Star outweighed any known in Milky Way

Supernova was a big blast, challenging theories of how stars live and die

3:25pm, December 2, 2009
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To make a truly spectacular stellar explosion, you gotta have heft. And according to a motherlode of data on a supernova dubbed SN 2007bi, the exploding star originally tipped the scales at more than 200 times the mass of the sun. That would make the star more massive than any known in the Milky Way.

If the researchers are correct in their mass determination, the supernova belongs to a type never before observed and predicted to be common only in the early universe. The presence of such an exploding star in the modern-day universe — the explosion was observed in 2007 as it appeared in a nearby dwarf galaxy a relatively recent 1.6 billion years ago — suggests that astronomers may have to revise their models of how extremely massive stars live and die, several researchers say. Avishay Gal-Yam of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues report their observations in the Dec. 3 Nature.

Several lines of evidence suggest that SN 2007bi was the explosion of a behemoth. The brightness of any exploding, massive star is governed by the glow generated by the radioactive decay of nickel-56, which is forged during the explosion. The supernova took about 70 days to reach its peak brightness, an indication that the glow from radioactive material had to punch through a huge amount of ejected material before it could be seen. In addition, the extraordinary brightness and duration of SN 2007bi, which lingered for about 1.5 years, indicates that the explosion generated a large amount of radioactive nickel — about three solar masses. The exploding star also had to be massive enough to gravitationally capture all that ejected nickel. Otherwise the supernova would not have been so bright.

Taken together, the data indicate that the star weighed at least as much as 200 suns, Gal-Yam says.

Theory predicts that any star heavier than the equivalent of 140 suns blows up in a very special way. Photons produced at the core of such a star provide an outward pressure that resists gravity’s inward pull. But when the core temperature exceeds about a billion kelvins, the photons suddenly become energetic enough to annihilate each other and produce pairs of electrons and positrons. Once the photons vanish, there’s nothing left to resist gravity and the core implodes, sparking a mammoth eruption.

Such an explosion, known as a pair-instability supernova, was first predicted in the late 1960s but until recently was believed to be limited to the early universe. That’s the era, it’s been thought, when extremely massive stars were not only easiest to form but were also most likely to retain their heft during their lifetimes.

“This is probably the best case yet for an observed example of a pair-instability supernova explosion,” comments Nathan Smith of the University of California, Berkeley. However, because of uncertainties in models of how stars lose mass, the estimated mass of the star is highly uncertain, he adds.

Smith notes that astronomers had assumed that stars in the universe today shed most of their mass differently and faster than in the early universe. But that thinking has recently changed, Smith notes. Astronomers have found evidence that massive stars in the modern universe shed less mass than thought.

“This changes the expectations that we have for how massive stars evolve and for how early stars differ from those we see today,” especially in dwarf galaxies, which have a low abundance of metals, Smith says. Gal-Yam adds that pair-instability supernovas may be beacons spotlighting nearby dwarf galaxies that could serve as laboratories for studying the early universe.

Stan Woosley of the University of California, Santa Cruz, who along with Alexander Heger of the University of Minnesota, Twin Cities provided the first detailed model of pair-instability supernovas, says that the new findings have a 50 percent chance of being correct. But if confirmed, the explosive death of very massive stars has important implications for how much material supernovas dump into space, and also for black hole formation and gamma-ray bursts, he says.

The core of the star that exploded as SN 2007bi had a mass equal to about 100 suns. According to Woosley, if a massive star’s core exceeds 133 solar masses a new phenomenon occurs. The collapsing star makes a big black hole, he says. With sufficient rotation, such an exploding star might produce a gamma-ray burst even more luminous than an ordinary burst, already considered to be the brightest explosions in the universe.

"That would be cool to see," says Gal-Yam.

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