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Megastar explodes in brightest supernova ever seen

Blast rips apart star some 150 times bigger than our sun in a relatively nearby galaxy. Astronomers predict similar event in our own galaxy. Images: X-rays document death of a star

A gargantuan explosion ripped apart a star perhaps 150 times more massive than our sun in a relatively nearby galaxy in the most powerful and brightest supernova ever observed, astronomers said on Monday.

And there is one such star in our own Milky Way galaxy that appears to be on the brink of dying in just such a supernova.

The exploding star's dramatic death may have come in a rare type of supernova reserved for "freakishly massive" stars that astronomers had speculated about but never previously witnessed.

The supernova, designated as SN 2006gy, occurred 240 million light years away in a galaxy called NGC 1260, and was studied using observations from NASA's orbiting as well as .

The explosion occurred long ago but was detected last year after its light traveled many, many trillions of miles before it could be observed from Earth.

"That sounds far away but it's actually quite nearby on the vast scale of the universe," astronomer Nathan Smith of the University of California at Berkeley, who led the research, told a news conference.

The supernova was discovered in September 2006, and stands as far and away the most powerful and brightest ever observed, Smith said.

"In fact, even after the better part of a year, well after 200 days, it has faded somewhat but it's still about as bright as a normal supernova at its peak," Smith said.

A supernova marks a star's death in a spectacular explosion. Scientists say these events play a crucial role in creating heavy elements through nuclear fusion and synthesis and then expelling them into space, seeding the cosmos with metals.

The scientists ruled out a possible alternative explanation that what they were witnessing was the explosion of a white dwarf star with a mass only a bit more than the sun.

Obliterated core
Astrophysicist Mario Livio said the supernova may have resulted from a type of explosion mechanism that had existed only in theoretical calculations. He said the first generation of stars in the universe may have died in such a manner.

In a normal supernova, the core of a star collapses when it exhausts its fuel, and forms either a neutron star or a black hole, with scant heavy elements blown into space.

But this supernova appears to be the result of the core not collapsing but being obliterated in an explosion blasting all its material into space, the scientists said.

Dave Pooley of the University of California, Berkeley said this star appears similar to Eta Carinae, a star perhaps 100 to 120 times the mass of the sun located 7,500 light years away within the Milky Way. There has not been a supernova in our galaxy in more than 400 years, Pooley said.

A light year is about 6 trillion miles, the distance light travels in a year.

If Eta Carinae were to burst into a supernova, Pooley said, "It would be so bright that you would see it during the day, and you could even read a book by its light at night."

Livio said Eta Carinae had an incredible eruption during the 19th century that left it in an hourglass shape. He said it could explode at any time.

"This could happen tomorrow, it could happen 1,000 years from now," Livio said. "Is there a risk to life on Earth as a result of this explosion? Well, not very likely."

Livio said Earth could be affected if there were a gamma ray burst that potentially could harm the atmosphere and life, but the chances of this aiming directly at Earth are slim.

Story Copyright © 2007 Reuters Limited. All rights reserved.