There are four possible kinds of "dead" or
"really really old stars"...
- A really low-mass star (less than the mass of the Sun), turns hydrogen
into helium so slowly that it never finishes during the entire age of the
- A medium-mass star (about the mass of the Sun) pulsates,
throwing off its outer layers, leaving a
white dwarf behind. (About the mass of
the Sun, but about the size of the Earth.)
- A high-mass star (more massive than the Sun) runs out of hydrogen,
then runs out of helium, then runs out of carbon and silicon, all the
way up to iron. Then, gravity wins, and the outer layers come crashing
down on top of it, creating a neutron star. (About the mass
of the Sun, but about the size of Chicago.) The neutrons can just
hold up agains gravity, so the outer layers
bounce off the neutron star and explode all over the place.
- A really massive star (much more massive than the Sun) runs through
hydrogen, helium, carbon, and silicon all the way to iron, but this
time, when the outer layers come crashing down on the core, gravity
REALLY wins, and the whole thing squshes down into infinite
density, creating a black hole. This thing is so dense
that nothing, not even light, can escape from it.