How does the rate of hydrogen burning in the shell compare to the rate during the star's main sequence core burning phase?
Answer
This shell burning is actually more vigorous than the original core burning was
Paradoxically, the hydrogen fusion occurring in the shell surrounding the contracting helium core is more intense than the fusion that took place in the core during the main sequence. The gravitational contraction of the dense helium core significantly raises the temperature and pressure within the adjacent hydrogen shell. This dramatic increase in conditions within that narrow region forces hydrogen fusion to proceed at a much higher rate. This intense, renewed energy production is what generates the massive outward thermal pressure required to overcome gravity and cause the star's outer layers to expand tremendously into the red giant configuration.
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