What fusion event triggers the expansion phase away from the main sequence for massive stars?
Answer
Hydrogen fusion in a shell surrounding the shrinking helium core.
The fundamental shift occurs when the primary fuel, hydrogen, is depleted in the star's core, halting core fusion. This core then contracts under gravity, generating intense heat. This heat eventually becomes sufficient to ignite hydrogen fusion in a surrounding shell. The energy output from this shell burning dramatically exceeds the previous core fusion rate, generating immense outward pressure that forces the star's vast outer envelope to expand spectacularly, leading to the supergiant phase.
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