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. Before this structure was worked out (which was in the first half of the twentieth century), the terms the galaxy and Milky Way both indicated the glowing portion of the night sky now known to be our view of the bulk of the Milky Way galaxy, and what we observe as other galaxies were then considered a type of nebula. The question of whether these nebulae were beyond the night sky's discernible stars arose centuries ago, was of research interest for considerable time, and even after that fact was established, galaxies were still often referred to as extragalactic nebulae. Galaxy classification based upon its observed shape (morphology):

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 does not match the early/late terminology of the stars in the galaxy: 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 the early/late implication regarding stellar evolution was being (properly) dismissed at the time, and while Hubble's use of the terms for galaxies implicitly suggest they form disk and spiral structures as they age, he explicitly said he adopted the terms merely as a convenience.

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 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 and gas 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 (SMBH) has been detected, and it is suspected this is normal or universal for substantial galaxies, but they are of far too little mass to explain the stars' motion. To explain it, the dark matter must be spread throughout the galaxy, extending beyond the visible galaxy in a larger, somewhat-spherical shape that is termed the galaxy's dark matter halo, which is considered an invisible part of the galaxy. Dynamics further demands the dark matter constitute on the order of 80-90% of a galaxy's total mass and galaxies are thought to have formed after gravity produced regions of high dark-matter density, which drew together the gas to form the stars.

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:
https://en.wikipedia.org/wiki/Galaxy
https://astronomy.swin.edu.au/cosmos/g/galaxy
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/galaxcon.html
http://sdssorgdev.pha.jhu.edu/dr1/en/astro/galaxies/galaxies.asp
https://pages.uoregon.edu/jimbrau/astr123/Notes/Chapter24.html
http://spiff.rit.edu/classes/phys240/lectures/gal_taxon/gal_taxon2.html

Referenced by pages:
1 Jansky Empty Field Survey (1 Jy)
21-cm experiment
21-cm line
2175 angstrom feature
2dF Galaxy Redshift Survey (2dFGRS)
3C 273
3C 295
3D-HST
6dF Galaxy Survey (6dFGS)
Abell 85 (A85)
Abell Catalog
active galactic nucleus (AGN)
active galaxy
adaptive mesh refinement (AMR)
AEGIS
Alcock-Paczyński effect (AP effect)
ALFALFA
ALMACAL
alpha CO (αCO)
alpha element
ammonia (NH3)
AMS-02
AMUSE²
Andromeda (M31)
angular distance
APM Galaxy Survey (APMUKS)
APOGEE
Arakelian Catalog (Ark)
ARCADE
Arecibo General Catalog (AGC)
Arp-Madore Catalog of Southern Peculiar Galaxies (AM)
Asiago Supernova Catalog (B/SN)
asterism
ASTHROS
astronomical catalog
astronomical object
astronomical quantities
astronomical survey
AT2018cow
ATHENA
Atlas 3D
Atlas of Peculiar Galaxies (APG)
axisymmetric
AzTEC-3
Balmer jump (BJ)
Balmer series (H)
Balmer-break galaxy (BBG)
band shifting
bar
baryon acoustic oscillations (BAO)
Baryon Oscillation Spectroscopic Survey (BOSS)
baryonic matter
BDF
binary SMBH (BSMBH)
binary star
black hole (BH)
black hole accretion rate (BHAR)
blind survey
Boltzmann transport equation (BTE)
brightest cluster galaxy (BCG)
bulge
Bullet Cluster
Butcher-Oemler effect (BOE)
calcium-rich gap transient (ca-rich gap transient)
Caldwell Catalog
CALIFA
Canada-France Redshift Survey (CFRS)
Canis Major Dwarf Galaxy
