What is the last step before a star collapses into a black hole?


Stars that have a higher mass than about 2.2 solar masses (the mass of our Sun) will typically collapse into black holes at the end of their life cycle when fusion has stopped and no longer pushes out against the inward pull of gravity. For stars of this mass, their inward pull of gravity is too strong to be resisted by other forces, thus their fate is to become black holes. However, stars under this limit and above the Chandrasekhar limit of 1.4 solar masses will end their lifecycles as neutron stars. Neutron stars are also the remnants of dead stars but since they have a smaller mass, their gravity is not enough for them to cross into becoming black holes. As the dead star compresses and becomes more and more dense, regular matter is turned into neutrons and the last force to resist the pull of gravity is called degenerate neutron pressure, which is a quantum effect that dictates that no two neutrons may be in the same quantum state. This creates an outward pressure that resists the pull of gravity and settles the star into a neutron star. If more mass is added later, from a star companion or other sources, once the star's mass crosses the limit of around 2.2 solar masses even degenerate neutron pressure isn't enough to balance gravity and the star will collapse into a black hole.