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Light from a cosmic “dark age”

As tro no mers are re port ing the dis cov ery of the most dis tant ob ject ev er dis cov ered, from a time when the first stars were form ing.

Two re search groups described a gamma-ray burst from a star that died when the un iverse was 640 mil lion years old, or less than 5 per cent of its pre s ent age, in this week’s issue of the sci ence jour nal Na ture. “This ob serva t ion al lows us to beg in ex plor ing the last blank space on our map of the Un iverse,” said Nial Tan vir of the Un ivers ity of Leices ter, who led one of the teams.

Dubbed GRB 090423, the rec ord-break er is an ex am ple of the bright est and most vi o lent ex plo sions known. The blast is thought to ac com pa ny the cat a stroph ic death of a huge star, and is trig gered by the cen ter of the star col laps ing to form a black hole.

Al though the burst it self oc curred about 630 mil lion years af ter the Big Bang be lieved to have giv en birth to our un­iverse, it is so far away that the light from the ex plo sion only ar rived at the Earth last April.

“It is tre men dously ex cit ing to be look ing back in time to an era when the first stars were just switch ing on,” said team mem ber An drew Levan of the Un ivers ity of War wick, U.K.

Much of the light from the burst was in the form of very high en er gy gamma-ray radia t ion, which trig gered the de tec tors on a NASA sat el lite called Swift.

Fol low ing an au to mat ic an nounce ment from Swift sev e ral of the world’s larg est tele scopes turned to the re gion of the sky with in the next min utes and hours and lo cat ed the af­terglow of the burst. Anal y sis re vealed that the af terglow was seen only in in fra red light and not in the nor­mal op ti cal. This was the clue that the burst came from very great dis tance, as tro no mers said.

Be yond the mere break ing of a rec ord, the age of the newly de tected ob ject opens a win dow in to a cos mo lo­g i cal era that has not pre vi ously been ac ces si ble to ob serva t ion. The cos mic “Dark Ages” are thought to have ended about 800-900 mil lion years af ter the Big Bang. That’s when light from stars and ga lax ies elec tric ally charged, or “re-ionized,” gas per vad ing the Un iverse. This pro cess al so made the gas, and thus the un iverse it self, more trans par ent, lead ing it to have the char ac ter is tics we see to day.

As more gamma-ray bursts are de tected from these early times, it should be pos si ble to trace the prog ress of this re-ion iz a tion, as tro no mers say.

Gamma-ray bursts are the Un iverse’s most lu mi nous ex plo sions. Most oc cur when mas sive stars run out of nu­clear fu el. As their cores col lapse in to a black hole or neu tron star, gas jet s—driven by pro cesses not fully un­der stood—punch through the star and blast in to space. There, they strike gas pre vi ously shed by the star and heat it, which gene rates short-lived af terglows.

The “redshift” of GRB 090423, a gauge of its dis tance, is estimated at 8.2. Red shift indicates how much the light from the object has been “stretched” be cause of the ex pan sion of the uni verse, which is most ap par ent at great dis tances. The pre vi ous rec ord hold er for a dist ant gam ma-ray burst had a red shift of 6.7, which placed it 180 mil lion light-years clos er than the newfound one. A light-year is the dis tance light trav els in a year.

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