster (CL)
galaxy environment
galaxy filament
galaxy formation
galaxy group
galaxy main sequence
galaxy merger
galaxy power spectrum
galaxy SED
galaxy subgroup
Galaxy Zoo
Galaxy Evolution Explorer (GALEX)
GALEX Arecibo SDSS Survey (GASS)
GAMA
gas flow
gas fraction
gas fraction estimation
gas streamer
Green Bank Telescope (GBT)
globular cluster luminosity function (GCLF)
G-dwarf problem
gravitational instability (GI)
giant elliptical galaxy
Gini/M20
GISMO
globular cluster (GC)
GOODS NICMOS Survey (GNS)
pea galaxy (GP)
grand design spiral galaxy
gravitational lensing
gravity
Great Attractor
great debate
GW170817
Galaxy Zoo 2 (GZ2)
habitable zone (HZ)
Hydrogen Accretion in Local Galaxies Survey (HALOGAS)
Hα survey
halo abundance matching (HAM)
Hawaii K-band Galaxy Survey
hypercompact stellar system (HCSS)
Hubble Deep Field (HDF)
Hubble Deep Field South (HDF-S)
heating
Herschel Redshift Survey (HeRS)
HFLS3
neutral atomic hydrogen (HI)
hierarchical assembly of galaxies
HII region (HII)
Hill radius
HIPASS
Parkes HI Zone of Avoidance Survey (HIZOA)
hyperluminous infrared galaxy (HLIRG)
Hoag's Object
Holmberg radius (RH)
hot DOG
H-R diagram (HRD)
Hubble constant (H0)
Hubble diagram
Hubble expansion
Hubble Ultra-Deep Field (HUDF)
high-velocity cloud (HVC)
hypervelocity star (HVS)
IC 342
intergalactic medium (IGM)
intensity mapping
intergalactic dust
interstellar astrophysics
interplanetary medium (IPM)
iron (Fe)
irregular galaxy
interstellar medium (ISM)
interstellar magnetic field (ISMF)
I Zwicky 18 (I Zw 18)
James Clerk Maxwell Telescope (JCMT)
jellyfish galaxy
Kapteyn universe
kinematic distance
KMOS3D
Kormendy Relation
Kiso Survey of Ultraviolet Excess Galaxies (KUG)
Lyman-alpha emitter (LAE)
Lambda-CDM model (ΛCDM)
LAMOST
Laniakea Supercluster
LasDamas
Lyman-break galaxy (LBG)
luminous blue variable (LBV)
Las Campanas Redshift Survey (LCRS)
lenticular galaxy (S0)
Lick indices
Limber approximation
line broadening
LINER
luminous infrared galaxy (LIRG)
Large Magellanic Cloud (LMC)
Large Millimeter Telescope (LMT)
Local Group (LG)
Local Sheet
luminous red galaxy (LRG)
low-surface-brightness galaxy (LSB galaxy)
large scale structure (LSS)
luminosity density
luminosity function (LF)
Local Volume (LV)
Lyman-alpha forest
Lyman series (L)
Messier 100 (M100)
Pinwheel Galaxy (M101)
Sombrero Galaxy (M104)
Messier 106 (M106)
Andromeda (M31)
Triangulum Galaxy (M33)
Whirlpool Galaxy (M51a)
M64
Messier 74 (M74)
M82
M87
MACHO
Magellanic clouds (MC)
magnetic field
magnitude
mass-to-light ratio (M/L)
mass transport
Morphological Catalogue of Galaxies (MCG)
Medium Deep Survey (MDS)
Meier paradox
Messier Catalog (M)
metallicity (Z)
Millennium Galaxy Catalogue (MGC)
gravitational microlensing
Milkomeda
Milky Way
Milky Way subgroup
Markarian galaxy (Mkn)
mock catalog
Morgan classification
morphology-density relation
M-sigma relation
Munich Near-Infrared Cluster Survey (MUNICS)
N-body problem
N-body simulation
nebula
NASA/IPAC Extragalactic Database (NED)
N galaxy
NGC 2363
NGC 253
NGC 2770
Spindle Galaxy (NGC 5866)
NGC 6946
ammonia (NH3)
N-point function
observable universe
1 Jansky Empty Field Survey (1 Jy)
Oort constants
open cluster (OC)
PAndAS
principle component analysis (PCA)
PCA analysis
