The Kelvin-Helmholtz mechanism (KH mechanism) is the increase in thermal and other kinetic energy and resulting radiation emission when something (e.g., self-gravitating bodies) shrinks, from the conversion/release of gravitational potential energy. The energy release can be significant when gravity is significant. It is considered a power source for stars, originally proposed as the mechanism for the Sun's present luminosity, but as fusion has been established as that mechanism, the KH mechanism is considered a secondary source, with significance before and after a star's main sequence. It can be very significant in the accretion of material by compact objects, and in the gas accretion during planet formation, and by accretion onto supermassive black holes, in the latter case with power that can exceed any known long-term fusion. The KH mechanism does provide some of the luminosity of gas giants (e.g., Jupiter), and brown dwarfs. The Kelvin-Helmholtz timescale approximates the length of time an existing luminous body (e.g., star) could support its current luminosity based upon this mechanism.