Asymptotic giant branch (AGB)

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.

The fate of an Asymptotic giant branch star depends on whether its envelope is removed before its core reaches the next ignition or instability threshold. The approximate envelope-loss time is $t_{\mathrm{env}}=M_{\mathrm{env}}/|\dot{M}|$, while the time to add a required core mass $\Delta M_c$ is $t_{\mathrm{grow}}=\Delta M_c/{\dot{M}}_c$. Shell stellar nuclear fusion grows the core; third dredge-up can reduce the net growth. If $t_{\mathrm{env}}\ll t_{\mathrm{grow}}$, shell fuel disappears first and a white dwarf remnant is favored. The opposite ordering permits further core growth but does not by itself specify whether carbon burning is stable, explosive, or followed by further evolution. Both rates change with time, so the estimates guide rather than replace a stellar model.

When the bottom of an Asymptotic giant branch star's convective envelope becomes hot enough for hydrogen burning, the CNO cycle can convert newly dredged-up carbon into nitrogen. Its onset and strength depend on mass, composition and envelope structure; it is not a universal property of all stars at one initial mass.

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.