A neutron star is the collapsed core of a star that is left over after a supernova explosion. It is, so to speak, matter’s last line of defense against gravity. In order to stay internally supported as an object and not be crushed into a singularity, the neutrons in the object press up against one another in a state known as neutron degeneracy. This state, which resembles the conditions within an atomic nucleus, is the densest known form of matter in the universe.