heliumburning

Heliumberning, in an astrophysical context, refers to the nuclear fusion of helium within the cores of stars. It is a nuclear, not chemical, process and marks the stage after a star exhausts its supply of hydrogen in the core. The primary reaction is the triple-alpha process, in which three helium-4 nuclei (alpha particles) fuse to form carbon-12. A small fraction of reactions can proceed via intermediate beryllium-8, but under stellar conditions the net result is production of carbon, with temperatures around 100 million kelvin being typical requirements. In some stars, helium can also be captured by existing carbon to form oxygen (12C + 4He → 16O), so the relative amounts of carbon and oxygen depend on reaction rates.

How helium burning begins depends on stellar mass. In low-mass stars, the core becomes electron-degenerate as it contracts, and helium ignition can occur explosively in a helium flash. This event lifts degeneracy and begins stable helium burning, after which the star typically settles onto the horizontal branch or red clump in the Hertzsprung–Russell diagram. In more massive stars, helium ignition proceeds without a flash and proceeds in a more gradual, non-degenerate manner during the core helium-burning phase.

Duration and significance vary with mass. For sun-like stars, the helium-burning phase lasts on the order of tens to a hundred million years. Helium burning is a major source of carbon and, to a lesser extent, oxygen in galaxies, and it sets the stage for subsequent late-life evolution, including later burning stages in more massive stars and the formation of white dwarfs, neutron stars, or black holes depending on initial mass. Helium burning is thus a central chapter in stellar evolution and chemical enrichment.