peak star-formation epoch
peculiar velocity
PG 1302-102
Principal Galaxies Catalog (PGC)
PHIBSS
planetary nebula luminosity function (PNLF)
POINT-AGAPE
point source
polar-ring galaxy
plane of the sky (POS)
Polar-ring Catalog (PRC)
primary
primordial gravitational waves
PRIMUS
projected separation
protogalaxy
pseudobulge
PSF fitting
quantum fluctuations
quenched galaxy
radio relics
ram pressure
RAMSES
rare designator prefixes
Reference Catalogue of Bright Galaxies (RC)
repulsive dark matter (RDM)
red and dead
reddening
redshift-angular size relation
redshift space
redshift survey
retrograde accretion
radio galaxy (RG)
RASS/Green Bank Catalog (RGB)
Radio Galaxy Zoo (RGZ)
ring galaxy
rotation curve
r-process
radio source (RS)
Shapley-Ames Catalog (SAC)
Sagittarius Dwarf Irregular Galaxy (SagDIG)
SAGE
satellite galaxy
satellite plane problem
surface brightness fluctuation (SBF)
Second Byurakan Survey (SBS)
scale height (H)
scattering
Scott effect
Subaru Deep Field (SDF)
Sloan Digital Sky Survey (SDSS)
spectral energy distribution (SED)
Spitzer Extended Deep Survey (SEDS)
Sérsic profile
Seyfert galaxy
star formation history (SFH)
synthetic field method (SFM)
star formation rate (SFR)
star-formation rate stellar-mass ratio
SHARDS
Spitzer HETDEX Exploratory Large Area Survey (SHELA)
shell
sidereal
signatures of formation
SIMBAD
SIMSTACK
The Survey for Ionization in Neutral Gas Galaxies (SINGG)
SINS
6dF Galaxy Survey (6dFGS)
supermassive black hole (SMBH)
Small Magellanic Cloud (SMC)
submillimeter galaxy (SMG)
supernova (SN)
SN 2014J
solid angle (Ω)
Soltan argument
spectral feature
spectral line
spectral line designation
spectral signature
spiral arm
spiral galaxy
Spitzer Space Telescope (SST)
stellar population synthesis (SPS)
stellar population synthesis code
South Pole Telescope (SPT)
standard candle
STARBURST99
starburst galaxy
star count
star formation (SF)
STARSMOG
star system
stellar dynamics
stellar evolution
stellar kinematics
stellar nucleus
stellar population
stellar stream
Subaru Telescope
subgrid-scale physics
submillimeter astronomy
supercluster
superluminal motion
surface brightness (SB)
synthetic photometry
synthetic spectrum
systemic velocity
TAIPAN
telluric star
Tully-Fisher relation (TFR)
thermal dust emission
3C 273
3C 295
3D-HST
three dimensional model
tidal arm
tidal force
tidal tail
timescale (t)
TiNy Titans (TNT)
21-cm line
TolTEC
Toomre sequence
2dF Galaxy Redshift Survey (2dFGRS)
Two Micron All-sky Survey (2MASS)
Type Ia supernova
ultra diffuse galaxy (UDG)
ultra-faint dwarf galaxy (UFD)
Uppsala General Catalogue (UGC)
ultraluminous infrared galaxy (ULIRG)
Ursa Major II Dwarf
van den Bergh galaxy classification
Virgo Cluster Catalog (VCC)
velocity-metallicity relation
VIMOS Public Extragalactic Redshift Survey (VIPERS)
Virgo Cluster
virial theorem
VISTA
void galaxy
Voyager
Catalogue of Interacting Galaxies (VV)
VIMOS-VLT Deep Survey (VVDS)
wide binaries (WB)
weak lensing (WL)
James Webb Space Telescope (JWST)
Westerhout Radio Survey (W)
Wide-field Multi-object Spectrograph (WFMOS)
WINGS
Wolf-Lundmark-Melotte (WLM)
Wolf-Rayet galaxy
Wolf-Rayet star
XBONG
FourStar Galaxy Evolution Survey (ZFOURGE)
zone of avoidance (ZOA)

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