Free-fall time

Summary

The free-fall time is the time a body of gas needs to collapse under its own gravity when nothing resists the collapse. For a uniform sphere starting at rest it depends only on the initial density, not on the size of the sphere. It is the basic time scale of gravitational collapse in star formation and the dynamical time scale of stars and galaxies. Comparing other time scales with it, such as the sound crossing time or the cooling time, shows which process controls a cloud.