Death star may pose threat to planet Earth

A highly unstable star at the end of its life could unleash a burst of gamma-ray radiation directed straight at Earth, any time between now and the next couple of hundred thousand years.

University of Sydney astronomer Peter Tuthill discovered an elegant rotating pinwheel system known as WR104 in the constellation Sagittarius eight years ago.

It contains a Wolf-Rayet star, the last stop in a star's life before it explodes in a massive supernova.

While studying WR104's striking and colorful shape, Dr Tuthill noticed that the perfect spiral he was observing could only occur when staring right down the centre of it.

"Viewed from Earth, the rotating tail appears to be laid out on the sky in an almost perfect spiral,'' Dr Tuthill said.

"It could only appear like that if we are looking nearly exactly down on the axis of the binary system.''

For most people, the word deathstar conjures images of the planet-destroying space station in Star Wars movies.

While Dr Tuthill's colorful spiral is far more beautiful than that rather ugly and menacing grey sphere, it is no less deadly.

Earth could be right in the firing line when the star eventually explodes.

"Sometimes, supernovae like the one that will one day destroy WR104 focus their energy into a narrow beam of very destructive gamma-ray radiation along the axis of the system,'' Dr Tuthill said.
"If such a gamma-ray burst happens, we really do not want Earth to be in the way.''

WR104 is about 8,000 light years away from Earth, which is right down the street in galactic terms and Dr Tuthill believes there is evidence that Earth may have been hit by one of these bursts before.

"Earlier research has suggested that a gamma-ray burst ... could be harmful to life on Earth out to these distances,'' he said.

"And scientists have speculated that, eons ago, a gamma-ray burst from a distant star could explain mass extinctions seen in the fossil record.''

A group of US scientists calculated that a 10 second burst of gamma-rays could deplete as much as 50 per cent of the Earth's protective ozone layer, allowing through potentially deadly radiation.

But don't go digging a shelter in your backyard and stocking up on sunscreen just yet, as Dr Tuthill says that the fully fledged gamma-ray burst is the worst case scenario.

"The two extremes are a very directed beam along the axis or a spherical explosion but the reality is that WR104 might lie somewhere in the middle of that, it has a preference for beaming things our way but maybe not as deadly as a fully fledged burst,'' Dr Tuthill said.

If the worst does happen, we can't expect much warning.

The first signs of the star's explosion would be the detection of the gamma-rays and it may have already happened.

"It could have gone off 8,000 years ago and we wouldn't know it, it could go off tomorrow, any time in the next couple of hundred thousand years is all we can say, but in astronomy terms that's imminent and we can't say any more than that,'' Dr Tuthill said.

"The question really boils down to whether or not WR104 could generate a gamma-ray burst when it goes supernova, and that's a question that I think the jury is still out on.

"But in any event we will have a ringside seat for a pretty impressive fireworks display.''

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