= 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>.
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