Triple-alpha process

The triple-alpha process is a sequence of nuclear reactions in which three helium-4 nuclei combine to form carbon-12 inside stars. It proceeds through the short-lived beryllium-8 nucleus and the excited Hoyle state of carbon-12.

Helium-4: A stable nucleus containing two protons and two neutrons, also called an alpha particle. Three helium-4 nuclei supply the particles that ultimately assemble into carbon-12.
Helium burning: A stellar fusion stage in which helium nuclei are converted into heavier elements. The triple-alpha process is the principal carbon-producing reaction during this stellar stage.
Fred Hoyle: A British astronomer who developed theories of stellar nucleosynthesis and predicted an excited carbon-12 state. His prediction of the required carbon resonance helped explain stellar carbon production.
Carbon abundance: The amount of carbon present in a specified region or material, often expressed as a fraction of its mass or atoms. The reaction rate helps set how much carbon stars produce and return to space.
Beryllium-8: An unstable isotope made of two helium-4 nuclei that decays back into them in about 10⁻¹⁶ seconds. It is the fleeting intermediate formed when the first two alpha particles collide.
Red giant: A luminous, expanded star that has exhausted hydrogen in its core and begun burning heavier fuels. Core helium fusion in many red giants relies on the triple-alpha process.
William Fowler: An American nuclear physicist who led experimental and theoretical work on stellar nuclear reactions. His research helped establish reaction rates and the role of the carbon-12 resonance.
Chemical evolution: The changing abundances of elements in the universe through successive generations of stars. Carbon made by helium-burning stars becomes material for later stars and planetary systems.
Hoyle state: An excited energy state of carbon-12 near the threshold for decay into three helium-4 nuclei. This resonance greatly raises the chance that beryllium-8 captures another alpha particle.
Asymptotic giant branch: A late stellar evolutionary phase characterized by a compact core and alternating hydrogen- and helium-burning shells. Helium-shell flashes on this branch activate the triple-alpha reaction.

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