The white dwarf star in the Ophiuchus constellation has exhausted its own nuclear fuel but is now stealing it from a neighbouring giant.

Every 20 years or so, it gathers sufficient material to explode with enough intensity to be seen from Earth with the naked eye.

The so-called recurrent nova event has now flared up six times in 108 years.

"It's a runaway nuclear bomb, basically, that has gone off on the surface of the white dwarf star," said Mike Bode.

The Liverpool John Moores University professor has been detailing the ongoing incident, along with Dr Tim O'Brien of Jodrell Bank Observatory, to the 2006 UK National Astronomy Meeting.

Amateur stargazers in Japan were the first to report the nova on 12 February, and the professional scientific community has responded by calling up some of the world's leading facilities to take a look.

These telescopes include the US space agency's flagship Swift and Spitzer missions; as well as ground-based observatories such as the United Kingdom Infrared Telescope on the summit of Mauna Kea in Hawaii, and the Merlin radio astronomy network in England.

"The level of co-operation and the range of telescopes involved has been extraordinary," said Professor Bode, the leader of the Swift's nova campaign.

Slow fade

The white dwarf star is about 5,000 light-years from Earth and is probably little bigger than our own planet, having burnt all its hydrogen and shed its outer layers.

It is now extremely dense and its strong gravity can pull gas off a companion, red giant star. The two objects orbit each other every 455 days.

What is very unusual in this arrangement is that the red giant is losing enormous amounts of gas in a wind that envelops the whole system (referred to as RS Ophiuchi, or RS Oph).

As a result, when the episodic thermonuclear explosions let go, they do so "inside" the companion's extended atmosphere.

"When this thing goes bang, it blows material out into the wind and sets off shocks that are at more than 100 million degrees, nearly 10 times the core temperature of our Sun," Professor Bode told the BBC News website.

The event remained visible in the sky to the unaided eye for about a week or so. Skywatchers would now need binoculars or a small telescope to see it.

However, the nova remains bright at wavelengths outside the visible spectrum - especially to the world's biggest telescopes.

Great opportunity

The space-borne Swift observatory was called in just three days after the outburst began. The Nasa-managed spacecraft carries UK-built components that detect X-rays.

Swift recorded how this high-energy light brightened, faded and then lit up again.

"About a month after the outburst, the X-ray brightness of RS Oph increased very dramatically," explained Dr Julian Osborne, a Swift scientist at the University of Leicester.

"This was presumably because the hot white dwarf, which is still burning nuclear fuel, then became visible through the red giant's wind.

"This new X-ray flux was extremely variable, and we were able to see pulsations which repeat every 35 seconds or so. Although it is very early days, and data are still being taken, one possibility for the variability is that this is due to instability in the nuclear burning rate on the white dwarf."

From high energies to lower-energy radio wavelengths, the astronomical community expects the observations to be a boon to its understanding of nova behaviour. All the data will be used to improve the current model for this type of event.

"This is our best chance yet of understanding what is truly going on," said radio astronomer Dr Stewart Eyres, of the University of Central Lancashire.

RS Oph's activity is expected to die down over the next few weeks and months - before it pulls enough gas off the red giant to explode again in 10-30 years' time.