X-rays reveal neutron star

Scientists at the US space agency Nasa have discovered an enormously dense star which, despite being only about 10 miles across, is beaming out a million times as much energy as our sun.

The object lies towards the centre of our galaxy, and is believed to be made of neutronium - the densest possible matter, which consists only of neutrons, the neutral particles usually found at the core of atoms such as helium. Because neutrons do not repel each other, they can be packed incredibly densely - in this case forming a "neutron star".

The scientists detected the star because it is radiating its energy as X-rays, shooting out about 20 bursts each day. Though such X-ray sources were first discovered 20 years ago, this is the most powerful one ever observed.

Fred Lamb, an astrophysicist at the University of Illinois, reckons the X-rays are caused by interstellar matter falling towards the star's surface, where gravity is roughly a hundred billion times greater than Earth's.

"First, matter is accelerated to half the speed of light because of the neutron star's enormous gravitational force," Professor Lamb explained. "Then, it crashes into the surface and is heated to nearly 1 billion degrees. Because it is so hot, the star radiates its energy almost entirely as X-rays rather than as visible light." X-rays are electromagnetic radiation, like light, but have a much higher energy and shorter wavelength.

The star - known as GROJ1744-28 - lies 30,000 light years away. The international team of scientists have been studying it since 5 December, and are now focusing an orbiting space telescope known as RXTE on it.

"With better measurements from the RXTE, we should be able to pin down the theoretical model," said Jean Swank, a scientists on the project.

It was suggested that X-ray sources were not natural phenomena - but the efforts of aliens to contact intelligent life, by manipulating stars' behaviour. This was discounted when more began to be discovered with improved radio telescopes.