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Stellar Heavyweight Breaks the Scales

21 July 2010 6:00 am
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ESO

Title holder. Lurking within a tightly packed stellar cluster, the supergiant R 136a1 is the most massive star ever found.

Using the world's most powerful ground-based telescope, astronomers have identified the seven heaviest stars ever found. One of these behemoths is so big, it's forcing researchers to rethink just how bulky stars can become.

When it comes to stars, mass is destiny. Our sun is an ordinary type of star called a yellow dwarf. It will probably continue burning for another 4 billion years or so, after which it will greatly expand—swallowing Earth in the process—and then shrink into a stellar ember that slowly fades away. Blue dwarfs, more massive but less common than yellow dwarfs, endure for only a fraction of that time, blowing themselves up in supernova explosions within about 30 million years.

Then there are the rarest of the rare, the blue supergiants, a hundred or more times more massive than our sun and whose light is millions of times stronger. Anything that bright and big is bound to live a very short life, going supernova within just a few million years.

Astronomers have been searching for blue supergiants for decades. Occasionally, they spot one indirectly because of the surpassing brilliance of its light. But because these supergiants live so close to other stars (they don't exist long enough to migrate away from their birthplaces within star-making clusters), they are usually impossible to pick out.

So a team of astronomers did some heavy digging. Led by astrophysicist Paul Crowther of the University of Sheffield in the United Kingdom, the group sifted through observations taken by the Hubble Space Telescope earlier in this decade and combined them with new images by the European Southern Observatory's Very Large Telescope (VLT) in Cerro Paranal, Chile. Using VLT's tremendous resolving power, the team was able to pick out three supergiants in the NGC 3603 star cluster and four in the R 136 cluster, located 22,000 light-years away and 165,000 light-years away, respectively.

Despite their scarcity, these monsters dominate their stellar neighborhoods. For example, the team reports (pdf) in the current Monthly Notices of the Royal Astronomy Society, the four supergiants in R 136 spew nearly half of the cluster's stellar wind particles and radiation. Those emissions would make it impossible for life to gain a foothold within many light-years.

But the big surprise among the supergiants was R 136a1, a star that might be called a hypergiant. Although already passing through stellar middle age at a mere 1.5 million years old, it started out with a mass equivalent to 300 suns—or twice as much mass as prevailing theory said a star could acquire. The discovery represents a challenge, says Crowther. That's because no one knows whether such supergiants grow from scratch within star-forming regions, or whether, like supermassive black holes and galaxies, they reach their enormous mass through mergers.

Crowther says the supergiants also remain as much of a puzzle at the end of their lives. Although all will eventually go supernova, the type of explosion they will generate is unknown. They could form neutron stars or black holes or obliterate themselves. Whatever their fate is, he says, "We still can't say."

It's a "state-of-the-art paper that's going to be a classic," says astronomer Nolan Walborn of the Space Telescope Science Institute in Baltimore, Maryland. For decades, he explains, astronomers have been arguing about the theoretical mass limit of stars. But this paper is "solid, because it presents all of the necessary data" showing that these are indeed viable candidates for the most massive stars known to date.

Astronomer Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, agrees. "It's a wonderful result," he says. The only possible hitch, he cautions, is that one or more of the supergiants could still turn out to be a pair of stars—something that will require additional observations to determine. Still, the stellar upper-mass limit seems to be "rising faster than our ability to understand how these stars form."

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