Long before that, Fritz Zwicky predicted that a supernova explosion gives birth to a tiny neutron star. Years later, Jocelyn Bell Burnell caught its pulses, discovering pulsars.
A neutron star is what remains after a massive star explodes. Its matter is compressed so tightly that a teaspoonful would weigh billions of tons. Under such conditions, protons and neutrons can rearrange, and new particles (hyperons, similar to neutrons but containing a 'strange' quark) can appear.