How can a star end in a black hole or neutron star?

Answer 1

Saving the full explanation of the processes of a star, just a short rundown: Stars are massive, so massive that their own gravity tries to collapse the star so it has to fuse hydrogen to exert an outward force to overcome its own gravity. During these processes, atoms are being stripped apart into free-floating nuclei, electrons and protons, accumulating in the core for the life of the star. When a star runs out of hydrogen fuel, it collapses and is stopped by the core getting hot enough to fuse helium (Product of hydrogen fusion), and that keeps happening up the atomic chain until the star starts fusing nickel, which requires more energy to fuse than is released, this is the end of a star as we know it.

If a star forms a black hole or a neutron star is dependant completely on the star's mass. If it is heavy enough, if will become a black hole, if not, then it will become a neutron star, where gravitational collapse is halted by the accumulated electrons in the core being compressed with free protons, bonding together to form neutron degenerate matter (Basically neutrons) and exerting an outward force that overcomes collapse. This force is known as neutron degeneracy pressure, courtesy of the Pauli exclusion principle of the Fermi-Dirac statistics. The principle states that no two fermions (Particles with a half-integer spin; quarks and leptons) can occupy the same energy state simultaneously, so you get an outward force.

Now if gravity were to overcome said force, the neutrons would then split into their individual quarks, resulting in quark degenerate matter and halting collapse, although very little is known about quark degeneracy or even how the matter splits into quarks. Then into the hypothetical preon degenerate matter and finally a gravitational singularity. Please note that very little is known about quark and preon degeneracy (The latter being generally not accepted as a viable model) as none have been discovered to date.

Finally, if the star wasn't heavy enough to form a neutron star, the core will decay into a large and hot ball of iron (Nickel decays into iron) and float, slowly cooling for the remainder of its existance or until acted upon by external forces.