Helium burning

Helium burning is stellar fusion that converts helium nuclei into heavier elements, chiefly carbon and oxygen. It begins when a star’s core becomes hot and dense enough for helium nuclei to fuse.

Triple-alpha process: A sequence in which three helium-4 nuclei combine to form carbon-12. This is the principal route by which helium burning builds carbon.
Helium-4: A helium nucleus containing two protons and two neutrons, also called an alpha particle. Helium-4 nuclei are the fuel consumed in the triple-alpha process.
Red giant branch: A phase of stellar evolution in which a star has a hydrogen-burning shell around an inert core. The core grows and heats on this branch until helium burning can begin.
Fred Hoyle: An English astronomer who developed influential theories of stellar nucleosynthesis. Hoyle predicted the carbon-12 energy level that explains carbon production in stars.
Hydrogen burning: Stellar fusion that converts hydrogen nuclei into helium and releases energy. It precedes helium burning and supplies the helium fuel in ordinary stellar evolution.
Hoyle state: An excited energy level of carbon-12 that enables efficient formation of carbon in stars. It makes the otherwise improbable triple-alpha reaction proceed at stellar temperatures.
Stellar core: The central region of a star, where gravity compresses matter and nuclear energy is generated. Helium burning occurs in stellar cores or in shells surrounding exhausted cores.
Horizontal branch: A stage of low-mass stellar evolution characterized by core helium fusion and a surrounding hydrogen-burning shell. It is the stable phase that follows helium ignition in many low-mass stars.
William Alfred Fowler: An American nuclear astrophysicist who helped establish how stars synthesize the elements. His laboratory and theoretical work helped test stellar fusion pathways, including helium burning.
Carbon burning: Stellar fusion reactions that consume carbon nuclei to produce heavier nuclei. It follows helium burning in sufficiently massive stars and requires higher core temperatures.

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