A team of astronomers from the University of Sydney have been keeping an eye on a binary star system called Wolf-Rayet 104, located in the constellation Sagittarius, which is considered by astronomers to be a highly unstable ticking bomb in its last phase of life because it will eventually explode in a cataclysmic supernova explosion, called a gamma-ray burst (GRB), which might be directed straight at the Earth.

Images of the rotating Wolf-Rayet 104 binary star system.

"When it finally explodes as a supernova, it could emit an intense beam of gamma rays coming our way", says Dr Peter Tuthill, who lead the research. Wolf-Rayet stars are at least twenty times the mass of our own Sun, and represent an evolutionary phase in the lives of giant stars whereby they rapidly loose mass by the means of a strong stellar wind up to a billion times more powerful than the Sun's. Nobody knows how long it takes from this point for the star to explode, but Dr Tuthill suspects we still have a few hundreds of thousands of years to wait before Wolf-Rayet 104 goes supernova.

Images of the rotating system of Wolf-Rayet 104 were taken by the Keck Telescope in Hawaii and revealed that the rotating tail, a stream of material jetting out from the two stars at the centre of the system, appears to be laid out on the sky in an almost perfect spiral. "It could only appear like that if we are looking nearly exactly down the axis of the binary system", says Tuthill. Dr Tuthill's team worry that at just 8,000 light years away, the Earth's box-seat view might put us in the firing line when the system explodes, because the destructive gamma-ray radiation expelled from the supernova explosion is likely to be focussed into a narrow beam along the axis of the system. This could have serious implications for the Earth, ionising the atmosphere and potentially destroying the protective ozone layer, exposing life on Earth to an extreme dose of radiation. Only the hardiest of microbes would survive.

Dr Nial Tanvir, expert on Gamma Ray Astronomy at the University of Leicester, is more optimistic: "Although the power of a GRB is astonishing, and it could well have some noticeable effect on the Earth, I think it's unlikely it would produce a global catastrophe." He adds, "looking on the bright side, we know that gamma-ray bursts are very rare events, and in particular hardly ever seem to occur in galaxies like the Milky Way."

Dr Tuthill admits there are plenty of uncertainties, "the beam could pass harmlessly to the side [of the Earth] if we are not exactly on the axis, and nobody is even sure if stars like Wolf-Rayet 104 are capable of producing a fully-fledged gamma-ray burst in the first place." Orbiting satellites such as Swift, which was launched in November 2004, are designed to detect GRBs and the accompanying supernovae. "This gives us further clues as to the kinds of stars which give rise to GRBs, although we're still a long way from knowing just how the ultra high-velocity outflows are generated," added Tanvir.

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