Kelvin–Helmholtz timescale

Summary

The Kelvin–Helmholtz timescale of a star is the time it could keep shining at its present luminosity using only its gravitational energy. It is of order $G M^2 / (R L)$ for a star of mass $M$, radius $R$ and luminosity $L$, with $G$ the gravitational constant. For the Sun it is about thirty million years. It is the time scale on which a star adjusts its thermal structure. It also sets the duration of the contraction of young stars towards the main sequence.