Strange dance begins as firework stars blast off

Amateur astronomers have found a nebula in explosive mood, writes Charles Arthur

The stars at the centre of the "Firework Nebula", pictured here, have begun shooting out light and X-rays after a four-year silence - a fact only picked up by an international team of amateur astronomers who keep in touch via the Internet.

Known officially as GK-Per, the nebula lies in the constellation Perseus, about 1,500 light years from Earth. At its centre are two stars - one hot and gassy, and another which is now a dense "white dwarf", about as heavy as our Sun but only as large as the Earth. It "went nova" in 1901, leading to the firework effect visible here as pieces of the star's outer shell were thrown off by a cataclysmic explosion within it.

Now, in part of a four-year cycle, gas is being sucked from the larger star towards the white dwarf "like water running down a plughole", said Julian Osborne, a research scientist at the University of Leicester. As the gas approaches the dwarf, it swirls around and heats up, giving off light, until finally a critical mass surrounding the star is dragged downwards - where it crashes into the hot surface at 5.4 million mph, accelerating so rapidly that it gives off X-rays.

Dr Osborne was contacted in February by the amateur group, numbering about 30 internationally, who regularly monitor the nebula and other variable stars. "It's not possible to predict when it will happen, so we rely on amateurs to tell us and to register its changes in brightness," he said yesterday. "They put the information on the Internet and have a daily mailing list so they can tell the professionals."

Once alerted, he was able to get an orbiting X-ray satellite to begin observing the starburst, which is expected to last two or three months. But so far nobody can predict how long the pair of stars will continue their strange dance. The white dwarf will gain mass, meaning that it will be able to suck gas more easily, but even so there is probably sufficient material for the cycle to last many millions of years.