Neutron Star

A supernova with a mass between about 1.5 and 3 times the mass of the Sun collapses under gravity until it consists almost entirely of neutrons. Nuclear forces prevent these squashing any closer. Neutron stars are only about 10 kilometers across and have a density of 1 hundred thousand billion (10¹⁴ ) times as dense as water.

Pulsars are rotating neutron stars. The rapid pulses of radiation they emit are generated by their very strong magnetic fields, which are characteristic of all neutron stars.

Note that a star smaller than this will collapse into a white dwarf, while a larger star will collapse into a black hole.