What prevents a neutron star from collapsing and becoming a black hole?

Answer 1

As Kris says, its neutron degeneracy pressure. Particles with half-integer spin such as neutrons obey Fermi-Dirac statistics and the Pauli Exclusion principle which states two particles cannot have the same quantum mechanical state. Particles with integer spin (such as photons) have no such restriction.

In degenerate matter, the extreme density means all the lower energy levels available for particles to occupy are filled up, meaning it takes a lot of energy to boost a particle to a high enough energy to reach an available level. When you compress a material, you do work on the material and therefore change the velocity and energy level of particles. Degenerate matter greatly resists such compression since changing the velocity of a particle is so difficult. There are no levels available at lower energies, and the available levels at higher energies are so far off. So the material is nearly incompressible.

Nearly.

Answer 2

Neutron degeneracy pressure. Basically, all the matter of a neutron star is in the form of neutrons (hence the name). You can only squeeze the neutrons so close together without destroying them. The neutrons don't want to get any closer together, so they resist - that's neutron degeneracy pressure.

Of course, that's a bit over-simplified. But it gets the general idea across.

Answer 3

Its small mass. It does not have enough mass or gravity to crush the neutrons any more.

Answer 4

Insufficient matter or gravity.