= Stellar nuclear fusion
{wiki}
<Stellar nuclear fusion> converts light <atomic nuclei> into more tightly bound products, releasing energy from the change in <nuclear binding energy>. In a <main-sequence star>, the net conversion of <hydrogen> to <helium> proceeds mainly through the <proton–proton chain> or the <CNO cycle>, depending on core conditions. The high <temperature> supplies energetic collisions, while <quantum tunnelling> allows reactions through the repulsive electrical barrier.
The released energy heats the interior and replaces energy lost through the <photosphere> and by <neutrinos>. A <stellar thermostat> arises because extra heating can expand the <star>, reduce core <temperature>, and lower the reaction rate. Later <stellar evolution> changes the available fuel and core structure, enabling <helium burning> and, in sufficiently massive <stars>, further burning stages.
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