What physical mechanism causes the initial helium-rich core to contract violently upon hydrogen fuel exhaustion?
Answer
The loss of outward pressure resisting gravity.
During the star's Main Sequence tenure, it maintained hydrostatic equilibrium, meaning the outward pressure generated by core hydrogen fusion perfectly balanced the inward crushing force of gravity. When the hydrogen fuel in the core is entirely consumed, this central outward pressure source vanishes. Gravity, which had been persistently active, is no longer counteracted in the core region. This imbalance allows gravity to dominate entirely, initiating the violent gravitational collapse of the now helium-rich core, which in turn generates the thermal energy necessary to heat the surrounding layers.
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