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 among the interesting questions is how and when 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, 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.
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 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.
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.