Why do we get steady fusion rates?
	• 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)