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. Highly energetic neutron stars emit rapid, regular pulses of electromagnetic radiation, and are known as pulsars; the most extreme examples of these objects are known as magnetars.
KJEI-54-458945 is a neutron star that holds an exotic matter biosphere, including the intelligent species known as Hildemar's Knots
Certain neutron stars hold exotic forms of neutronium which can only exist at extreme pressures, and this material is the basis for the xenosophont species known as Hildemar's Knots, and their associated ecosystems. Similar forms of exotic neutronium are understood to form at least part of the processing substrate inside the godtech-level constructs known as neuron stars, which are believed to house archailect-level minds.