A neutron star is a collapsed core of a massive supergiant star. The stars that later collapse into neutron stars have a total mass of between 10 and 25 solar masses (M☉), possibly more if the star was especially rich in elements heavier than hydrogen and helium. Except for black holes, neutron stars are the smallest and densest known class of stellar objects. Neutron stars have a radius on the order of 10 kilometers (6 mi) and a mass of about 1.4 M☉. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.