Neutron Stars

After expending its energy, a supernova might collapse under its own gravitational pull till its protons and electrons were crushed together to form neutrons and the neutrons were forced into contact. Such a neutron star might possess all the mass of a full-sized star and be only 8 miles across. These would be difficult to detect and were not observed for decades.
 Fritz Zwicky Switzerland 1934 Neutron Stars

After expending its energy, a supernova might collapse under its own gravitational pull till its protons and electrons were crushed together to form neutrons and the neutrons were forced into contact. Such a neutron star might possess all the mass of a full-sized star and be only 8 miles across. These would be difficult to detect and were not observed for decades.