 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| • |
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, ˝ of gravitational energy goes into heating
|
|
|
up star, ˝ gets radiated away
|
|
|
| • |
Rate
at which it radiates energy, so rate at which it shrinks,
|
|
|
is
limited by how “insulating” intermediate layers are
|
|
