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it is depleted, leaving a helium-rich core. Hydrogen fusion continues in a shell surrounding the core, which causes continued accumulation of helium in the core. This makes the core denser, but its temperature is still unable to reach the level required for helium fusion, as happens in more massive stars. Thus the thermal pressure from fusion is no longer sufficient to counter gravitational pressure and achieve the hydrostatic equilibrium found in most stars. This causes the star to start contracting

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