Neutron stars are the most compact form of matter that we know about; they’re even denser than the nucleus of an atom.
Neutrons, protons and electrons are fermions, meaning they must obey the Pauli exclusion principle. No two neutrons (or protons or electrons) can be in the same quantum mechanical state. If you take ordinary star matter (plasma made of dissociated protons and electrons) and squeeze it, eventually the electrons will nearly overlap in their states. You’d have two electrons with nearly the same energy, spin and location. They cannot overlap though, so this creates a repulsive force that prevents the matter from further compression; this is called the electron degeneracy pressure.
If the compressive force overcomes this pressure, then the electrons can capture on the protons to form neutrons. Neutrons and protons also have degeneracy pressures, but they can be packed much more densely than electrons. This is because their wavelength is shorter. The wavelength of a massive particle is inversely proportional to its mass, and protons and electrons are about 2000 times the mass of electrons. So compressed ordinary matter will inevitably become pure neutrons, simply because this is the most compact form.
A pure electron or pure proton star wouldn’t be as compact because both are charged particles so there would be Coulomb repulsion (this isn’t an issue in ordinary matter since the number of electrons and protons is roughly equal). You’d also need to somehow separate the electrons from the protons, and this isn’t a process that would naturally occur in a collapsing star.
Is that the form matter takes at the center of a black hole? Hmmm, that would make sense but I guess it doesn't necessarily have to, as the gravitational force isn't directly dependent on the density, just the mass.
To add on, definitely not a "normal" degenerate matter, no. If there's enough mass there to form a black hole, the energy liberated from shrinking exceeds the energy needed to decrease the wavelength of the particles, and continues doing so perpetually. Doing that calculation is why we were so sure black holes must exist in the first place.
There are no accepted answers to your question. However, I like to think that string theorists have it right and matter is sort of crushed so completely that you're just left with a roiling sea of primordial energy - a scalar field, aka, a new universe. It is in my opinion likely that black holes are the holographic medium on which a universe can be projected. String theory supports this thought; it posits that black holes are "fuzz balls", aka: a mass of unimaginable energy vibrating in many dimensions, aka: a universe. It is a simple, elegant explanation for why the universe exists, and where it came from.
But that is all extremely hypothetical. Like I said, there are no answers here... yet. There's a lot of good science to back up that thought, but none of it is falsifiable with our current understanding of physics. We probably need a theory of quantum gravity to start answering these questions.
A black hole is believed to contain a singularity with all of the mass as a single point. So this is well past the point of baryonic matter and in a region where our physics models break down.
If you just take the total mass of a black hole and divide it by the volume of the Schwarzschild radius (aka event horizon) you get a density MUCH greater than a neutron star. This isn’t a useful measure of the black hole density though, since all of the mass is at a single point of presumably infinite density.
Because neutrons are what you get when an elecron and a proton love each other very very much...
So basically if you sqeeze anything (made of atoms) hard enough you get neutrons.
It's not like it's "impossible" to sqeeze a bunch of protons, but stuff is usually made of atoms, not of protons, hence you don't get such a weird thing as a giant segregation of protons just lying there.
Because neutrons are what you get when an electron and a proton love each other very very much...
What occurs to me now that you put it that way, is from way back during the very early days of galactic formation, when so much hydrogen gas got blasted by radiation (was that caused by Population I stars?), stripping the electron away and leaving most of the hydrogen ionized - a fancy way of saying "lone protons floating in space".
Now what would happen if some ionized hydrogen clouds happened to collapse into massive stars during this window of time, before the universe became re-ionized? Massive stars with mostly protons and very few electrons?
Is this a valid hypothetical object? Then if it collapses under it's own proton weight, where are you going to get the electrons to merge with the protons to transform into neutrons?
Charge is (basically) conserved, so if you start with a non-charged star you need a non-charged result. All ordinary matter is close to balanced in charge. (Even things on earth with a really high voltage)
As for why not a mix of both, neutrons are just more compact. Literally, as the start collapses, the massive pressure "squishes" protons and electrons together, and they undergo the reverse of a radioactive decay. It's just because that's more energetically favourable than taking up the extra space, and a neutron results.
There's actually a great XKCD "What If?" post that covers this: https://what-if.xkcd.com/140/
It's down to the way they are formed. Basically, once gravity starts winning in the internal forces within a star, it collapses on itself. At that point, it's made up of protons, neutrons and electrons, like most visible matter. However, if the star is large enough, the force from the gravity compresses the atoms so much that the protons and electrons get forced together. And the tl;dr version is that when this happens, their charge cancels out, and you get more neutrons.
I'm starting here: https://physics.stackexchange.com/questions/116921/can-there-be-electron-and-or-proton-stars
I don't have the physics chops to understand most of what is being said, but if I can bang rocks together for a moment it sounds like it would fly apart, or, if you forced it all together somehow they would literally become neutrons.
Protons are positively charged. Like charges repel, so protons fly apart unless there's an equal number of electrons (or other negatively charged particles) to keep them happy.
Electrons are negatively charged, and fly apart unless there's a more-or-less equal number of protons. Electrons are a lot lighter than protons so move easier, and electrons deciding to be somewhere with more protons and less electrons is basically lightning.
Neutrons don't have an electrical charge. They just sit there being gravitational.
Sounds like I wasn't far wide of the mark. Cheers
