Published online 6 March 2008 | Nature | doi:10.1038/news.2008.653

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Star-system explosion could threaten life on Earth.

The clear dust spiral shows that a beam from this star would be aimed at Earth. Courtesy of the researchers

Astronomers have spotted a binary star system that could collapse to produce a massive gamma-ray burst at any point during the next few hundred thousand years — and it is pointing at Earth.

The binary star system WR 104, some 8,000 light-years from Earth in the Sagittarius constellation, is made up of two stars that complete an orbit of one another every 8 months. Both stars are massive and have strong solar winds that spew out material, resulting in a spiralling trail of hot gas and dust.

Peter Tuthill at the University of Sydney in Australia and his colleagues watched WR 104 for 6 years, during which time they saw 10 full orbits and captured them on camera using the Keck telescopes on Mauna Kea, Hawaii. The research is published in the Astrophysical Journal1.

The resulting pictures show the spiralling dust in all its glory, and reveal a potentially devastating fact: Earth looks down the axis of the system. “It is pointing at us within a range of about 0–16º,” says Tuthill, who estimates it is most likely 12º off-centre. “It would take on a different appearance, looking foreshortened, if we were looking at it from a wider angle.”

The massive stars are certain to explode, and soon, astronomically speaking. The question is whether this will happen in a supernova that bursts in all directions, spreading out the damage, or whether the stars are spinning fast enough for this explosion to be directed in a gamma-ray burst. If so, the angle of the system means that we are in the firing line for one half of the burst jet.

It is unclear how direct the burst would have to be to have an effect, says Tuthill. It has been variously postulated that a burst angle of 2–20º might put us outside the danger zone, but Tuthill says that even a miss of 12º would be dangerous for life on Earth.

Caught in the beam

A mass-extinction event on Earth some 450 million years ago might have been triggered by a gamma-ray burst. Adrian Melott at the University of Kansas in Lawrence, who suggested this in 2003, says that the new observations of WR 104 are big news because this is the first candidate system spotted that could produce a similar Earth-walloping gamma-ray burst in the future. “If it were a full gamma-ray burst and we were caught in the beam, the effects would be pretty severe,” says Melott. “My guess is that there would be a lot of death from it, rather like a small-scale nuclear war.”

A gamma-ray beam might not kill us all immediately. First there would be a bright flash, possibly blinding people, says Melott, then after a few hours the effects would begin in earnest.

The gamma rays would break up molecules in the atmosphere, producing particular oxides of nitrogen that would start to eat up the ozone layer after a few hours, says Melott. Within a few days a quarter of the ozone layer would be destroyed, he suggests.

The ozone destruction would allow through enough ultraviolet light to cause severe radiation damage to plants and people. The nitrogen oxides would also cause acid rain that could kill off plants and algae.

In a spin

But it is impossible to predict whether WR 104 will produce a gamma-ray burst, or when it might happen. For it to occur, the star has to be spinning fast enough to make an axis for the gamma-ray beam to emerge. “Here is where things get a bit more complicated and murky,” says Tuthill, who says he doesn’t know whether WR104 is spinning fast enough.

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The fact that this is a binary system could mean that the spin is high, says gamma-ray expert Craig Wheeler at the University of Texas at Austin. “In a binary system the two stars yank each other around and spin each other up,” he says. But Wheeler says he isn't yet scared of what is rapidly becoming known as the Death Star. “I’m not going to worry about it at all.”

There are, certainly, more concrete dangers to worry about, and many questions to answer before we all duck for cover. “WR 104 is now in the last-known stable phase of a massive star. It should explode as a supernova within a few hundred thousand years. Of course, this could mean tomorrow, we have no way of telling,” says Tuthill.

Watch a video of the spinning star system.