The CNO cycle catalyses hydrogen fusion through carbon, nitrogen, and oxygen nuclei. Near , its specific energy generation is approximately proportional to and is therefore strongly concentrated near a star's centre.
If proton–proton chain heating scales as and CNO cycle heating as , their ratio at fixed hydrogen abundance scales as . Equating the rates proves the displayed crossover-temperature relation. It applies to a local power-law approximation, with unchanged coefficient normalizations apart from the catalyst abundance.
A branch of the CNO cycle proceeds through proton captures on nitrogen-15 and oxygen-16, beta-plus decay of fluorine-17, and . Returning through oxygen-15 and nitrogen-15 closes the cycle. It also catalyzes the conversion of four protons into helium-4, with branch-dependent neutrino loss.
The principal CNO cycle follows , beta-plus decay to carbon-13, proton capture to nitrogen-14, proton capture to oxygen-15, beta-plus decay to nitrogen-15, and . The carbon seed is regenerated. The ordinary bottleneck is , so nitrogen-14 accumulates in cycle equilibrium.
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The CNO cycle, or carbon-nitrogen-oxygen cycle, is one of the two main fusion processes that stars use to convert hydrogen into helium, the other being the proton-proton chain reaction. The CNO cycle is particularly important in stars that are more massive than the Sun, where temperatures and pressures are sufficiently high to facilitate the reaction.