When core hydrogen is depleted, what ignites in a shell surrounding the inert helium core?
Answer
Remaining hydrogen
When the hydrogen fuel in the star's core is completely consumed, fusion ceases there, and gravity takes over, compressing the core. This compression significantly raises the temperature in the layers immediately surrounding the now inert helium core. This increased heat is sufficient to ignite the remaining hydrogen present in that surrounding shell. Simultaneously, in massive stars, the core continues to compress until the helium itself ignites via the triple-alpha process deeper inside. This layered burning structure, where successively heavier fuels burn in shells around progressively heavier inert cores, is characteristic of the giant and supergiant phases.
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