First stars may have been supergiants, researchers say
Jan 05,2007 00:00 by

New tel­e­scope ob­ser­va­tions have bol­stered a claim that as­tro­no­mers have seen the uni­verse’s first lu­mi­nous ob­ject­s—pos­sib­ly gar­gan­tu­an stars, re­search­ers say.

If the find­ings prove cor­rect, sci­en­tists add, they might fit with a the­o­ry that such stars seeded the growth of the big­gest, so-called su­per­mas­sive, black holes. Black holes are ob­jects so heavy and com­pact that their grav­i­ty sucks in eve­ry­thing near­by, in­clud­ing light.

The bot­tom pan­el is an im­age from NASA's Spitzer Space Tel­e­scope, of stars and galax­ies in the Ur­sa Ma­jor con­stel­la­tion. This in­fra­red im­age co­vers a re­gion of space so large that light would take up to 100 mil­lion years to trav­el across it. The top pan­el is the same im­age af­ter stars, galax­ies and oth­er sources were masked out. The re­main­ing back­ground light, ac­cord­ing to some as­tro­no­mers, is from a time when the uni­verse was less than a bil­lion years old, and prob­a­bly orig­i­nat­ed from the uni­verse's first groups of ob­jects. Darker shades in the top im­age cor­re­spond to dim­mer parts of the glow; yel­low and white show the bright­est.
But some re­search­ers said they’re not con­vinced the find­ings are cor­rect.

Ac­cord­ing to those who reached them, their new ob­ser­va­tions, from NASA’s Spitzer Space Tel­e­scope, strongly sug­gest clumps of the pri­mord­ial ob­jects—pos­sib­ly stars or black holes—are res­pon­si­ble for in­fra­red light seen in an ear­li­er stu­dy. 

In­fra­red is a form of light too low in en­er­gy to be di­rect­ly vis­i­ble, but de­tect­a­ble with suit­a­ble in­stru­ments.

The new da­ta show this patchy light is splat­tered sky-wide and comes from clus­ters of bright, mon­s­trous ob­jects more than 13 bil­lion light-years away, the as­t­ro­no­mers said. A light-year is the dis­tance light trav­els in a year. 

This would mean the light from those bo­dies has been tra­v­el­ing 13 bil­lion years, im­ply­ing in turn that we see them as they were that many years ago.

“We are push­ing our tel­e­scopes to the lim­it and are tan­ta­liz­ing­ly close to get­ting a clear pic­ture of the ve­ry first col­lec­tions of ob­jects,” said Al­ex­an­der Ka­sh­lin­s­ky of NA­SA’s God­dard Space Flight Cen­ter in Green­belt, Md. 

“What­ever these ob­jects are, they are in­t­rin­si­cal­ly in­c­red­i­bly bright and very dif­fer­ent from an­y­thing in ex­is­t­ence to­day,” added Ka­sh­lin­s­ky, the lead au­thor of two re­ports on the work to ap­pear in As­t­ro­phys­i­cal Jour­nal Let­ters, a re­search pub­li­ca­tion.

The ob­jects, he argued, are ei­ther the first stars—ti­tanic ones weigh­ing more than 1,000 times our sun—or black holes vo­ra­cious­ly con­sum­ing gas, a pro­cess that would al­so pro­duce in­tense light in their area.

If they’re stars, the clus­ters might be the first mini-galax­ies, weigh­ing less than about one mil­lion suns, he added; merg­ers of such galax­ies prob­a­bly made big­ger ones like our Milky Way, which holds the equi­v­a­lent of some 100 bil­lion suns.

The ear­li­er stu­dy, al­so by Ka­sh­lin­sky’s team, ap­peared in the jour­nal Na­ture in No­v­em­ber 2005.

Sci­en­tists es­ti­mate that the uni­verse be­gan 13.7 bil­lion years ago in an ex­plo­sion, the “Big Bang.” Stars formed a few hun­dred mil­lion years lat­er, end­ing the so-called cos­mic dark age. Kash­lin­sky’s group stud­ied the “cos­mic in­fra­red back­ground” light, a dif­fuse glow that they said comes from this ear­ly ep­och.

“There’s on­go­ing de­bate about what the first ob­jects were and how galax­ies formed,” said God­dard’s Har­vey Mose­ley, a co-au­thor of the pa­pers. “We are on the right track to fig­ur­ing this out.”

If the ob­jects are stars, they could be a first gen­er­a­tion of stars long sought by as­tro­no­mers and termed “Pop­u­la­tion III” stars. Some the­o­rize that their burnt-out rem­nants gave rise to the su­per­mas­sive black holes, which lurk at the hearts of most galax­ies. The stars, once spent, would col­lapse in­to smaller “seed” black holes, which then swell in­to huge ones by eat­ing up other mat­ter near­by. 

In or­der to form black holes big enough and fast enough to fit with ob­ser­va­tions, these the­o­ries re­ly on the in­i­tial stars them­selves be­ing “su­per­mas­sive,” weigh­ing hun­dreds of suns. Those found in the new stu­dy, if they’re stars, might fit the bill, re­search­ers say.

“There would be quite a link” to the black hole the­o­ry, said Mar­tin Haehnelt, a cos­mol­o­gist with the Uni­ver­si­ty of Cam­bridge, U.K. But he said this would de­pend on Kash­lin­sky’s team hav­ing in­ter­preted its re­sults cor­rectly, and he’s far from sure of that.

Con­tam­i­nat­ing light from ob­jects in the fore­ground can be­dev­il at­tempts to meas­ure the “in­fra­red back­ground,” Haehnelt said. Al­so, he said, Kash­lin­sky’s stu­dy in­volved com­par­ing sig­nals in dif­fer­ent parts of the sky, rath­er than re­solv­ing in­di­vid­u­al ob­jects, and it’s hard to say what such cor­re­la­tions mean.

Kash­lin­sky said his team care­ful­ly erased light from fore­ground stars and galax­ies, leav­ing on­ly the most an­cient light; then stud­ied fluc­tu­a­tions in the bright­ness, re­veal­ing clus­ters of ob­jects. “Imag­ine try­ing to see fire­works at night from across a crowd­ed city,” he said. “If you could turn off the city lights, you might get a glimpse at the fire­works. We have shut down the lights of the uni­verse to see the out­lines of its first fire­works.” 

If they’re stars, they’re prob­a­bly ex­treme­ly mas­sive, Mose­ley said, as small stars shine too in­ef­fi­cient­ly to ex­plain the light seen; also, there are the­o­ret­i­cal rea­sons to be­lieve su­per­mas­sive stars would form. A fu­ture tel­e­scope planned by NASA, the James Webb Space Tel­e­scope, should be able to iden­ti­fy what the clus­ters are, ac­cord­ing to mem­bers of Kash­lin­sky’s group.