What precise force balance must a star achieve to leave core ignition and settle onto the main sequence?
Answer
Hydrostatic equilibrium
Hydrostatic equilibrium is the defining state of stability for a star during its longest phase of life, the main sequence. This equilibrium is established when the immense inward crushing force of the star's own gravity is perfectly counteracted by the massive outward pressure generated by the energy released from sustained nuclear fusion in the core. When these two opposing forces are balanced, the star maintains a constant size and stable energy output. If the balance is disturbed slightly—for example, if the core cools—gravity compresses it, raising the temperature and consequently speeding up fusion until the outward pressure restores the original balance.
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