Supernova’s death throes revealed

In archival images, astronomers see activity in weeks before giant star’s explosion

Just before a giant star blew up in a spectacular supernova explosion, it gave hints to its imminent demise. The pre-explosion activity of this star, detailed online February 6 in Nature, could enable astronomers to predict a star’s coming supernova and then watch it in real time.

“It’s a very fascinating study,” says Jon Mauerhan, an astronomer at the University of Arizona who was not involved in the research. Astronomers have only rarely witnessed the activity of a massive star before its explosion, he adds.

The star came to astronomers’ attention in August 2010 thanks to a computer program. The program scans sky survey images from a 48-inch telescope at the Palomar Observatory in Southern California and flags regions that show sudden brightening, which astronomers take as potential signs of supernovas. The researchers followed up on one such brightening 500 million light-years away and confirmed that it was a type II supernova, an explosion of a massive star whose core runs out of fuel and collapses. Then they looked at images of the same star from the prior weeks and months to see whether the star showed signs that it was about to blow.

Sure enough, they spotted a slight brightening of the fated star 37 days before its death. The scientists calculated from the brightening that the star sent a shell of gas equivalent to about one one-hundredth the mass of the sun hurtling into space at some 2,000 kilometers per second.

“The star was making a little burp,” says Alex Filippenko, an astronomer at the University of California, Berkeley who was on the team that made the discovery. When the star finally exploded, it ejected material that spread so quickly that it took just three weeks to overtake the previous outburst.

That stars shed mass before exploding is not surprising, Filippenko says, but this is the first time astronomers have spotted an ejection so shortly before a star’s demise. The tight time frame suggested to the team that these cosmic burps are more than 100 times more likely to occur just before a supernova than at any other point in a star’s lifetime.

According to Filippenko, astronomers could potentially use this correlation to hunt for other burps and achieve something long desired but never accomplished: Predict an impending supernova and observe it as it occurs. “This type of ejection could be a cosmic lamppost for a final explosion that’s about to happen,” he says.

 Further work will help scientists distinguish between these ejections that predate supernovas and more-routine eruptions. Other massive stars have burped material into space and lived on. The most famous example is Eta Carinae, a star 7,500 light-years from Earth which, for a brief time in the mid-1800s, unleashed into space a shell of gas 10 times the mass of the sun and became the second-brightest star in the night sky. Since then it has quieted down, and astronomers have no idea whether it will explode in 100 days or 100,000 years.

Astronomers certainly have the tools to explore these questions. Automated sky surveys like the Palomar project have popped up worldwide, and by the end of the decade the 8.4-meter Large Synoptic Survey Telescope in Chile will scan the entire sky every few nights with its 3,200-megapixel camera. The Chilean telescope, Mauerhan says, “will find tons of these things. It’s an exciting time for supernova science.”

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