See my copyright notice for fair use practices. 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. Within a certain distance of the point mass, the escape velocity is greater than the speed of light. Remember from the gravity chapter that the escape velocity is the speed an object needs to avoid being pulled back by the gravity of a massive body. Since the mass is in the top of the fraction, the escape velocity is greater for larger masses.