Neutron stars are extremely dense, compact stars consisting mostly of neutrons. They are thought to be the hot remains of a supernova explosion — when an old massive star collapses in on itself. Most known neutron stars are either pulsating (pulsars) or have a close companion.
There are supposed to be as many as a billion lone neutron stars zooming about the Milky Way, but the first isolated, non-pulsating neutron star was discovered just five years ago by Fred Walter of the State University of New York at Stony Brook (Nature 379, 233–235; 1996). The initial X-ray discovery was confirmed by optical data from the Hubble Space Telescope (Nature 389, 358– 360; 1997). Last week, at a meeting of the American Astronomical Society in Honolulu, Hawaii, Walter revealed the latest Hubble observations of this extraordinary object (The Astrophysical Journal ; in the press).
The extreme densities of neutron stars offer a natural laboratory for exploring the 'equation of state' of nuclear matter — the relationship between pressure, temperature and radius in the atomic nucleus. Although the first optical measurements of the neutron star gave Walter a fairly good idea of its radius and distance, he needed to know its parallax to be sure. Parallax is the apparent motion of nearby objects, when seen against the background of more distant ones. As the Earth orbits the Sun, the positions of nearby stars shift slightly, with a maximum every six months.
From observations over three years (see image), Walter calculated a distance of 200 light years for the star, making it the closest known neutron star to Earth. He found its radius to be just 11 km; together with the mass this places constraints on the nuclear equation of state. Walter's results also show that the star is speeding away (at about 108 km s−1) from a group of stars known as the Upper Sco OB association, from which it was probably ejected a million or so years ago. When combined with the X-ray and optical observations, this date allows us to estimate how quickly neutron stars cool, which for this star seems to be faster than theoretical expectations. Quite a legacy for what was once just another mystery X-ray source.