Question: How does quantum degeneracy pressure stop a neutron star from further gravitational collapse?

Evan Keane answered on 20 Jun 2011:

This is the most advanced question I’ve had in I’m a Scientist! 🙂 I will answer you but it might be a bit advanced, to match the question. If you can understand it you should be a physicist!

In a normal star there is nuclear fusion happening and that creates energy which comes out in the form of radiation (light). This radiation acts outwards. Gravity of course acts inwards, and the two forces find a balance. When a star has no more stuff with which to perform nuclear fusion it suddenly cannot produce any more outward force (Actually some neutrinos try to support the star but they are rubbish at that and can only hold things together for like a second). Uh oh. So there is only gravity acting inwards and nothing acting outwards. The star then collapses.

But the star does not collapse to nothing. At some point the collapse halts and the core of the star becomes robust again and all the stuff that is falling on it bounces off out into space. This is a supernova explosion – and all those pretty images of supernova remnants you might have seen are of this stuff which has been bounced into space. Ok, but why did the star not just collapse all the way? The reason is degeneracy pressure. Ok, here’s where it gets tricky:

In Quantum Mechanics there is something called the “Heisenberg Uncertainty Principle”. This says that if you are pretty certain about where a particle (like an electron or neutron) is you have a very poor idea of how fast it is moving, and vice versa. IT IS NOT POSSIBLE to know exactly where a particle is and know exactly how fast it is moving. Ok, so what? Well when the star collapses all the particles get squashed together and they are in a more and more confined space. But if they are in a confined space then we have a good idea where they are -> that means (according to the Heisenberg Principle) that we are very uncertain as to their speeds. The uncertainty in their speeds is so high that it is “relativistic”. This means that the (uncertainty of the) speeds are close to the speed of light (the fastest speed you can go). All these fast moving particles create a pressure which supports the collapsed star against gravity. Remember the pressure of a gas is just due to the gas particles moving about hitting against things – the faster they move around the more pressure.

There is electron degeneracy pressure which (obviously) involved electrons and this is what supports white dwarfs. There is also neutron degeneracy pressure and this is what supports neutron stars (and this is why they have “neutron” in their name). When you squish things so tight that electron degeneracy pressure can’t keep it together neutron degeneracy pressure kicks in -> you can pack neutrons tighter together than electrons. But if you still squish the neutrons further you will either squeeze so hard that the quarks inside the neutrons come out (but nobody knows if this happens for sure) or you will just get a black hole.

I hope some of that was understandable 🙂