What process drives the massive bloating of the star into a red giant?
Answer
Intense hydrogen fuel burning in a shell surrounding the inert core.
The expansion characterizing the red giant phase is fueled by hydrogen fusion that ignites in a shell surrounding the inert helium core. This shell burning is significantly more intense than the previous core burning because the collapsing core acts as a highly efficient furnace, dumping immense energy outward. This massive energy output pushes the star's outer layers outward dramatically, causing the radius to swell perhaps hundreds of times its original main-sequence size, while the surface cools down, leading to the characteristic red color.
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