Harvard-Smithsonian Center for Astrophysics

Edge of time. This 23 April gamma ray burst (center) is nearly as old as the universe.

A Blast From the Deep, Dark Past

Staff Writer

Astronomers have imaged the most distant object ever seen--a star collapsing into a black hole more than 13 billion light-years away. The resulting burst of gamma rays, detected by NASA's SWIFT satellite on 23 April and reported tomorrow in two Nature papers, shows that stars were already forming and dying a mere 600 million years after the big bang.

Every week, astronomers sight two or three gamma ray bursts (GRBs), short flashes of high-energy radiation from collapsing stars. Most turn out to be from the nearby universe. But GRB 090423--named, by convention, after the date it was detected--shatters the previous record for farthest object, a galaxy 12.8 billion light-years away that was discovered in 2006. "It brings us close to that magical point of first light," says Volker Bromm, an astrophysicist at the University of Texas, Austin, who was not involved in the study. "We don't have to get much farther to catch the earliest stars."

Researchers are interested in spotting and studying such far-out objects because they provide a window on the early universe. Theoretical models predict that blobs of gas began collapsing into massive stars within a few hundred million years of the big bang. These stars burned for a while before exploding as supernova. The limited information obtained from GRB 090423's spectra indicates that despite its proximity to the big bang, it does not belong to the first generation of stars, the two teams report.

One team was led by Nial Tanvir of the University of Leicester in the United Kingdom, the other by Ruben Salvaterra of the Italian Institute of Astrophysics in Merate. Soon after the burst was detected by SWIFT, both groups scrambled to observe it before the signal faded away using ground-based telescopes. The story of that race appeared in the 18 September issue of Science.

Tanvir and his colleagues estimate that the density of stars in the early universe was great enough for astronomers to expect the detection of even more distant GRBs in the future. "Finding such events is not an unreasonable hope," they write, noting that they took the first spectra from GRB 090423 15 hours after the burst. Taking spectra more quickly would have yielded more information about its composition, and that's exactly what researchers aim to do the next time they spot an ancient flare.

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