Hydrogen burning

Hydrogen burning is the net conversion of hydrogen into helium through nuclear fusion. The proton–proton chain and CNO cycle provide different sequences with the same principal fuel and final helium product. In the CNO cycle, carbon, nitrogen and oxygen act as catalysts rather than being consumed as the principal fuel.

The released nuclear binding energy supplies the long-term luminosity of a main-sequence star, with some energy escaping in neutrinos. Core exhaustion ends that evolutionary phase even if the outer envelope still contains abundant hydrogen. A hydrogen-burning shell can subsequently sustain an evolved star. The word burning denotes nuclear reactions here; it does not require combustion with molecular oxygen.

A hydrogen-burning shell converts hydrogen into helium in a layer surrounding a hydrogen-exhausted stellar core. It moves outward in enclosed mass as it supplies fresh helium to that core; its movement in radius need not have the same sign.