The Kelvin-Helmholtz mechanism is an astronomical event that occurs when the surface of a star or a planet cools. As a result of this cooling, the pressure drops, and the star or planet compresses to compensate. This compression, in turn, heats up the core of the star/planet. This mechanism is evident on Jupiter and Saturn.
The mechanism was originally proposed by Kelvin and Helmholtz in the late 1800s to explain the source of energy of the sun. As we now know, the amount of energy generated by the Kelvin-Helmholtz mechanism is far too low to power the sun.