New Type of Supernova Discovered

November 6, 2009

An odd star explosion 160 million light-years away might be the first proof of a theoretical new class of supernova, astronomers suggest.

Dubbed SN 2002bj, the blast was witnessed in 2002 and originally classified as a common Type II supernova. These explosions are thought to occur when the dense iron core of a massive star collapses.

But earlier this year Dovi Poznanski, an astronomer at the University of California, Berkeley, took a closer look at the light signatures recorded during and after the explosion.

He found that the blast didn't send out the right kind of chemical elements to have been a Type II event.

Instead the explosion looked more like a Type Ia supernova, a known type of blast that involves a pair of stellar corpses called white dwarfs.

But unlike Type Ia explosions, SN 2002bj blazed as bright as ten billion suns but faded away into invisibility within 20 days.

"Other supernova types typically last about three times longer," taking three to four months to totally fade, Poznanski said.

He and his colleagues therefore think the outburst might be the first known "Type .Ia" supernova—so named because the blasts are only a fraction as bright as Type Ia supernovae and last for just a fraction as long.

Exploding Shell

Lars Bildsten, a physicist at the University of California, Santa Barbara, predicted the existence of Type .Ia supernovae two years ago.

According to Bildsten, a Type .Ia blast requires two white dwarfs of unequal masses circling each other.

The more massive star is made of carbon and oxygen, while its companion consists mostly of helium.

Gravity from the larger white dwarf slowly pulls helium from the surface of its neighbor, and the siphoned gas builds up into a shell around the star.

After tens of millions of years, that shell reaches a critical mass threshold, at which point it detonates in a luminous but short-lived explosion.

Since only the helium shell explodes, both white dwarfs remain intact after the blast, and the cycle could potentially repeat.

SN 2002bj fits this description almost perfectly, based on the new analysis of its chemical signatures and duration.

The find adds to the likelihood that other supernova mechanisms are awaiting discovery, study author Poznanski said.

(Related: "'Brightest Supernova' Reveals New Kind of Star Death.")

"The universe is big," he said. "If something is physically possible, it probably exists somewhere."

Findings appear this week in the journal Science.

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