This is the remnant
of a star which depleted its nuclear fuel long ago. It now
consists primarily of neutrons, but with a small portion of protons and
electrons. It may be possible to work out some of the same
research data by manufacturing a small amount of neutronium in a
laboratory, although that would be pretty dangerous to the habitat the
lab is in. A neutron star that hasn't run out of energy and
strong magnetic field is a
Pulsar.