If the core mass is between 1.4 and 3 solar masses, the compression from the star's gravity will be so great the protons fuse with the electrons to form neutrons. The core becomes a super-dense ball of neutrons. Only the rare, massive stars will form these remnants in a supernova explosion. Neutrons can be packed much closer together than electrons so even though a neutron star is more massive than a white dwarf, it is only about the size of a city. If the core remnant has a mass greater than 3 solar masses, then not even the super-compressed degenerate neutrons can hold the core up against its own gravity.
Gravity finally wins and compresses everything to a mathematical point at the center.
The point mass is a black hole. As the core implodes it briefly makes a neutron star for just long enough to produce the supernova explosion. Copyright 2001 - Samuel J. Wormley, Iowa State University. All rights reserved.