Jul 22, 2010

Newfound stars seen shattering known size limits

As­tro­no­mers have iden­ti­fied the hug­est stars yet known, in­clud­ing one esti­mated to weigh the equi­va­lent of 265 Suns.

If it took our Sun’s place, it would out­shine our star by as much as the Sun cur­rently out­shines the full Moon, sci­en­tists said. “It would bathe the Earth in in­credibly in­tense ul­tra­vi­o­let radia­t­ion,” shut­ting off any chance of life, said Raph­a­el Hirschi from Keele Uni­vers­ity, U.K., a mem­ber of the re­search team.

The star clus­ter RMC 136a in three in­crea­sing­ly close-up views from top to bot­tom. (Courtesy ESO)

The group, led by Paul Crow­ther of the Uni­vers­ity of Shef­field, U.K., used the Eu­ro­pe­an South­ern Ob­ser­va­to­ry’s Very Large Tel­e­scope and ar­chi­val da­ta from the Hub­ble Space Tel­e­scope to closely study two rel­a­tively young star clus­ters

The first, des­ig­nat­ed NGC 3603, is a cos­mic fac­to­ry where stars form fran­ti­cally out of ex­tend­ed clouds of gas and dust. The sec­ond, known as R136, is an­oth­er clus­ter of young, mas­sive and hot stars, lo­cat­ed in zone called the Ta­ran­tu­la Neb­u­la, in one of our neigh­bour­ing ga­lax­ies, the Large Mag­el­lanic Cloud.

The two clus­ters lie 22,000 and 165,000 light-years away, re­spec­tive­ly. A light-year is the dis­tance light trav­els in a year.

The re­search­ers found sev­er­al stars with esti­ma­ted sur­face tem­per­a­tures over 40,000 de­grees Cel­si­us, more than sev­en times hot­ter than our Sun, and a few tens of times larg­er and sev­er­al mil­lion times brighter.

The­o­ret­i­cal mod­els imply that sev­er­al of these stars were born with mass­es over 150 so­lar mass­es, in­ves­ti­ga­tors ex­plained. One found in the clo­ser clus­ter is the heav­i­est, cur­rently weigh­ing in at about 265 so­lar mass­es and with an es­ti­mat­ed birth­weight of up to 320.

Not only is that star the most mas­sive ev­er found, it’s al­so the bright­est, al­most 10 mil­lion times more lu­mi­nous than the Sun. “Ow­ing to the rar­ity of these mon­sters, I think it is un­likely that this new rec­ord will be bro­ken any time soon,” said Crow­ther.

Gi­ant stars pro­duce pow­er­ful out­flows that make them shrink as they give up some of their con­tents to space. “Un­like hu­mans, these stars are born heavy and lose weight as they age,” said Crow­ther. “Be­ing a lit­tle over a mil­lion years old, the most ex­treme star, R136a1, is al­ready ‘middle-aged’ and has un­der­gone an in­tense weight loss pro­gramme, shed­ding a fifth of its in­i­tial mass.”

These monstrous stars form only in the most closely packed star clus­ters, mem­bers of the re­search team said. They added that dis­tin­guish­ing the in­di­vid­ual stars, a new achieve­ment, re­quired the ex­quis­ite re­solv­ing pow­er of the in­fra­red light in­stru­ments on the Very Large Tel­e­scope in Pa­ra­nal, Chile.

The team al­so es­ti­mat­ed the max­i­mum pos­si­ble mass for the stars with­in these clus­ters and the rel­a­tive num­ber of the most mas­sive ones. “The small­est stars are lim­ited to more than about eighty times more than Ju­pi­ter, be­low which they are ‘failed stars’ or brown dwarfs,” said team mem­ber Oliv­i­er Schnurr from the As­t­ro­physikalis­ches In­sti­tut Pots­dam in Germany.

“Our new find­ing sup­ports the pre­vi­ous view that there is al­so an up­per lim­it to how big stars can get, al­though it raises the lim­it by a fac­tor of two, to about 300 so­lar mass­es.”

With­in R136, only four stars weighed more than 150 so­lar mass­es at birth, yet they ac­count for nearly half of the wind and radia­t­ion pow­er of the whole clus­ter, about 100,000 stars in all, ac­cord­ing to as­tro­no­mers. R136a1 alone en­er­gises its sur­round­ings by more than a fac­tor of fif­ty com­pared to the Ori­on Neb­u­la clus­ter, the clos­est re­gion of mas­sive star forma­t­ion to Earth.

Un­der­stand­ing how high mass stars form was al­ready puz­zling, re­search­ers said, so that the iden­ti­fica­t­ion of such ex­treme cases raises the chal­lenge to the­o­rists still fur­ther. “Ei­ther they were born so big or smaller stars merged to­geth­er to pro­duce them,” said Crowther.

The findings are described in the re­search jour­nal Month­ly Notices of the Roy­al Astro­nom­ical So­ciety.

Stars be­tween about eight and 150 so­lar mass­es ex­plode at the end of their short lives as so-called su­pernovas, leav­ing be­hind ex­ot­ic rem­nants known as ei­ther neu­tron stars or black holes. The new find­ings raise the pros­pect of the ex­ist­ence of ex­cep­tion­ally bright su­pernovas that to­tally b­low them­selves apart, leav­ing no rem­nant and dis­pers­ing up to ten so­lar mass­es of iron in­to their sur­round­ings, ac­cord­ing to re­search­ers. Such ex­plo­sions may have al­ready been de­tected in the past, they added, with a few can­di­date events ripe for fur­ther in­ves­ti­ga­t­ion.



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