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The biggest bang of them all

May 8, 2007 By Lucy Odling-Smee This article courtesy of Nature News.

Bright supernova reveals secrets of star death.

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Astronomers have witnessed the brightest stellar explosion ever recorded — a supernova called SN 2006gy.

New observations suggest that a very massive star, some 150 times the mass of our Sun, must have exploded to form the supernova. Events triggering the demise of the Universe's massive stars, the data indicate, may differ from those known to destroy smaller, more commonly observed stars.

"The supernova was so powerful that it may require a new type of mechanism that's been predicted theoretically but never seen before," says Nathan Smith of the University of California, Berkeley, who led a team of astronomers from California and the University of Texas in Austin.

Although pictures of the supernova were obtained late last year — by ground-based optical telescopes and NASA's orbiting Chandra X-ray Observatory — interpretations of the data were released on Monday.

For where SN 2006gy is in the Universe — some 240 million light-years from Earth — "it's far and away the brightest supernova ever seen," says Smith. One possible explanation was that it was an exploding white dwarf — a star of a mass similar to the Sun that has exhausted its nuclear fuel.

But for an exploding white dwarf to produce that much light, the star would need to have interacted with other matter, such as a surrounding cloud of hydrogen. An explosion of that nature would have been about 1,000 times brighter in X-rays compared with what the Chandra telescope detected, says David Pooley of the University of California, Berkeley.

Instead, the team concluded that it must have been an incredibly rare, very massive star. "We didn't think we'd see anything like this in the local Universe," says Pooley.

Supernovas typically occur when stars exhaust their fuel and collapse under their own gravity — a process that results in stellar material being sucked up forever into a black hole. But in the case of SN 2006gy, the light emitted by the explosion was so intense that the team thinks a very different process may have triggered it.

All stars are sustained by a balance between the emission of radiation, which exerts an outward pressure, and the pull of gravity inwards. In stars as massive as the parent to SN 2006gy, the inside is predicted to have been hot enough to split photons apart. This equates to a loss in radiation energy, and so to a loss in the internal energy source that stabilizes the star against the effect of gravity. The result is that the whole thing explodes.

"In terms of the effect on the early Universe, there's a huge difference between these two possibilities," said Smith in a statement. "One pollutes the galaxy with large quantities of newly made elements, and the other locks them up forever in a black hole."

The earliest stars in the Universe were thought to be massive — maybe 150-200 times the mass of the Sun. That means that SN 2006gy may provide a unique insight into how the Universe's first generation of stars met their end, says Mario Livio of the Space Telescope Science Institute in Baltimore, Maryland, who was not involved in the research.

Before it exploded, the star that produced SN 2006gy expelled a large amount of mass. A similar kind of eruption can be seen spewing from Eta Carinae, a massive star that is only about 7,500 light years away in our own Milky Way.

So the discovery of SN 2006gy also raises the possibility that Eta Carinae could explode in a similar way at any time — perhaps tomorrow or in 1,000 years from now. Pooley predicts that if that happened, the light emitted from the event would be so bright that you could read a book by its light in the middle of the night.


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