Disclaimer: I’m not a career astronomer. I don’t own a telescope. I have no professional credentials. But I do find this stuff fascinating, and I consume all astronomy documentaries I can.
So, I’ve watched lots of documentaries describing stellar evolution. I understand that below a certain threshold, stellar death does not involve supernovae. I understand that above that threshold, supernovae may create neutron stars, magnetars, or (if the supernova qualifies as a hypernova) black holes.
However, for a long time, I was curious about why stars below the supernova threshold—like our own Sun—become Red Giants.
From documentaries, I have been instructed that (for stars below the supernova threshold), when the star’s core’s fusion cannot continue…fusion ceases, and the star begins to collapse under gravity.
As gravity crushes the star, I understand that the star heats up as gravity crushes it. As a result, although the stellar core remains “dead” (no fusion occurs), a “shell” of gas around the stellar core becomes hot enough to begin fusing helium. Since the fusion occurs as a “shell” around the stellar core, the outward-push from the fusion is what pushes the star’s outer layers further. The result is that the star grows into a Red Giant.
My question is this: Why does fusion cease in the core?! It seems to me that as gravity crushes the star, stellar fusion would reignite in the core itself—not in a sphere around the core. Why does the stellar core remain “dead” while its “shell” begins fusion???