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Light from a cosmic “dark age”

Oct. 28, 2009
Courtesy Nature
and World Science staff

As­tro­no­mers are re­port­ing the dis­cov­ery of the most dis­tant ob­ject ev­er dis­cov­ered, from a time when the first stars were form­ing.

Two re­search groups described a gamma-ray burst from a star that died when the un­iverse was 640 mil­lion years old, or less than 5 per­cent of its pre­s­ent age, in this week’s issue of the sci­ence jour­nal Na­ture.

A sche­mat­ic of how a gamma-ray burst orig­i­nates. Stars shine by burn­ing hy­dro­gen, in a pro­cess called nu­clear fu­sion. Hy­dro­gen burn­ing pro­duces he­li­um "ash." As the star runs out of hy­dro­gen (and nears the end of its life), it be­gins burn­ing he­li­um. The ash­es of he­li­um burn­ing, such as car­bon and ox­y­gen, al­so get burned. The end re­sult of this fu­sion is iron. Iron can­not be used for nu­clear fu­el. With­out fu­el, the star no long­er has the en­er­gy to sup­port its weight. The co­re col­lapses. If the star is mas­sive enough, the co­re will col­lapse in­to a black hole. The black hole quick­ly forms jets; and shock waves re­ver­ber­at­ing through the star ul­ti­mate­ly b­low apart the out­er shells. Gamma-ray bursts are the bea­cons of star death and black hole birth. (Cred­it: Ni­colle Ra­ger Ful­ler/NSF)


“This ob­serva­t­ion al­lows us to beg­in ex­plor­ing the last blank space on our map of the Un­iverse,” said Nial Tan­vir of the Un­ivers­ity of Leices­ter, who led one of the teams.

Dubbed GRB 090423, the rec­ord-break­er is an ex­am­ple of the bright­est and most vi­o­lent ex­plo­sions known. The blast is thought to ac­com­pa­ny the cat­a­stroph­ic death of a huge star, and is trig­gered by the cen­ter of the star col­laps­ing to form a black hole.

Al­though the burst it­self oc­curred about 630 mil­lion years af­ter the Big Bang be­lieved to have giv­en birth to our un­iverse, it is so far away that the light from the ex­plo­sion only ar­rived at the Earth last April. 

“It is tre­men­dously ex­cit­ing to be look­ing back in time to an era when the first stars were just switch­ing on,” said team mem­ber An­drew Levan of the Un­ivers­ity of War­wick, U.K.

Much of the light from the burst was in the form of very high en­er­gy gamma-ray radia­t­ion, which trig­gered the de­tec­tors on a NASA sat­el­lite called Swift. 

Fol­low­ing an au­to­mat­ic an­nounce­ment from Swift sev­e­ral of the world’s larg­est tele­scopes turned to the re­gion of the sky with­in the next min­utes and hours and lo­cat­ed the af­terglow of the burst. Anal­y­sis re­vealed that the af­terglow was seen only in in­fra­red light and not in the nor­mal op­ti­cal. This was the clue that the burst came from very great dis­tance, as­tro­no­mers said.

Be­yond the mere break­ing of a rec­ord, the age of the newly de­tected ob­ject opens a win­dow in­to a cos­mo­lo­g­i­cal era that has not pre­vi­ously been ac­ces­si­ble to ob­serva­t­ion. The cos­mic “Dark Ages” are thought to have ended about 800-900 mil­lion years af­ter the Big Bang. That’s when light from stars and ga­lax­ies elec­tric­ally charged, or “re-ionized,” gas per­vad­ing the Un­iverse. This pro­cess al­so made the gas, and thus the un­iverse it­self, more trans­par­ent, lead­ing it to have the char­ac­ter­is­tics we see to­day.

As more gamma-ray bursts are de­tected from these early times, it should be pos­si­ble to trace the prog­ress of this re-ion­iz­a­tion, as­tro­no­mers say.

Gamma-ray bursts are the Un­iverse’s most lu­mi­nous ex­plo­sions. Most oc­cur when mas­sive stars run out of nu­clear fu­el. As their cores col­lapse in­to a black hole or neu­tron star, gas jet­s—driven by pro­cesses not fully un­der­stood—punch through the star and blast in­to space. There, they strike gas pre­vi­ously shed by the star and heat it, which gene­rates short-lived af­terglows.

The “redshift” of GRB 090423, a gauge of its dis­tance, is estimated at 8.2. Red­shift indicates how much the light from the object has been “stretched” be­cause of the ex­pan­sion of the uni­verse, which is most ap­par­ent at great dis­tances. The pre­vi­ous rec­ord hold­er for a dist­ant gam­ma-ray burst had a red­shift of 6.7, which placed it 180 mil­lion light-years clos­er than the newfound one. A light-year is the dis­tance light trav­els in a year.

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Astronomers are reporting the discovery of the most distant object ever discovered, from a time when the first stars were forming and previously out of reach to telescopes. Two research groups reported their observations of a gamma-ray burst from a star that died when the Universe was 640 million years old, or less than 5 percent of its present age. The findings are published in this week’s edition of the science journal Nature. “This observation allows us to begin exploring the last blank space on our map of the Universe”, said Nial Tanvir of the University of Leicester, who led one of the teams. Dubbed GRB 090423, the record-breaker is an example of the brightest and most violent explosions known to exist. The explosion is thought to accompany the catastrophic death of a huge star, and is triggered by the center of the star collapsing to form a black hole. Although the burst itself occurred about 630 million years after the Big Bang believed to have given birth to our universe, it is so far away that the light from the explosion only arrived at the Earth in April of this year. “It is tremendously exciting to be looking back in time to an era when the first stars were just switching on,” said team member Andrew Levan of the University of Warwick, U.K. Much of this light was in the form of very high energy gamma-ray radiation, which triggered the detectors on a NASA satellite called Swift. Following up on the automatic announcement from Swift several of the world’s largest telescopes turned to the region of the sky within the next minutes and hours and located the faint, fading afterglow of the GRB. Analysis revealed that the afterglow was seen only in infrared light and not in the normal optical. This was the clue that the burst came from very great distance, astronomers said. Beyond the mere breaking of a record, the age of the newly detected object opens a window into a cosmological era that has not previously been accessible to observation. The cosmic “Dark Ages” are thought to have ended about 800-900 million years after the Big Bang. That’s when light from stars and galaxies electrically charged, or “re-ionized,” gas pervading the Universe. This process also made the gas, and thus the universe itself, more transparent, leading it to have the characteristics we see today. As more gamma-ray bursts are detected from these early times, it should be possible to trace the progress of this re-ionization, astronomers say. Gamma-ray bursts are the Universe’s most luminous explosions. Most occur when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets—driven by processes not fully understood—punch through the star and blast into space. There, they strike gas previously shed by the star and heat it, which generates short-lived afterglows in other wavelengths. The previous record holder was a burst with a redshift of 6.7, which places it 180 million light-years closer than GRB 090423.