June 29, 2008

Our Own Death Star

By Luntz, Stephen

A beautiful star system may eventually be a threat to Earth, although its discoverer stresses that the risk is small. Wolf-Rayet stars are considered the time bombs of the galaxy, likely to explode as supernovae in astronomically short timescales. They are enormous - generally tens of solar masses - and shedding material 100 billion times as fast as the Sun.

Being close to a supernova is a health hazard for a living planet, but from a few hundred light-years away all one can normally expect is a spectacular view and enough data to keep astronomers happy for years.

WR104 is a particularly interesting Wolf-Rayet star because it has a companion in relatively close orbit that is also large enough to eventually become a supernova. The pair's motion and stellar winds create a tail as material is blown away. When Dr Peter Tuthill of the University of Sydney discovered WR104 8 years ago its location 8000 light-years away in Sagittarius suggested it was safe, but more recent and detailed images taken by the Keck Telescope suggest otherwise.

"Viewed from Earth," Tuthill says, "the rotating tail of the binary 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 on the axis of the system." The very circular nature of the stars' orbits strongly suggests that the axis of spin of WR104 is the same as the pair's orbital spin, suggesting that we're virtually over the stars' poles.

The problem with this is that a minority of supernovae are accompanied by very powerful gamma-ray bursts (AS, April 2005, p.4) emerging from the pole. "Earlier research has suggested that a gamma- ray burst - if we are unfortunate enough to be caught in the beam - could be harmful to life on Earth out to these distances," Tuthill says. "Scientists have speculated that, eons ago, a gamma-ray burst from a distant star could explain mass extinctions seen in the fossil record."

The narrowness of these beams means you have to be very unlucky to be caught in one, particularly at distances close enough to be harmful, but Tuthill believes our angle to WR104's pole is between 0[degrees] and 16[degrees], making the danger much higher than usual.

Tuthill doubts we can count on prior warning when WR104 does become a supernova. "It has already used up its hydrogen and is burning helium," he says. "Prior to exploding it will burn exotics, but that is only for a few days or hours so I'm not sure there will be any precursors."

Nevertheless Tuthill says that we're far from the eve of destruction. "There are still plenty of uncertainties," he says. "The beam could pass harmlessly to the side if we are not exactly on the axis, and nobody is even sure if stars like WR104 are capable of producing a fully-fledged gamma-ray burst in the first place."

While it is inevitable that both WR104 and its companion will eventually become supernovae, gamma ray bursts are thought to require very rapid stellar spin. Some astronomers question whether the conditions for a gamma ray burst can exist in a galaxy as evolved and metal-rich as ours. Most reassuringly Tuthill adds that "we probably have hundreds of thousands of years before it blows, so we have plenty of time to come up with some answers".

The first step to doing so is to more accurately measure our angle to WR104's pole. Tuthill says this can be done using spectroscopy to determine whether the stars are moving towards and away from us as they orbit. He believes the required data has been collected but not analysed. Other research on just how likely gamma ray bursts are in our galaxy, and the width of their beams, will probably prove more difficult since they depend on a number of factors with substantial uncertainties.

The pinwheel shape of WR104 indicates we are looking at it from above its poles. The dust spirals are 200 times as long as the distance from the Earth to the Sun.

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