Adding weight. Like their smaller siblings, massive stars appear to form by accretion of gas from a circumstellar disk.

Solar Mergers Are Out

Small companies can grow gradually by hiring more people or expand rapidly by merging with other businesses. For years, astronomers have thought stars grow in much the same way: low-mass, sunlike stars form through accretion of gas in the dense core of a collapsing interstellar cloud, while massive stars arise through the merger of smaller stars. But two new studies of massive protostars--stars in the process of being born--indicate that the merger model may be wrong, at least for all but the most extreme heavyweights.

There was a lot of logic behind the merger idea. Accreting material has never been observed around stars more than 10 times the mass of the sun. Moreover, theorists think accretion cannot produce the most massive stars in the Milky Way because any protostar above 30 solar masses would radiate so fiercely that infalling matter would be blown away.

But two papers in this week's issue of Nature present strong evidence for accretion as the driving force in the formation of two massive stars. A team led by Nimesh Patel of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, used the Submillimeter Array at Mauna Kea, Hawaii, to obtain high-resolution infrared images of a rotating accretion disk surrounding a 15 solar-mass protostar over 2000 lightyears away known as Cepheus A HW2.

Meanwhile, measurements taken by Zhibo Jiang of the Purple Mountain Observatory in Nanjing, China, and colleagues using the 8.2-meter Japanese Subaru telescope, also at Mauna Kea, suggest that a massive protostar in Orion is also surrounded by an accretion disk. The protostar, known as the Becklin-Neugebauer object, is 1500 lightyears away, and probably 7 to 10 times more massive than the sun. Since accretion disks would be destroyed in a violent stellar merger, the findings indicate that mergers played no role in the formation of these massive stars.

"It's quite exciting," says Ian Bonnell of the University of St. Andrews in the United Kingdom. "Looking for disks around high-mass protostars has been like a Holy Grail, and these results appear fairly convincing." Still, Bonnell warns that the new observations do not rule out the merger scenario altogether. It's hard to stop mergers from happening in dense star-forming regions, he says. In addition, he says, mergers may still be the only way for stars to grow beyond 30 solar masses.

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