A relatively small, dense object racing across the sky and heading our way at more than 100 times the speed of a Concorde jet has been identified as our solar system's closest known neutron star.

The compact remains of an ancient explosion, less than 12 miles (19 kilometers) in diameter but 10 trillion times denser than steel, the neutron star zips along at roughly 240,000 miles per hour (108 kilometers per second). Most neutron stars are found in paired or binary star systems but this runaway object has broken free of its larger companion, giving astronomers a rare treat.

"The scientific importance of this object lies in the fact that the neutron star is isolated," says Frederick M. Walter of the State University of New York in Stony Brook. Walter said the object is surprisingly hot, probably due to the fact that it is relatively young and still cooling off.

"Since we know its approximate age, we can test how fast neutron stars cool off," Walter said.

The object, first spotted in 1992, was confirmed to be a neutron star in 1996. But only now has its distance from Earth been determined, using data provided by the Hubble Space Telescope. The object, also described as the corpse of a star, currently is about 200 light-years away. It is due to pass by Earth in about 300,000 years, but will safely miss by about 170 light-years.

Because the neutron star, called RX J185635-3754, is relatively nearby and all by itself, astronomers will be able to glean valuable information about star formation by studying it. Most neutron stars are hard to study as they are a billion or more light-years away.

Researchers say that RX J185635-3754 was probably shot like a cannonball when its larger companion star exploded as a supernova a million years ago. The other star, now a hot blue star, has been seen racing in another direction. Calculations show that the two were likely orbiting each other before the explosion in a configuration known as a binary star system.

The explosion would have been visible to our earthly ancestors in about 1 million B.C., scientists say. Supernova explosions are very complex and not well understood. Nor is the structure of a neutron star known in any detail.

The results were presented November 9 at a meeting of the High-Energy Astrophysics Division of the American Astronomical Society in Honolulu. A paper on the finding will be published in the Astrophysical Journal.

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