The star formation rate (SF rate or SFR) is the rate at which gas and dust is turned into stars. The term can be used in describing a galaxy or globular cluster, for example. A typical unit is solar masses per year. The existence of hot stars, whose lifetimes are very short, is considered a sign of recent star formation as are indicators of some types of clouds associated with star formation or short-lived hot stars. The rate can only be estimated, which can be based on a number of measurements, including:
These are generally signs of short-lived stars (early, such as O-type star, B-type star, A-type star) or their effects (e.g., H-alpha from their ionization of nearby hydrogen), necessarily indicating recent star formation. When citing a galaxy's star formation rate, often the method of detecting it is included, e.g., H-alpha star formation rate, UV star formation rate, IR star formation rate, optical star formation rate. The methods differ in the sources of error and differ in how long ago the is the "recent" star formation is indicated. Ratios of the rates (e.g., H-alpha to UV ratio) can be used to refine techniques (such as developing correction factors) and to give some indication of when the recent star formation took place.
The molecular star formation law states that the SFR is proportional to the density of the molecular gas, i.e.,
ΣSFR ∝ ΣH2
The star formation rate density (SFRD), the star formation rate per unit volume, is observed to have risen, then fallen, peaking on the order of redshift z = 1.9, i.e., 3 billion years after the Big Bang. The comoving star formation rate density (Or comoving SFR density or comoving SFRD or comoving density of star formation) is the SFRD per unit volume expressed in comoving units, i.e., units that expand with time according to Hubble expansion, assuming they will be adjusted with the Hubble parameter. The specific star formation rate (SSFR), e.g., of a galaxy, is a measure of the star formation per unit mass, e.g., solar masses of formed stars per solar masses of the galaxy. The star formation efficiency (SFE), is the star formation rate per unit of mass of a specific gas in a galaxy, e.g., hydrogen gas.
The SF rate of galaxies over time is of interest, there having been a peak star-formation epoch. UV and/or IR can indicate SF (e.g., a ultraluminous infrared galaxy), but for the oldest galaxies, there are ways to detect redshifted UV but detecting IR presents challenges.
Note: the abbreviation SFR is also used for star-forming region.