What is the physical trigger that initiates the hydrogen shell burning phase in a red giant star?
Answer
The gravitational collapse of the helium core heating the surrounding layers
When core hydrogen fusion stops, the outward pressure supporting the star against gravity ceases in the center, leading to the gravitational collapse of the helium core. This contraction causes the core material to heat up intensely. This intense heating, transferred to the layer of fresh hydrogen situated just outside the contracting helium center, raises that shell's temperature sufficiently high to overcome the required ignition threshold, thereby igniting the hydrogen shell burning process. This mechanism dictates the shift from core to shell as the primary energy source.
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