How does a black hole form from a dying star?

When a massive star (typically more than 20 times the mass of our Sun) exhausts its nuclear fuel, it can no longer support itself against gravity. The core collapses suddenly and catastrophically in a fraction of a second. If the remaining core mass exceeds about 3 solar masses, not even neutron degeneracy pressure can halt the collapse, and it continues collapsing into a singularity. The outer layers of the star rebound and explode outward as a supernova. What remains is a region where gravity is so extreme that the escape velocity exceeds the speed of light, forming a black hole with an event horizon marking the point of no return.