CARMA EDGE
Catalog of Galaxies and Clusters of Galaxies (CGCG)
Catalogue of Interacting Galaxies (VV)
celestial mechanics
Centaurus A
Cepheid variable (CEP)
Cerro Tololo Inter-American Observatory (CTIO)
CfA Redshift Survey
CFHTLS
chemical tagging
chemodynamics
Circinus Galaxy (ESO 97-G13)
circumgalactic medium (CGM)
cloud
cluster radius
CMB dipole
CO ladder
cold dark matter (CDM)
cold gas
COLD GASS
color-color diagram (CCD)
color-magnitude diagram (CMD)
Comet Galaxy (PGC 3234374)
computational astrophysics
conditional luminosity function (CLF)
conditional stellar mass function (CSMF)
conservation law
constellation
cooling function
COPSS
Coriolis force
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS)
cosmic distance ladder
Cosmic Evolution Survey (COSMOS)
cosmic gamma ray background (CGB)
cosmic infrared background (CIB)
Cosmic Lens All-sky Survey (CLASS)
cosmic string
cosmic web
cosmological distance
cosmological redshift
cosmological simulation
cosmological zoom simulation
Council of Giants
Cygnus A (3C 405)
damped Lyman alpha absorber (DLA)
dark energy (Λ)
Dark Energy Survey (DES)
dark flow
dark galaxy
dark matter (DM)
dark matter detector
dark matter filament
dark matter halo
dark-matter-free galaxy
David Dunlap Observatory Catalog (DDO)
de Vaucouleurs' profile
DECaLS
deep field (DF)
Deep Lens Survey (DLS)
Deep Multicolor Survey (DMS)
deep survey
Deep Underground Neutrino Experiment (DUNE)
DEEP2
delay time distribution (DTD)
DEVILS
disk galaxy
dispersion measure (DM)
Doppler shift
Drake equation
dropout
dust echo
dusty galaxy
dwarf galaxy
dwarf galaxy problem
dwarf spheroidal galaxy (dSph)
Dwingeloo Obscured Galaxy Survey (DOGS)
dynamical friction
dynamical instability
dynamo
EAGLE
effective radius (Re)
ejection
elliptical galaxy
emission line galaxy (ELG)
EMPIRE Survey
epicycle
epicyclic frequency
epoch of galaxy formation
epoch of reionization (EOR)
escape fraction
escape velocity (Ve)
ESO Nearby Abell Cluster Survey (ENACS)
ESO/Uppsala Survey of the ESO (B) Atlas
evaporation
Event Horizon Telescope (EHT)
Extended Groth Strip (EGS)
extended source
extinction
extragalactic astronomy
Extragalactic Distance Database (EDD)
extreme mass ratio inspiral (EMRI)
extremely metal poor galaxy (XMPG)
extremely red object (ERO)
f(R) gravity
F4
Faber-Jackson relation (FJR)
faint blue galaxy (FBG)
Faint Infrared Grism Survey (FIGS)
Fairall Galaxy List (Frl)
fast blue optical transient (FBOT)
fast radio burst (FRB)
field galaxy
final parsec problem
FIRE Simulations
first galaxies
flux-weighted gravity-luminosity relationship (FGLR)
Fokker-Planck equation
FORNAX
fossil group
FourStar Galaxy Evolution Survey (ZFOURGE)
frame of reference
fundamental plane
fuzzy dark matter (FDM)
G-dwarf problem
GADGET-2
Gala
galactic archaeology
galactic astronomy
galactic bulge
galactic disk
galactic electron density
galactic halo
galactic plane
galactic tide
galactic wind
galaxy age determination
galaxy bias
galaxy classification
galaxy cloud
galaxy cluster (CL)
galaxy environment
Galaxy Evolution Explorer (GALEX)
galaxy filament
galaxy formation
galaxy group
galaxy harassment
galaxy interaction
galaxy main sequence
galaxy merger
galaxy power spectrum
galaxy SED
galaxy strangulation
galaxy subgroup
Galaxy Zoo
Galaxy Zoo 2 (GZ2)
GalDNA
GALEX Arecibo SDSS Survey (GASS)
GAMA
gas flow
gas fraction
gas fraction estimation
gas streamer
gas-to-dust ratio (GDR)
giant elliptical galaxy
Giant Gemini GMOS survey (GGG survey)
Giant Stellar Stream (GSS)
Gini/M20
GISMO
GLADE
GLARE
globular cluster (GC)
globular cluster luminosity function (GCLF)
GN-z11
GOODS NICMOS Survey (GNS)
grand design spiral galaxy
gravitational instability (GI)
gravitational lensing
gravitational microlensing
