•Limiting
case: Assume no nuclear fusion,
only energy source is gravity.
•
•Star
is “almost” in hydrostatic equilibrium
–Star
radiates energy: If nothing else happened T would drop, P would drop, star would shrink.
–Star
does shrink, but in doing so gravitational energy is converted to heat, preventing T from continuing to
drop.
–In
fact, since star is now more compact, gravity is stronger and it actually needs higher P (so higher T) to
prevent catastrophic collapse
–
•As
star shrinks, ½ of gravitational energy goes into heating up star, ½ gets radiated away
•
•Rate at which it radiates energy, so rate at which it
shrinks, is
limited by how “insulating” intermediate layers are