Asymptotic giant branch

The asymptotic giant branch is a late evolutionary phase of low- to intermediate-mass stars, with a cool, luminous envelope around a contracting carbon–oxygen core. Shell burning, thermal pulses, and strong mass loss shape this phase.

Thermal pulse: A brief helium-shell flash caused by unstable helium burning in a shell around a stellar core. Repeated helium-shell flashes interrupt steady burning and reshape the star’s structure.
Stellar evolution: The sequence of structural and chemical changes a star undergoes over its lifetime. The asymptotic giant branch is a late stage in this sequence for suitable stars.
S-process: A sequence of slow neutron captures that builds many stable elements heavier than iron. Neutron captures in branch stars create heavy elements that can reach their surfaces.
Red giant branch: An evolutionary phase in which a star expands after hydrogen in its core is exhausted, before core helium burning begins. It is an earlier giant phase, before the later double-shell-burning branch.
Hydrogen burning shell: A thin stellar layer where hydrogen fusion produces helium around an inert or contracting core. Hydrogen-shell burning supplies much of the luminosity between helium-shell flashes.
Red giant: A luminous, expanded star with a cool surface, produced as its core fuel supply changes. The branch is a later giant phase, reached after earlier expansion.
Carbon star: A cool giant star whose atmosphere contains more carbon than oxygen. Dredged-up carbon can make an asymptotic giant branch star a carbon star.
Horizontal branch: A post-helium-flash phase in which a star burns helium in its core and hydrogen in a surrounding shell. It precedes the branch in many low-mass stars’ evolution.
Helium burning shell: A stellar layer where helium fusion, chiefly through the triple-alpha process, produces carbon. Its unstable burning triggers the thermal pulses characteristic of this phase.
Carbon–oxygen core: A dense stellar core composed mainly of carbon and oxygen left by helium fusion. This contracting core lies beneath the branch star’s burning shells.

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