gravitationally bound
gravity
GRB 060505
Great Attractor
Great Debate
Green Bank Telescope (GBT)
GW170817
Hα survey
H-R diagram (HRD)
habitable zone (HZ)
halo abundance matching (HAM)
halo model
Hawaii K-band Galaxy Survey
HD1
heating
Hercules-Corona Borealis Great Wall (Her-CrB GW)
Herschel Stripe 82 Survey (HerS)
HFLS3
HI gas mass
HI supershell
hierarchical assembly of galaxies
high-velocity cloud (HVC)
HII region (HII)
Hill radius
HIPASS
Hoag's Object
Holmberg radius (RH)
hot DOG
Hubble constant (H0)
Hubble Deep Field (HDF)
Hubble Deep Field South (HDF-S)
Hubble diagram
Hubble expansion
Hubble Ultra-Deep Field (HUDF)
Hydrogen Accretion in Local Galaxies Survey (HALOGAS)
hypercompact stellar system (HCSS)
hyperluminous infrared galaxy (HLIRG)
hypervelocity star (HVS)
I Zwicky 18 (I Zw 18)
IC 342
infrared excess (IRX)
initial fluctuation spectrum
intensity
intensity mapping (IM)
intergalactic dust
intergalactic HI cloud
intergalactic medium (IGM)
interplanetary medium (IPM)
interstellar astrophysics
interstellar magnetic field (ISMF)
interstellar medium (ISM)
interstellar radiation field (ISRF)
ionized carbon fine structure line ([CII])
ionizing radiation
IRAC Shallow Survey (ISS)
iron (Fe)
irregular galaxy
isotropy
James Clerk Maxwell Telescope (JCMT)
James Webb Space Telescope (JWST)
Jeans anisotropic modeling (JAM)
Jeans equations
Jeans length
jellyfish galaxy
jet
JINGLE
Kapteyn universe
kinematic distance
kinemetry
Kiso Survey of Ultraviolet Excess Galaxies (KUG)
KMOS3D
Kormendy Relation (KR)
Lambda-CDM model (ΛCDM)
LAMOST
Laniakea Supercluster
Large Magellanic Cloud (LMC)
Large Millimeter Telescope (LMT)
large scale structure (LSS)
Las Campanas Redshift Survey (LCRS)
LasDamas
LEDA
lenticular galaxy (S0)
Lick indices
Limber approximation
line broadening
LINER
Local Group (LG)
Local Group Galaxy Survey (LGGS)
Local Sheet
Local Volume (LV)
low Earth orbit (LEO)
low-surface-brightness galaxy (LSB galaxy)
luminosity class
luminosity density
luminosity function (LF)
luminous blue variable (LBV)
luminous infrared galaxy (LIRG)
luminous red galaxy (LRG)
Lyman alpha (Ly-α)
Lyman break (LB)
Lyman continuum (LyC)
Lyman series (L)
Lyman-alpha blob
Lyman-alpha emitter (LAE)
Lyman-alpha forest
Lyman-break galaxy (LBG)
Lynden-Bell mechanism
M-sigma relation
M64
M82
M87
M87*
MACHO
Magellanic Clouds (MC)
magnesium lines (Mg lines)
magnetic field
magnitude
MaNGA Stellar Library (MaStar)
Markarian galaxy (Mkn)
maser
mass fraction
mass transport
mass-luminosity relation
mass-to-light ratio (M/L)
MATCH
Medium Deep Survey (MDS)
Meier paradox
Messier 100 (M100)
Messier 106 (M106)
Messier 74 (M74)
Messier 77 (M77)
Messier Catalog (M)
metallicity (Z)
metallicity gradient
Milkomeda
Milky Way (MW)
Milky Way subgroup
Millennium Galaxy Catalogue (MGC)
missing baryon problem
mock catalog
modified Newtonian dynamics (MOND)
MORA
Morgan classification
Morphological Catalogue of Galaxies (MCG)
morphology
morphology-density relation
Munich Near-Infrared Cluster Survey (MUNICS)
N galaxy
N-body problem
N-body simulation
N-point function
NASA/IPAC Extragalactic Database (NED)
natural astronomical telescopes
nebula
neutral atomic hydrogen (HI)
neutrino hotspot
New General Catalogue (NGC)
NGC 1052
NGC 2363
NGC 253
NGC 2770
NGC 3314
NGC 6946
NGC 7331
NGC 7727
nova (N)
observable universe
Ogle Galaxy Catalog (OGC)
Oort constants
open cluster (OC)
Orphan stream
PAndAS
Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS)
Parkes HI Zone of Avoidance Survey (HIZOA)
PCA analysis
pea galaxy (GP)
peak star-formation epoch
peculiar galaxy (p)
peculiar velocity
Perseus Cluster (Abell 426)
PG 1302-102
PHANGS
PHIBSS
Pinwheel Galaxy (M101)
plane of the sky (POS)
