Kelvin-Helmholtz contraction is the contraction of a ball of gas under gravity, accompanied by the radiation of the lost potential energy as heat. This was proposed by William Thomson (Lord Kelvin) and Heinrich Helmholtz as the most efficient means by which the Sun could remain hot for a long period. Unfortunately, it isn't long enough. The so-called Kelvin-Helmholtz timescale of the Sun is only 20 to 30 million yr. Although stars are now known to shine by nuclear fusion, rather than by gravitational collapse, Kelvin-Helmholtz contraction is still believed to be a valid description of the way infant stars behave during a portion of their pre-main-sequence evolution.