Astrophysics (Index)About

star formation

(theorized collapse of gas and dust into stars)

Star formation (SF), the process by which stars are born, is thought to be understood in outline, but questions remain, perhaps more than for models of subsequent stellar evolution. Clouds sufficiently dense will undergo gravitational collapse to achieve the density to trigger fusion, but among the remaining questions is how and when this cloud density arises.

Molecular clouds and interstellar dust apparently sometimes develop dense patches that collapse into stars. Areas of such activity are known as stellar nurseries or star-forming regions. Triggering events include collisions of clouds, nearby supernovae, radiation pressure from nearby early stars (radiation driven implosion or RDI), or the collision of galaxies. Accretion of cold gas from the intergalactic medium increases the probability of star formation and can trigger it as well. Nearby dust will be heated, producing a lower-temperature black-body spectrum, with longer wavelength electromagnetic radiation such as radio. As a general rule regarding galaxies, the more infrared it emits (a sign of heated dust), the higher the star formation rate. Star formation in distant galaxies is of interest as the amount of star formation appears to have grown and diminished over the history of the universe (star formation history) and mechanisms that might do that are of interest. H-alpha and the HI line are used to measure SF in distant galaxies. Lyman alpha can also indicate distant SF and Lyman-alpha emitters are presumed to have a very high SF rate.

Star formation of early stars is not as well understood as for lower-mass stars because radiation pressure would seem to limit the process. Theories include merging lower-mass stars, or that the radiation is anisotropic, i.e., less in some direction(s), the lesser directions constituting a "hole in the wall" through which gas accretion can continue.

As reflected in the initial mass function's greater-than-2 exponent, the vast majority of star formation, by mass, is in later, lower mass stars, e.g., much more mass amongst M-type stars than O-type stars. This means that one useful sign of star formation in distant galaxies, the colors of early stars, is imperfect because if some star formation regions don't trigger whatever it is that creates high mass stars, they could be missed.

The term quenching is used to indicate the cessation of star formation, e.g., in a galaxy, which would be said to become quiescent. For the long term, a star forming region is assumed to cease due to gas heating from hot stars and supernovae (star formation feedback), followed by cooling and settling and perhaps triggers from nearby events. Thus a long-term star formation rate must be time-averaged over periods of high and low star formation. Such an oscillation is termed episodic star formation. Dwarf galaxies show evidence of this, e.g., through their stellar demographics and the mechanism by which this happens is of interest.

The term star and planet formation (SPF) covers star formation and planet formation as well, which is believed to happen during the first few million years of a star's life.

Further reading:

Referenced by pages:
accretion rate
active galactic nucleus (AGN)
Balmer-break galaxy (BBG)
binding energy
bipolar outflow
blue horizontal branch (BHB)
Butcher-Oemler effect (BOE)
Bok globule
brown dwarf (BD)
cosmic gamma ray background (CGB)
ionized carbon fine structure line ([CII])
Canadian Institute for Theoretical Astrophysics (CITA)
cloud fragmentation
cold gas
Carbon Monoxide Mapping Array (COMA)
computational astrophysics
core collapse supernova (CCSN)
cosmological simulation
chemically peculiar star (CP star)
dense core
disk galaxy
Drake equation
delay time distribution (DTD)
dwarf spheroidal galaxy (dSph)
EF Eridani
evaporating gas globule (EGG)
emission line galaxy (ELG)
elliptical galaxy
ESO 137-001
faint blue galaxy (FBG)
star formation feedback
FIRE Simulations
galactic wind
galaxy formation
galaxy main sequence
galaxy merger
galaxy strangulation
Galaxy Evolution Explorer (GALEX)
gas fraction
gas fraction estimation
giant planet formation
giant molecular cloud (GMC)
Gould's Belt
habitable zone (HZ)
Hall effect
Hydrogen Accretion in Local Galaxies Survey (HALOGAS)
Hα survey
Haro 29
ionized hydrogen (HII)
HII region (HII)
inside-out growth
intensity mapping
interstellar medium (ISM)
Institute of Theoretical Astrophysics (ITA)
I Zwicky 18 (I Zw 18)
jellyfish galaxy
Kennicutt-Schmidt law
Kelvin-Helmholtz instability (KHI)
Lyman-alpha emitter (LAE)
Lyman-break galaxy (LBG)
Lynds Catalog of Dark Nebulae (LDN)
lenticular galaxy (S0)
luminous infrared galaxy (LIRG)
Large Millimeter Telescope (LMT)
low mass star (LMS)
Lyman alpha (Ly-α)
Lyman-Werner photon
molecular cloud turbulence
Milky Way
molecular cloud
Molecular Deep Field
Spindle Galaxy (NGC 5866)
Orion Molecular Cloud Complex
peak star-formation epoch
quenched galaxy
radial mixing
red and dead
radio galaxy (RG)
ring galaxy
RT instability
SBS 0335-052
Sloan Digital Sky Survey (SDSS)
spectral energy distribution (SED)
Serpens Cloud
star formation history (SFH)
star formation rate (SFR)
star-forming region (SFR)
star-formation rate stellar-mass ratio
shock wave
SMBH formation
submillimeter galaxy (SMG)
Smith Cloud
sound speed
spectroscopic parallax
spiral arm
spiral galaxy
starburst galaxy
stellar association
stellar astronomy
stellar demographics
stellar evolution
stellar kinematics
submillimeter astronomy
thermal dust emission
tidal arm
Tomographic Ionized-carbon Mapping Experiment (TIME)
Toomre Q parameter (Q)
ultra diffuse galaxy (UDG)
ultraluminous infrared galaxy (ULIRG)