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Solitary Neutron Star Reveals Its True Colors

Astronomers have taken their first direct look at a lone neutron star in visible light. Because they are normally found paired with other, much brighter stars, which complicate interpretation of their light spectra, or as twinkling radio or x-ray pulsars, these strange objects have been hard to study in detail. But in this week's issue of Nature, a team confirms finding a nonpulsing, lone example by observing the visible component of its light.

Neutron stars, which are the remains of large, old stars that have blown up in supernovae, are the most dense objects known to exist in the universe. Their exotic matter--a handful weighs as much as a fleet of battleships--is of great interest to astrophysicists, and several competing theories attempt to describe their composition and structure. But neutron stars are typically spotted as fluctuating sources of radio waves or x-rays emitted by particles caught in their magnetic fields, which makes it hard to observe their hot, glowing surfaces directly.

Frederick Walter and Lynn Matthews, of the State University of New York, Stony Brook, now confirm that a bright and steady x-ray source they first observed last year is a neutron star with a visible surface. When looking at the location of the x-ray source, called RX J185635-3754, with the Hubble Space Telescope, they saw a dim blue light that proved the source was a neutron star. "There's really nothing else, no [other] astronomical source that could account for it," says Matthews, as nothing else could be so x-ray bright while being so dim in visible light.

Because the star is in front of a relatively close gas cloud, whose distance from us is known, Walter and Matthews could set an upper limit on the star's distance, and they guessed its maximum possible size based on that distance and the amount of light from its glowing surface that reaches Earth. They estimate it to be no bigger than 28 kilometers across, although it has more mass than the sun. If the expected, more precise distance measurements put the star much closer than the cloud, the star's diameter estimate will be significantly smaller, challenging some current neutron star models.

Astrophysicist Adam Burrows, of the University of Arizona, Tucson, says that if scientists are to understand neutron stars in as much detail as they do ordinary stars, radiation from a star's surface must be observed cleanly. "This is nice, new progress in a subject that has been in need of this type of data for a long time," he says.