Why do some stars become neutron stars instead of black holes?

When a massive star runs out of fuel, its core can collapse after a supernova explosion. If the remaining core is not too massive, quantum pressure from densely packed neutrons can support it against further collapse, forming a neutron star. If the remnant is heavier, gravity overwhelms that support and a black hole forms instead. The dividing line is not fixed perfectly, but observations suggest the heaviest neutron stars are around two solar masses, while the transition to black holes occurs at higher remnant masses. This boundary is one of the active research areas in astrophysics.