During which specific stage of a Sun-like star's Red Giant lifetime does the star fuse helium into carbon in its core via the triple-alpha process?
Answer
Horizontal Branch/Red Clump
The Red Giant phase involves multiple distinct internal fusion stages following core hydrogen depletion. Initially, on the Red-Giant Branch (RGB), hydrogen fuses in a shell around the inert helium core. Once sufficient temperature and pressure are achieved in that core, helium ignition occurs through the triple-alpha process, causing the star to enter the Horizontal Branch or Red Clump phase, where helium is actively fused into carbon within the core itself. This is followed by the Asymptotic Giant Branch (AGB), where the star again has dual shells—one fusing helium and an outer one fusing hydrogen—surrounding an inert carbon-oxygen core.
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