A very small dense star that is composed mostly of tightly-packed neutrons (neutronium).
This hard-to-see body, the remnant of a star after it has exploded as a supernova, has a thin atmosphere of superhot hydrogen plasma and a crust made up mainly of iron and other heavy nuclei. It has a diameter of about 5-16 km and a density of roughly 1015 gm/cm3. Beneath the crust lies a mantle of superfluid neutronium, which can become mixed with quark matter at the core. Most neutron stars rotate rapidly, with periods ranging from a few seconds down to milliseconds.