neutron degeneracy pressure

What stops a dead star from collapsing all the way to nothing? In a white dwarf, electrons refuse to be squeezed together and hold the line. But push harder — beyond the Chandrasekhar limit — and even the electrons give way, getting crushed into the protons to make neutrons. Now the neutrons take up the fight, refusing in turn to be packed any tighter. The outward push they exert is neutron degeneracy pressure, and it is what holds up a neutron star, the densest stable matter that exists.

The underlying rule is the same one that supports white dwarfs: a quantum law called the Pauli exclusion principle forbids two identical particles from occupying the same state. Cram neutrons together and they are forced into ever-faster motion to stay out of each other's way, and that motion is a pressure — one that, remarkably, does not need any heat. It works at near-absolute-zero just as well. But neutrons are nearly 2,000 times heavier than electrons and can be packed far closer, so neutron degeneracy holds out against gravity at staggering densities — comparable to an atomic nucleus, hundreds of trillions of times denser than water.

Like electron degeneracy, this support has its own breaking point. There is a maximum neutron-star mass (the Tolman-Oppenheimer-Volkoff limit, somewhere around 2 to 2.3 solar masses) above which not even the neutrons can win, and the star collapses into a black hole. An important honest caveat: neutron-star interiors are so extreme that pure neutron degeneracy is only part of the story — strong nuclear forces between the neutrons also contribute, and the exact behaviour of matter at that density is still an open research question.