 A supernova explosion blows away the star 's outer layers , leaving a remnant such as the Crab Nebula . The core is compressed into a neutron star , which sometimes <unk> itself as a pulsar or X @-@ ray <unk> . In the case of the largest stars , the remnant is a black hole greater than 4 M ☉ ) s . In a neutron star the matter is in a state known as neutron @-@ degenerate matter , with a more exotic form of degenerate matter , <unk> matter , possibly present in the core . Within a black hole , the matter is in a state that is not currently understood . 
