Red Giants & Helium Burning: Physical Mechanisms

The red-giant core is much denser and hotter than the expanded envelope. Hydrogen fusion occurs in a shell around the core, while the photosphere expands to many times the main-sequence radius and cools. Helium burning converts helium primarily into carbon through the triple-alpha process.

When core hydrogen fusion stops, gravity contracts the core and releases energy. The hotter surrounding shell burns hydrogen rapidly, increasing luminosity and forcing the envelope outward. In low-mass stars, electron degeneracy can support the helium core until helium ignition becomes rapid in a helium flash; more massive stars ignite helium more gradually.

Cluster H-R diagrams show populated red-giant branches and helium-burning groups at predicted luminosities. Asteroseismology can distinguish red giants with inert helium cores from stars actively burning helium because their internal oscillation patterns differ.

Core contraction raises temperature until helium fusion becomes possible. In a degenerate low-mass core, pressure is weakly temperature-dependent, so helium ignition can run away temporarily in the helium flash. Expansion then lifts degeneracy and restores stable regulation.