The mass of the neutron star is _not_ the same as the entire star before it. The neutron star is (mostly) core material from its parent star, the rest of the ma

The mass of the neutron star is _not_ the same as the entire star before it. The neutron star is (mostly) core material from its parent star, the rest of the matter was likely flung out upon that stars death. Gravity is not the only thing at play, strong nuclear forces between atoms prevent the star from collapsing under its own gravity, as well as the angular momentum of the stars spin. The neutron star is one giant atomic nuclei in the sense that all of its atoms are incredibly densely packed together. Imagine a styrofoam ball that you put under immense pressure from all angles (say by putting it deep under water). The air/free space in the styrofoam is squeezed out, and this in a sense is the same thing that happens to the plasma inside the parent star's core as it collapses. All of the atoms are squeezed together, and a balancing act is created whereby the incredible density of the core is trying to pull the matter inwards (into a black hole), while the strong nuclear force/momentum are pushing back to prevent the star from collapsing further.