Degenerate matter and Core Bounce during Supernovas

[Yora]

Yes, as the title implies, this is a bit of an advanced question:

Neutron stars and black holes are endlessly fascinating, but there is something about the core collapse that I've been wondering about that always appears to be glossed over in all explanations of the process that I can find.

During a core collapse, the iron core rapidly contracts, turns into degenerate matter, and then instantly stops its contraction because of neutron degeneracy pressure. Infalling material from the lower shells impacts on the neutron core, gets compressed, and acts as a spring that kicks the outher shells out into space. The lower shells have transfered their kinetic energy to the outer shells, so they can't bounce away into space and have to fall back onto the neutron core.

So far, so good.

But now the material of the lower shells also turns into degenerate matter, forming a single ball of neutrons. To my knowledge, there are no stars with a neutron core and outer shells of oxygen and silicon. All the material that is not ejected into space becomes degenerate matter.

But if the gravity at the surface of the neutron core is enough to turn all matter into neutrons, then why didn't the material of the lower shells also turn into degenrate matter instantly? Why didn't the entire star collapse into one big neutron star (and supsequently into a black hole)?
How can matter bounce off the surface of a neutron star?

[Chronos]

It's not gravity itself that causes matter to become degenerate, but pressure. Neutron stars do in fact have outer layers composed of more normal matter, because there isn't enough matter above them to build up the pressure. But these outer layers are very thin, because with that kind of gravity, the pressure builds up fast.

[shawnhcorey]

When a supernova collapses, the lower layers fall on to the surface of the core and transferred thru the core to the matter on the other side. This flings the matter away from the core where it encounters more falling matter. They come together and get very hot. A cloud of very hot plasma builds up around the core until it explodes.

After the supernova, there is still a thinner layer of plasma above the surface of the neutron star. This plasma will slow lose energy as light and eventually come to rest on the surface of the star. Of course, it could have its life extended by matter falling into the star, like the dust and particles in our galaxy.

[Fat Rooster]

Yes, as the title implies, this is a bit of an advanced question:

Neutron stars and black holes are endlessly fascinating, but there is something about the core collapse that I've been wondering about that always appears to be glossed over in all explanations of the process that I can find.

During a core collapse, the iron core rapidly contracts, turns into degenerate matter, and then instantly stops its contraction because of neutron degeneracy pressure. Infalling material from the lower shells impacts on the neutron core, gets compressed, and acts as a spring that kicks the outher shells out into space. The lower shells have transfered their kinetic energy to the outer shells, so they can't bounce away into space and have to fall back onto the neutron core.

So far, so good.

But now the material of the lower shells also turns into degenerate matter, forming a single ball of neutrons. To my knowledge, there are no stars with a neutron core and outer shells of oxygen and silicon. All the material that is not ejected into space becomes degenerate matter.

But if the gravity at the surface of the neutron core is enough to turn all matter into neutrons, then why didn't the material of the lower shells also turn into degenrate matter instantly? Why didn't the entire star collapse into one big neutron star (and supsequently into a black hole)?
How can matter bounce off the surface of a neutron star?

'Bounce' is a simplification of what happens. Most of the energy transfer is photonic, rather than pressure waves. The material that is collapsing onto the seed of the neutron star gets absurdly hot. It is surrounded by an insulating layer of star though, so that energy cannot immediately escape. Instead it produces pressure.

It is similar to how a red giant gets larger than a hydrogen burning star. The temperatures required for helium burning are much higher than hydrogen, which causes the star to be much larger. A neutron star forming produces runaway temperatures though, and instead of transitioning slowly, it does it fast. That means that the star cannot swell fast enough, and the inertia of the outer layers produces a further inward pressure term. Instead of the star swelling and the pressure dropping, the pressure just increases and accelerates the collapse. It is a bit like gunpowder burning in the open compared to a confined space. The pressure accelerates the burning, causing a much more impressive bang. By the time the outer layers of the star are where the steady state would be they are moving far above escape velocity, so just keep going.

You might be interested in Thorne Zythow objects. They are what happens when a neutron star merges with another star relatively peacefully. The neutron star gets to the high pressure area gradually, letting the temperatures ramp up much more slowly. Material collapsing onto the neutron star produces enough heat to prevent rapid collapse, but not so much that it shreds the star. The outer layers have time to slowly move to their steady state position, so it doesn't just blow up. If the neutron star then suddenly collapses further though, then things get messy. Observing the remnant of such a collapse would be extremely interesting, as it might produce a quark star that we know couldn't be a neutron star.