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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, for example, in describing a particular galaxy or a particular star-forming region. A typical unit is solar masses per year (MSun/yr); one estimate of the Milky Way's current (average) SF rate is two MSun/yr. Rates of different galaxies vary from a tiny fraction of an MSun/yr to hundreds of MSun/yr. 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 with short-lived hot stars. The rate can only be estimated, which can be based on a number of measurements, including:
Most of these are signs of short-lived stars (early, such as O-type stars, B-type stars, and maybe A-type stars) 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 determining the rate 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 "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 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., of its hydrogen gas. Galaxies generally fall within three distinct classes, each with a distinct the power-law relation between their star formation rate and their total mass: a typical rate (termed a main sequence galaxy), distinctly high (a starburst galaxy), or distinctly low (a quenched galaxy); the Milky Way is atypical, having a merely somewhat-low rate, such a galaxy termed a green valley galaxy.
The variation of SF rate of the entire universe over time (star formation history) 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, redshifted UV can be detected whereas detecting redshifted IR presents challenges. The star formation rate density (SFRD), the star formation rate per unit volume; that of the entire universe 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 the Hubble expansion, assuming they will be adjusted with the scale factor.
Note: the abbreviation SFR is also used for star-forming region.