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 (cold gas, gas that has cooled sufficiently
to form molecules) apparently sometimes develop patches dense enough
to collapse into stars, the areas of such activity known as stellar
nurseries or star-forming regions.
Some such dense regions may result from collisions of clouds, or
even whole galaxies.
Movement of cold gas, which would (sometimes) produce such high density can
be due to nearby supernovae and/or radiation pressure from nearby
early stars (radiation driven implosion or RDI).
For any of this to happen, there must exist available cold gas
within the galaxy and galaxy-accretion of cold gas from the
intergalactic medium increases the probability of star formation and can trigger
it as well.
As stars form, any nearby dust will be heated.
This spreads the heat energy, giving it a larger surface
and lower temperature, with a black-body spectrum concentrated
at longer wavelengthelectromagnetic radiation such as radio.
As a consequence, the presence of recent star formation can result
in short wavelengths from the early stars produced, but what is
sometimes observable is longer wavelengths from heated dust surrounding
the star formation.
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.
The specifics of star formation of early stars are 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 quenched aka 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 very
soon after the host star's formation, during the first
few million years of its life.