CNO cycle

A sequence of nuclear reactions in which carbon, nitrogen, and oxygen nuclei catalyze hydrogen fusion into helium inside stars. The catalysts are regenerated, while some cycle variants create additional isotopes.

Proton–proton chain: A set of stellar fusion reactions that converts hydrogen into helium without carbon, nitrogen, or oxygen catalysts. It is the competing hydrogen-fusion process that dominates in the Sun.
Nuclear fusion: A process in which light atomic nuclei combine to form heavier nuclei, releasing energy when the products have greater binding energy per nucleon. The CNO cycle is one way stars release energy through fusion.
Stellar nucleosynthesis: The creation of chemical elements through nuclear reactions inside stars and stellar explosions. The CNO cycle processes hydrogen and changes the isotope inventory inside stars.
Arthur Eddington: A British astrophysicist who established nuclear fusion as the likely source of stellar energy. His stellar-energy argument framed the problem that the CNO cycle later helped solve.
Carbon-12: The most abundant stable carbon isotope, containing six protons and six neutrons. It captures the first proton in the main CNO-I cycle.
Nuclear reaction notation: A symbolic notation for nuclear transformations that identifies reactants, products, and conserved quantities. It makes the cycle’s proton captures and radioactive decays explicit.
Main sequence: The long-lived phase in which a star fuses hydrogen into helium in its core. CNO fusion supplies core energy in sufficiently hot main-sequence stars.
Hans Bethe: A German-American physicist who developed foundational theories of stellar nuclear energy generation. He identified the carbon–nitrogen cycle as a mechanism for stellar hydrogen burning.
Nitrogen-14: A stable nitrogen isotope containing seven protons and seven neutrons. Its proton capture is the slowest step in the main CNO-I cycle.
Nuclear binding energy: The energy required to separate a nucleus into its constituent protons and neutrons. The net fusion reactions release energy because their products are more tightly bound.

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