planetary nebula luminosity function (PNLF)
plate measuring machine (APM)
point source
POINT-AGAPE
Polar-ring Catalog (PRC)
polar-ring galaxy
Population I (Pop I)
Population II (Pop II)
Population III (Pop III)
post-starburst galaxy (PSB galaxy)
Press-Schechter formalism
primary
PRIMUS
principal component analysis (PCA)
Principal Galaxies Catalog (PGC)
projected separation
proto-cluster
protogalaxy
proximity effect
pseudobulge
PSF fitting
quantum fluctuations
quenched galaxy
r-process
radial mixing
radiance
radio galaxy (RG)
Radio Galaxy Zoo (RGZ)
radio relics
radio source (RS)
ram pressure
RAMSES
rare designator prefixes
RASS/Green Bank Catalog (RGB)
red and dead
reddening
redshift space
redshift survey
redshift-angular size relation
Reference Catalogue of Bright Galaxies (RC)
REFINE
repulsive dark matter (RDM)
RESOLVE­
retrograde accretion
ring galaxy
Roman Space Telescope (RST)
rotation curve
Sérsic profile
S-Star Cluster
SAGE
Sagittarius Dwarf Irregular Galaxy (SagDIG)
satellite galaxy
satellite plane problem
scale height (H)
scattering
Scott effect
Second Byurakan Survey (SBS)
secular
self-gravitation
Seyfert galaxy (Sy)
Shapley-Ames Catalog (SAC)
SHARDS
shell galaxy
sidereal
signatures of formation
SIMBAD
SIMSTACK
SINS
Sloan Digital Sky Survey (SDSS)
SN 2014J
solid angle (Ω)
Soltan argument
Sombrero Galaxy (M104)
South Pole Telescope (SPT)
Southern Sky Redshift Survey (SSRS)
specific intensity
spectral energy distribution (SED)
spectral feature
spectral line
spectral line designation
spectral resolution
spectral signature
Spindle Galaxy (NGC 5866)
spiral arm
spiral galaxy
Spitzer Extended Deep Survey (SEDS)
Spitzer HETDEX Exploratory Large Area Survey (SHELA)
Spitzer Space Telescope (SST)
standard candle
star count
star formation (SF)
star formation feedback
star formation history (SFH)
star formation rate (SFR)
star system
star-formation rate stellar-mass ratio
starburst galaxy
STARBURST99
STARSMOG
stellar dynamics
stellar evolution
stellar halo
stellar kinematics
stellar nucleus
stellar population
stellar population synthesis (SPS)
stellar population synthesis code
stellar stream
Stephan's Quintet
structure formation
Subaru Deep Field (SDF)
Subaru PFS
Subaru Telescope
subgrid-scale physics
submillimeter astronomy
submillimeter galaxy (SMG)
supercluster
supergalactic coordinate system
superluminal motion
supermassive black hole (SMBH)
supernova (SN)
supershell
surface brightness (SB)
surface brightness fluctuation (SBF)
surface brightness profile
surface density (Σ)
synthetic field method (SFM)
synthetic photometry
synthetic spectrum
systemic velocity
TAIPAN
telluric star
The Survey for Ionization in Neutral Gas Galaxies (SINGG)
thermal dust emission
thin disk
three dimensional model
tidal arm
tidal force
tidal tail
timescale (t)
TiNy Titans (TNT)
tip of the red-giant branch (TRGB)
TolTEC
Toomre sequence
Triangulum Galaxy (M33)
Tully-Fisher relation (TFR)
Two Micron All-sky Survey (2MASS)
Type Ia supernova
ultra diffuse galaxy (UDG)
ultra-faint dwarf galaxy (UFD)
ultraluminous infrared galaxy (ULIRG)
Uppsala General Catalogue (UGC)
Ursa Major II Dwarf
UVJ diagram
van den Bergh galaxy classification
velocity dispersion (σ)
velocity-metallicity relation
VIMOS Public Extragalactic Redshift Survey (VIPERS)
VIMOS-VLT Deep Survey (VVDS)
Virgo Cluster
Virgo Cluster Catalog (VCC)
virial theorem
VISTA
void galaxy
Voyager
warp
weak lensing (WL)
Wesenheit function
Westerhout Radio Survey (W)
Whirlpool Galaxy (M51a)
WHISP
wide binaries (WB)
Wide-field Multi-object Spectrograph (WFMOS)
wind-momentum luminosity relationship (WLR)
WINGS
Wolf-Lundmark-Melotte (WLM)
Wolf-Rayet galaxy
Wolf-Rayet star
X-SHOOTER Lyman α survey at z = 2 (XLS-z2)
XBONG
zone of avoidance (ZOA)

Index