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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 gravitationally collapse, but an interesting question is when and how this 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, 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 as well as 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 should be enough to start limiting 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.

Given 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 indirect in the sense that 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.

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.


Referenced by:
accretion rate
active galactic nucleus (AGN)
Balmer-break galaxy (BBG)
binding energy
bipolar outflow
blue horizontal branch (BHB)
Bok globule
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
dense core
disk galaxy
Drake equation
dwarf spheroidal galaxy (dSph)
emission line galaxy (ELG)
elliptical galaxy
ESO 137-001
star formation feedback
FIRE Simulations
galaxy main sequence
galaxy merger
Galaxy Evolution Explorer (GALEX)
gas fraction
gas fraction estimation
giant molecular cloud (GMC)
Gould's Belt
habitable zone (HZ)
Hall effect
Hα survey
Haro 29
ionized hydrogen (HII)
HII region (HII)
intensity mapping
interstellar medium (ISM)
Institute of Theoretical Astrophysics (ITA)
I Zwicky 18 (I Zw 18)
Kennicutt-Schmidt law
Kelvin-Helmholtz instability (KHI)
Lyman-alpha emitter (LAE)
Lynds Catalog of Dark Nebulae (LDN)
luminous infrared galaxy (LIRG)
Large Millimeter Telescope (LMT)
molecular cloud turbulence
Milky Way
molecular cloud
Molecular Deep Field
Spindle Galaxy (NGC 5866)
Orion Molecular Cloud Complex
peak star-formation epoch
quenched galaxy
ring galaxy
RT instability
SBS 0335-052
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
submillimeter galaxy (SMG)
Smith Cloud
sound speed
spectroscopic parallax
spiral arm
spiral galaxy
starburst galaxy
stellar association
stellar demographics
stellar evolution
submillimeter astronomy
tidal arm
Tomographic Ionized-carbon Mapping Experiment (TIME)
Toomre Q parameter (Q)
ultraluminous infrared galaxy (ULIRG)