A neutron star is the densest known object in the universe, with the mass of about 1.4 Suns packed into a sphere about 12 miles (20 kilometers) across. Its density is about the same as a bare atomic nucleus. It is a stellar corpse, formed during a supernova explosion. When a massive star (more than about 8 times the mass of the Sun) explodes, its core implodes forming a very dense state of matter. On Earth or in the Sun, most of the volume of atoms is occupied by clouds of electrons. In neutron star matter, the electrons are squeezed into the protons (forming neutrons), allowing the matter to be condensed by about a factor of 100,000 (a one thousand trillion times reduction in volume). These neutron stars are born hot, and most appear to be born with strong magnetic fields and rotate very rapidly.
If we could see the thermal emission from the surface of a neutron star, we could determine its radius, temperature, and what it was made of. This would permit tests of theories of the structure of neutron stars and would let astronomers probe the characteristics of the densest stable state of matter known. About 1,500 neutron stars are now known, but most are pulsars (highly magnetized rapid rotators). The rest reside in close binary systems called X-ray binaries, where they emit X-rays as they accrete matter from their companions. In the pulsars and accreting sources, the surface emission is swamped by emission from other sources (such as the magnetosphere in a pulsar and the mass accretion process in the binaries). Only in an isolated neutron star can we measure the details of its surface, much as astronomers would study any star in the sky.
Of the isolated, non-pulsing neutron stars now known, RX J185635-3754 is the nearest and brightest. Hence, it is an excellent test bed for studies of the physics of neutron stars.
Why RX J185635-3754 is not a pulsar, despite its young age, is not yet known.