I've heard this explained numerous times, most recently in my General Relativity course today, where he talked about how smaller stars will collapse into white dwarfs, while more massive ones will overcome the electron fermi gas pressure, effectively forcing the electrons into the protons, so they make neutrons to go to a lower energy state. Then, as a neutron star, there is a fermi gas pressure (or pauli exclusion) that keeps the star from collapsing further. This is where it gets vague. If the star is more massive, the gravitational pull will larger than the neutron pressure... but what exactly happens to the neutrons? How is it that Pauli exclusion apparently just doesn't apply? Can anyone make this a little clearer? Thanks.