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"Brightest Supernova" Reveals New Kind of Star Death

Victoria Jaggard in Washington, D.C.
National Geographic News
May 8, 2007
 
The brightest star explosion ever seen has been spotted about 240 million light-years away in the constellation Perseus, researchers announced yesterday.

The distant event, which so far has remained brighter than an ordinary supernova for more than 200 days, likely represents a new and extremely rare type of star death that occurs only in supermassive stars.

"It's no surprise that a very massive star will eventually collapse," David Pooley, an astrophysicist at the University of California, Berkeley, and co-author of a new study on the supernova, said during a press briefing.

What surprised scientists is that the brightness of the explosion couldn't be explained by the faint amount of x-rays emitted by the blast.

Normally when a large star dies, the explosion sends shockwaves through surrounding cooler gases, creating regions that emit large amounts of x-rays—the source of a supernova's light.

But the explosion of SN 2006gy, which is thought to be nearly 150 times as massive as the sun, showed few x-rays.

This suggests that the light is being produced from hot material being ejected into space.

"This would require a new type of [explosion] mechanism that has been produced theoretically but never observed," study leader Nathan Smith said at the briefing.

Cosmic Instability

The finding has ramifications for Eta Carinae, the most massive star in our galaxy, which lies just 7,400 light years away.

This star, estimated to be 100 to 120 times the sun's mass, has been experiencing preliminary eruptions that could mean it will explode in a manner similar to SN 2006gy.

If such a supernova occurred in our galaxy, "it would be so bright that you could see it [from Earth] during the day and you could even read a book by its light at night," Pooley said.

For their study, Smith, Pooley, and colleagues examined images of SN 2006gy taken from space by NASA's Chandra X-Ray Observatory and from the ground by the Lick Observatory in California and the Keck Observatory in Hawaii.

(See more images from the Chandra X-Ray Observatory.)

The results will appear in an upcoming issue of the Astrophysical Journal.

The team surmised that the explosion was the first recorded example of a type of supernova called a pair-instability mechanism, which was first proposed in scientific literature in the 1960s.

The phenomenon has likely never been seen until now because today such extreme stellar bodies are few and far between.

"Like people over seven feet [two meters] tall, stars that are over a hundred times the mass of our sun are very rare," Smith said.

In such heavy stars, radiation created by fusion in the core balances the pressure of the star's gravity.

When fusion stops, the star begins to collapse, crunching its core into a black hole or neutron star.

In the subsequent explosion, the star emits material equal to only about 5 percent of the sun's mass.

The pair-instability theory states that the gamma radiation that normally holds very massive stars together can start to produce pairs of particles and anti-particles when the core gets hot enough.

(Related news: "Massive Cosmic Explosion Has Astronomers Stumped" [December 20, 2006].)

"The minute the radiation starts putting energy into creating particles, the star loses support," Mario Livio, of the Space Telescope Science Institute, said at the briefing.

Such a scenario would allow the star to "jump the gun" and explode before its core collapses.

"Instead of the core of [SN 2006gy's star] collapsing, the core was completely obliterated and the explosion spewed matter out into space," UC Berkeley's Smith said. This material was equal to roughly 20 times the mass of the sun.

Scientists believe that the early universe was populated with similar supermassive stars that died by the pair-instability mechanism.

Instead of creating black holes that sucked up their material, these dying stars "polluted" the space around them, seeding the universe with the elements needed for life.

Neighborhood Radiation

Closer to home, the massive star Eta Carinae was observed in the 1840s ejecting lobes of material that resemble the bulbs of gases seen around SN 2006gy.

Does this mean the star's death could eject radiation that would be harmful to life on Earth?

"That's not very likely," Livio said.

If jets of gamma rays were aimed directly at the planet, that could affect us, he said. But the chances of that happening are very slim.

What's more, there's no guarantee that the star will explode in the same way. And even if it did, it could happen tomorrow or a hundred thousand years from now.

"We can sleep quietly tonight," Livio said.

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