Helium flash

A rapid onset of helium fusion in a low-mass star’s degenerate core, triggered when the core reaches helium-ignition conditions. It lifts the core out of electron degeneracy without causing a stellar explosion.

Electron degeneracy pressure: Pressure produced by the exclusion principle when electrons are packed into high-density matter. It supports the core while making its temperature initially insensitive to added heat.
Red giant: A luminous, expanded star with a cool surface, produced by changes in its interior fuel supply. The helium flash occurs near the end of low-mass stars’ red-giant ascent.
Red clump: A concentration of core-helium-burning stars in the color–magnitude diagrams of many stellar populations. It contains many low-mass stars that have completed a helium flash.
Carbon ignition: The onset of carbon fusion in a sufficiently hot and dense stellar core. It occurs in more massive stellar evolution and does not define the low-mass helium flash.
Triple-alpha process: A sequence of nuclear reactions that fuses three helium-4 nuclei into carbon-12. Its sharply temperature-sensitive rate drives the sudden energy release.
Stellar core: The central region of a star where its density, temperature, and nuclear activity are greatest. The flash begins in the helium-rich core beneath the hydrogen-burning shell.
Mira variable: A pulsating, long-period variable star on the asymptotic giant branch or nearby evolutionary phases. Such stars descend from low-mass stars whose core helium burning began after a flash.
Shell helium flash: A temporary instability in helium-burning shells during the thermal-pulsing asymptotic giant branch phase. It is a recurring shell event, unlike the core’s initial helium flash.
Thermal runaway: A positive-feedback process in which heating accelerates heat-producing reactions faster than energy can escape. Degenerate helium burning amplifies small temperature rises into rapid fusion.
Hydrogen shell burning: Fusion of hydrogen in a layer surrounding a star’s exhausted or inert core. It adds helium to the core until that core reaches ignition conditions.

Linked from 8 pages