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Astronomical Artifact: Most Distant Object Yet Detected Carries Clues from Early Universe

A stellar explosion spotted in April took place 13 billion years ago



Gemini Observatory/NSF/AURA, D. Fox and A. Cucchiara (Penn State University) and E. Berger (Harvard University)

A violent explosion picked up by a NASA satellite earlier this year is the oldest object ever seen by astronomers, its light having been emitted some 13 billion years ago. At that time the universe was roughly 5 percent of its present age and the big bang was a fairly recent occurrence, having taken place just 600 million years earlier.

NASA's Swift Gamma-Ray Burst spacecraft spotted the flash signaling a massive stellar explosion on April 23. The explosion was officially designated GRB 090423, after its type (a gamma-ray burst) and date of detection; the space agency quickly announced it as the new record holder for cosmic distance. Now, two papers in the October 29 Nature present detailed analyses of the burst and afterglow, confirming the initial distance assessments and providing a few clues as to conditions in the early universe.

"We don't know much" about the event that produced GRB 090423, says Bing Zhang, an astrophysicist at the University of Nevada, Las Vegas, who wrote a commentary for Nature accompanying the new studies. (Scientific American is part of the Nature Publishing Group.) "What we know is that at that early time we already had a star and that star exploded, and that star is not that different from the nearby stars that produce gamma-ray bursts."

Specifically, the progenitor star appears to belong to the second or third generation of stars, rather than the first generation. The earliest stars are thought to have been massive, short-lived balls of hydrogen and helium, whereas their offspring incorporated heavier elements formed in the first generation's explosive demise. The burst and afterglow of GRB 090423 is not unlike that of closer (and hence more recent) gamma-ray bursts, pointing to a later-generation progenitor. "We know that this star is not first-generation, because its observational properties are quite similar to its nearby cousins," Zhang says. The first stars, long a target for researchers, have eluded detection thus far.

Still, the burst puts scientists closer to the first generation than ever before: It is about 150 million years older than any other known astrophysical object, says astronomer Nial Tanvir of the University of Leicester in England, the lead author of one of the new Nature papers. "On the one hand, that doesn't sound all that significant, given that they're all sort of 13 billion years or so of age," Tanvir says. "But in terms of pushing back closer to the very first stars that formed in the universe, we think this is quite a significant leap."

Swift's X-Ray Telescope trained itself on GRB 090423 just 73 seconds after the satellite's Burst Alert instrument picked up the signal of the event, and Tanvir's team had the U.K. Infra-Red Telescope on Mauna Kea in Hawaii looking at the afterglow just 20 minutes after the burst. But the weather in Hawaii was not ideal, and this particular gamma-ray burst was not especially bright, limiting the amount of additional information astronomers could glean from the explosion.

In principle, Tanvir says, a bright, well-observed GRB at great distances could expose the makeup of the intergalactic medium as well as the chemistry of the star's host galaxy, which would in turn indicate the products of previous generations of stars.

As for the galaxy to which GRB 090423 belongs, Tanvir says that his group will be using the Hubble Space Telescope next year to seek it out, now that its location has been marked. "We hope to locate the host galaxy," he says. "We have very little idea of what galaxies were like at that time. We have only very sketchy ideas."

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