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Limiting case:
Assume no nuclear fusion so only energy source is gravity.
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Star is “almost” in hydrostatic equilibrium
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Star radiates energy: If
nothing else happened T would drop, P would
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drop, star would shrink.
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Star does shrink, but in doing so gravitational energy
is converted to heat,
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preventing T from continuing to drop.
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In fact, since star is now more compact, gravity is
stronger and it actually
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needs higher P (so higher T) to prevent catastrophic collapse
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As star shrinks, ˝ of gravitational energy goes into heating up
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star, ˝ gets radiated away
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Rate
at which it radiates energy, so rate at which it shrinks, is
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limited
by how “insulating” intermediate layers are
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