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Star is “almost” in hydrostatic equilibrium
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Star radiates energy:
If nothing else happened T would
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drop, P would drop, star would shrink.
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Star does shrink, but in doing so gravitational energy is
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converted to heat, preventing T from continuing to
drop.
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In fact, since star is now more compact, gravity is
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stronger and it actually needs higher P (so higher T) to
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prevent catastrophic collapse
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As star shrinks, ˝ of gravitational energy goes into
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heating up star, ˝ gets radiated away
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Rate
at which it radiates energy, so rate at which it
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shrinks,
is limited by how “insulating” intermediate
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layers
are
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