If it has between 1.4 and 3 solar masses of neutrons, neutron degeneracy pressure will stop its collapse and the core becomes a stable neutron star. If the neutron core contains more than 3 solar masses, even that force can't prevent the neutrons from overlapping, collapsing further, and creating a black hole. These are concentrations of matter so dense that their only effect on the outside comes from their gravitational attractions. Once matter or energy enters a black hole, it cannot escape. However, the details of what happens to the material entering the black hole are not yet known.
Supernova remnants are among the most spectacular objects in the galaxy.
In 1987, a supernova in the Large Magallenic Cloud, a galaxy 160,000 light-years from Earth, was visible to the naked eye in the night sky of the Southern Hemisphere. The gas and dust that compose supernova remnants expand free of the core and spread out into the galaxy. Many of the radioactive isotopes in the remnant decay to stable elements, creating even more of the heavy elements that exist on Earth. Eventually enough of this gas cools or clumps together with other interstellar gas to begin recollapsing and forming a new generation of stars and planets. Earth is an especially concentrated residue of a supernova.