What process triggers the dramatic expansion into a red giant phase after core hydrogen depletion?

Answer

Hydrogen fusion igniting in a shell surrounding the contracting core

When the hydrogen fuel in the stellar core is completely exhausted, fusion ceases at the center, and gravity causes the inert helium core to contract and heat up dramatically. This intense compression generates sufficient temperature and pressure in the adjacent layer of unused hydrogen fuel to ignite fusion there, creating a hydrogen-burning shell. Crucially, this shell burning generates energy at a rate significantly greater, or *more energetic*, than the core burning it replaced. This massive surge in outward thermal pressure overwhelms the inward pull of gravity in the outer regions, forcing the star's extensive outer layers to inflate rapidly to enormous proportions, thus defining the transition to the red giant phase.

What process triggers the dramatic expansion into a red giant phase after core hydrogen depletion?

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