Lone neutron star speeds through space

RX J185635-3754 is 200 light-years away

By BBC News Online science editor Dr David Whitehouse

Astronomers have identified a runaway neutron star that is hurtling in our direction at more than 100 times the speed of a supersonic jet.

Neutron stars are small, extremely dense, rapidly rotating spheres of neutrons that are created when some of the biggest stars in the Universe explode at the end of their fuel-burning lives.

This object, designated RX J185635-3754, is smaller than the Isle of Wight but 10 trillion times denser than steel. Astronomers say it would have been seen to blow up in the night sky by our distant ancestors in one million BC.

Now just 200 light-years away in the southern constellation Corona Australis, the neutron star will pass the Earth at a more than safe distance of 170 light-years in about 300,000 years. A light-year is the about 10 trillion km (6 trillion miles).

Lone star

RX J185635-3754 is the closest known neutron star - its distance has been well measured by the Hubble Space Telescope. Astronomers will now study the object's physical properties, such as size, brightness and age, and use it to test their theories about neutron stars.

"The scientific importance of this object lies in the fact that the neutron star is isolated," said Frederick Walter of the State University of New York.

"It appears to be hot, not because it is accreting hydrogen gas as it moves through space but because it is still young and cooling off. Since we know its approximate age, we can test how fast neutron stars cool off."

"Because this is the closest and brightest of the few known isolated neutron stars, it is the easiest to study and is an excellent test-bed for nuclear astrophysical theories."

The neutron star's trajectory was caught in three Hubble images taken in 1996 and 1999. The three images show it moving across the sky.

In 5,400 years, RX J185635-3754 travels an apparent angular distance across the sky equal to the diameter of the Moon. Although this apparent motion may seem slow, it is actually one of the fastest moving stars in the sky.

Binary system

This neutron star may be moving away from a grouping of young stars in the constellation Scorpius.

About one million years ago, a massive star in a binary system exploded as a supernova, releasing its companion star, an ultra-hot, blue star now known as Zeta Ophiuchus. This is also moving away from the region.

One million years ago, the neutron star and Zeta Ophiuchus would have been in about the same location in space. This makes astronomers think the neutron star may be remnant of the original binary companion to Zeta Ophiuchus which exploded.

The Hubble image at the top of the page is the sum of three images showing the motion of the star across the sky.