hydrogenburning

Hydrogen burning is the nuclear fusion process in which hydrogen nuclei fuse to form helium, releasing energy that powers stars during their main-sequence phase. It occurs in stellar cores at temperatures around 10 million kelvin for Sun-like stars; more massive stars achieve core temperatures where the CNO cycle dominates, typically above 15–20 million kelvin.

There are two main pathways. The proton-proton chain is the dominant mechanism in smaller stars and proceeds through steps that build deuterium and helium-3 before producing helium-4 and energy. In more massive stars, the CNO cycle dominates; carbon, nitrogen, and oxygen isotopes act as catalysts to convert protons into helium, releasing energy while leaving the catalytic elements unchanged.

Hydrogen burning is the first major stage of stellar nucleosynthesis. It converts core hydrogen into helium, raising the mean molecular weight and influencing the star’s internal structure. The energy generated supports hydrostatic equilibrium, counteracting gravity, and powers the transport of energy to the surface via radiation and convection.

Hydrogen burning lasts for a star’s main-sequence lifetime, which spans billions of years for Sun-like stars. The precise duration depends on stellar mass. When the core hydrogen is exhausted, the star leaves the main sequence and hydrogen burning shifts to shells or ends in later evolutionary phases, with outcomes that depend on the star’s mass.

Note: Hydrogen burning is a nuclear fusion process, distinct from chemical burning, requiring extreme temperatures and densities inaccessible to ordinary chemical reactions.