An asymptotic-giant-branch star has an inert carbon-oxygen core surrounded by helium- and hydrogen-burning shells and a deep convective envelope. Alternating shell burning produces thermal pulses and dredge-up, while strong mass loss removes the envelope.
Stellar dredge-up occurs when a convective envelope penetrates into layers altered by nuclear burning and transports their material to the surface.
First dredge-up occurs as a star ascends the red-giant branch. Its deepening convective envelope exposes material processed by hydrogen burning, especially enhanced helium and nitrogen and depleted carbon.
Second dredge-up occurs after core helium exhaustion in sufficiently massive intermediate-mass stars. The convective envelope moves inward, mixes helium- and hydrogen-burning products to the surface, and reduces the hydrogen-exhausted core mass.
Third dredge-up follows a thermal pulse on the asymptotic giant branch, when the convective envelope reaches into the intershell and can carry freshly made carbon and slow-neutron-capture elements to the surface.
A thermal pulse is a helium-shell flash caused by thermally unstable burning in a thin shell. It briefly drives intershell convection, expands and cools the hydrogen-burning shell, and can be followed by third dredge-up.
The Härm–Schwarzschild instability is the thin-shell thermal instability of helium burning in an asymptotic-giant-branch star. Heating greatly raises the nuclear rate while the geometrically thin shell initially expands too little to provide stabilizing cooling.
A super-AGB star is massive enough to ignite carbon and form a degenerate oxygen-neon core, but initially too light to proceed directly through all advanced burning stages. Its fate depends on competition between core growth and envelope mass loss.
An oxygen-neon-magnesium core is the degenerate product of carbon burning, composed mainly of oxygen and neon with a smaller magnesium abundance. It can become an oxygen-neon white dwarf or collapse after electron captures if it approaches the Chandrasekhar mass.
An electron-capture supernova occurs when electron captures, especially on neon and magnesium nuclei in a degenerate oxygen-neon core, remove pressure support and trigger collapse before ordinary iron-core formation.
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The Asymptotic Giant Branch (AGB) is a phase in the evolution of stars, particularly those with initial masses between approximately 0.6 and 8 times that of the Sun. This stage occurs after a star has completed the hydrogen and helium burning phases in its core and is characterized by significant changes in the star's structure and composition.