What process immediately follows the exhaustion of the core's usable fuel in Sun-like stars, causing them to expand dramatically?

Answer

Shell fusion begins in the surrounding hydrogen layer

Once hydrogen in the core is exhausted, gravity contracts the core, heating the surrounding hydrogen layer until fusion ignites there, which then powers the star to expand into a red giant.

What process immediately follows the exhaustion of the core's usable fuel in Sun-like stars, causing them to expand dramatically?

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