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Strange counterintuitive result:
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As star radiates away thermal energy it actually
heats up
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(because as it shrinks gravity supplies even more
energy)
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Star continues to shrink till it gets hot enough
inside for
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fusion (rather than gravity) to balance energy being
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radiated away.
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Nuclear thermostat
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If fusion reactions took place in a box with fixed walls:
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Fusion more energy higher T more fusion (explosion)
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If fusion reactions take place in sun with soft gravity walls:
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If fusion rate is too high T tries to go up
but star expands and actually
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ends up cooling off slowing down fusion. (steady rate)
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