Anatomy of a Black Hole
If a neutron star has a mass greater than about 3 Solar Masses, neutron degeneracy is insufficient to support the degenerate neutron matter from futher collapse. Within approximately 1/100,000 sec (based on the equations of general relativity), the neutron star collapses into a singularity of infinite density and zero volume. What remains is a "hole" in spacetime with a curvature so severe that not even electromagnetic radiation can escape the gravitational well of the singularity.
The event horizon marks the original surface of the neutron star prior to collapse—approximately 6 miles in radius, or the size of a city such as Washington, D.C.—and also marks the boundary between the universe as we know it and the forever hidden region inside the black hole.
Moving outward from the singularity, the region inside the black hole is marked by an escape velocity greater than the speed of light. The further the distance from the singularity, the lower the escape velocity. Eventually, one would theoretically reach a distance called the Schwartzchild radius (Rs)—the distance from the singularity to the event horizon—where the escape velocity exactly equals the speed of light. Outside of the event horizon, the escape velocity is lower than the speed of light and it would theoretically be possible to escape the gravitational clutches of the black hole.