•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