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Helium burning is stellar nuclear fusion in which helium nuclei are converted into heavier nuclei. The Triple-alpha process combines three helium-4 nuclei through an unstable beryllium-8 intermediate to form carbon-12; capture of another alpha particle can produce oxygen-16.

The strong temperature dependence and sensitivity to density make ignition conditions important for stellar evolution. In a degenerate core, expansion may initially provide little temperature regulation, permitting a helium flash. Burning means nuclear energy release here, rather than chemical combustion with oxygen.

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.