Kelvin–Helmholtz mechanism

The release of gravitational potential energy as a self-gravitating body contracts, converting part of that energy into heat and radiation.

Gravitational contraction: The inward shrinking of matter under its own gravity, often accompanied by heating. The mechanism begins as gravity compresses the body and releases binding energy.
Protostar: A young stellar object still gathering mass and contracting before stable hydrogen fusion. A protostar radiates energy released by contraction during its early evolution.
William Thomson, 1st Baron Kelvin: A Scottish physicist who developed thermodynamic theory and estimated the Sun's age from energy constraints. Kelvin argued that gravitational contraction could supply the Sun's heat for a limited time.
Solar luminosity: The total radiant power emitted by the Sun, about 3.8 × 10²⁶ watts. At this output, the Sun would exhaust its contraction energy far sooner than its known age.
Virial theorem: A relation between average kinetic and potential energies in a bound system in equilibrium. It explains why contraction heats a body while allowing roughly half the released energy to be radiated.
Pre-main-sequence star: A young star contracting toward the main sequence before core hydrogen fusion becomes stable. Its luminosity is substantially powered by gravitational contraction.
Hermann von Helmholtz: A German physicist who applied energy conservation to explain heat generated by gravitational contraction. Helmholtz independently advanced contraction as a source of solar energy.
Stellar lifetime: The duration of a star's evolution, determined by its mass, fuel supply, and energy-production processes. The mechanism sets a short energy-limited lifetime when contraction is the only power source.
Gravitational binding energy: The energy required to disperse a bound mass against its gravitational attraction. Its decrease during contraction supplies the energy that powers radiation.
Sun: The star at the center of the Solar System, powered primarily by hydrogen fusion in its core. The Sun's Kelvin–Helmholtz timescale shows contraction alone cannot explain its age and luminosity.

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