Star formation efficiency

Star formation efficiency is the fraction of a gas reservoir converted into stars over a defined region or time interval. Its value depends on how the reservoir and interval are specified.

Molecular cloud: A cold, dense interstellar cloud composed mainly of molecular hydrogen, where most stars form. It is the gas reservoir whose conversion into stars the efficiency measures.
Star formation rate: The mass of new stars formed per unit time in a region or galaxy. Together with gas mass, it yields a rate-based measure of conversion efficiency.
Star formation efficiency per free-fall time: The fraction of gas converted into stars during one gravitational free-fall time. It removes the arbitrary duration of a measurement by using a collapse timescale.
Star formation law: An empirical or theoretical relation connecting gas properties to the rate of star formation. Efficiency is embedded in the normalization and scatter of these relations.
Gravitational collapse: The contraction of matter under its own gravity when internal support cannot prevent collapse. Collapse gathers cloud gas into the dense structures that form stars.
Kennicutt–Schmidt law: An empirical relation between gas surface density and star formation rate surface density in galaxies. Its normalization expresses how efficiently observed gas forms stars.
Star formation efficiency per unit time: The fraction of a gas reservoir converted into stars per unit time, often normalized by a dynamical timescale. This rate-based definition distinguishes slow ongoing conversion from a total cloud yield.
Toomre stability criterion: A criterion for the local gravitational stability of a rotating disk against axisymmetric perturbations. Disk stability may regulate which gas becomes dense enough to form stars.
Turbulence: Irregular, multiscale fluid motion that transports energy and momentum through a medium. Turbulent motions can support a cloud against collapse while also creating dense pockets.
Star formation rate surface density: The mass of stars formed per unit time and projected area. It allows efficiencies to be compared across regions with different sizes.

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