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