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Newfound stars seen shattering known size limits

July 21, 2010
Courtesy of the European Southern Observatory
and World Science staff

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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Astronomers have discovered the hugest stars yet known, including one weighing at birth more than twice the currently accepted upper limit on star weight of 150 Suns. If that star took our Sun’s place, it would outshine it by as much as the Sun currently outshines the full Moon, scientists said. “It would bathe the Earth in incredibly intense ultraviolet radiation, rendering life on our planet impossible,” said Raphael Hirschi from Keele University, U.K., a member of the research team. 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 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 R136 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 super heavyweight stars form solely only 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. Stars between about 8 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, they added, with a few candidate events ripe for further investigation.