How does the energy output from shell hydrogen fusion compare to the star's previous core fusion rate?
Answer
It is significantly greater than the previous core fusion rate.
When hydrogen fusion ceases in the core and shifts to a shell surrounding the contracting helium core, the nature of the star's energy generation changes drastically. This new shell burning process produces energy at a much higher rate than the steady fusion that occurred during the main sequence phase. This surge in energy output translates directly into intense outward pressure, which is powerful enough to inflate the star's immense outer envelope to supergiant proportions.
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