In astronomy, the Kennicutt–Schmidt law is an empirical relation between the surface gas density and star formation rate (SFR) in a given region. ^[1] The relation was first examined by Maarten Schmidt in a 1959 paper ^[2] where he proposed that the SFR surface density scales as some positive power ${\displaystyle n}$ of the local gas surface density. i.e.

${\displaystyle \Sigma _{SFR}\propto (\Sigma _{gas})^{n}}$.

In general, the SFR surface density ${\displaystyle (\Sigma _{SFR})}$ is in units of solar masses per year per square parsec ${\displaystyle (M_{\odot }~{\textrm {yr}}^{-1}{\textrm {pc}}^{-2})}$ and the gas surface density in grams per square parsec ${\displaystyle ({\textrm {g}}~{\textrm {pc}}^{-2})}$. Using an analysis of gaseous helium and young stars in the solar neighborhood, the local density of white dwarfs and their luminosity function, and the local helium density, Schmidt suggested a value of ${\displaystyle n\approx 2}$ (and very likely between 1 and 3). All of the data used were gathered from the Milky Way, and specifically the solar neighborhood.

In 1989, Robert Kennicutt found that the H${\displaystyle \alpha }$ intensities in a sample of 15 galaxies could be fit with the earlier Schmidt relations with a power law index of ${\displaystyle n=1.3\pm 0.3}$. ^[3] More recently, he examined the connection between surface gas density and SFR for a larger set of galaxies to estimate a value of ${\displaystyle n=1.4\pm 0.15}$. ^{[4] [5]}