A Neutron star is a collapsed core of a supergiant star, which has 10 to 25 solar masses, or a radius of 10-20 kilometers,
it can even be more if it is metal-rich. They are also some of the densest stellar objects in space known.
They result from the supernova explosion of a massive star, with gravitational collapse (where a object's gravity collapses on itself) that compresses the core past white dwarf star density to that of atomic nuclei
(is the small, dense region consisting of protons and neutrons at the center of an atom)

For more information click this link https://en.wikipedia.org/wiki/Neutron_star

(source Wikipedia "Neutron star")