The Kennicutt–Schmidt law relates star-formation-rate surface density to gas surface density, commonly as . Disk-averaged total-gas measurements give near , while resolved molecular-gas relations are often closer to linear.
The gas-depletion time is the time required to consume a gas reservoir at its current star-formation rate, . It ignores replenishment, returned stellar mass, and changes in the rate.
The star-formation efficiency per free-fall time is the fraction of gas converted into stars during one gravitational free-fall time, . Molecular clouds commonly have values of order one per cent.
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The Kennicutt–Schmidt law, formulated by B. D. Kennicutt in the early 1990s, describes the empirical relationship between the surface density of star formation and the surface density of gas in galaxies. This law is significant in astrophysics as it provides insight into how stars form and evolve across different types of galaxies.