After the type II supernova , only the collapsed core is left behind. If it's less than 2 or 3 solar masses, it is what's known as a neutron star , named thus because it's made almost entirely of degenerate neutrons.
Neutron stars are really dense.
They have between 1.4 and 3 times as much mass as the Sun, but are compressed into a ball with a radius of about 10 km. They were predicted to exist in the 1930s, but weren't discovered until 1967. A graduate student named Jocelyn Bell was monitoring radio wavelengths from space. She discovered a really regular signal. At first, no one could believe that anything natural could produce such a regular, repeating signal.