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Darwinian Star Formation Weeds Out Wimps
By Robert Roy Britt
Senior Science Writer
posted: 10:11 am ET
24 April 2002

When stars are born on the same cosmic block, the principle of survival of the fittest rules like nowhere else in the universe, a new study suggests.

Star formation may be more rambunctious than thought when it occurs in bursts, generating crowded stellar neighborhoods. Stars sometimes form so closely together that they must compete for the available gas in their natal cloud of material. More massive stars are more capable of grabbing more stuff, and smaller objects are sometimes simply shoved right out of the neighborhood before they get a chance to grow.

The findings are the result of computer simulations led by Matthew Bate of the University of Exeter and which were presented in early April at the UK National Astronomy Meeting.

The simulation followed the collapse of an interstellar gas cloud more than 1 light-year in diameter and containing a mass of gas and dust equal to 50 Suns. The end result, after 266,000 years, was the formation of a cluster of stars typical to our galaxy.

Yet of the 50 objects that were formed, only about half became stars. The rest were gravitationally ejected from the central cluster before they gathered enough mass to trigger thermonuclear fusion, the fusing of hydrogen into helium that powers a real star.

The ostracized failures are considered brown dwarfs.

The computer model might help explain recent findings of brown dwarfs wandering alone through space. Astronomers have been unsure how these objects developed and why they are alone. Brown dwarfs have also been found orbiting stars.

Other surveys in recent years have begun to suggest, in fact, that there may be as many brown dwarfs in the Milky Way galaxy as there are stars. The new study supports that notion, Bate and his colleagues said in a statement, adding that "the high frequency of brown dwarfs is a natural consequence of the competition between stars during their formation."

The model also revealed that the chaotic interaction stripped material away from the environments around the new stars.

Where disks of gas and dust as large as our solar system might have developed, far smaller disks resulted. These disks are thought to be the primary birthplaces of planets. And because most of the stars in the galaxy form in star clusters, the findings may put a damper on hopes that planets are ubiquitous throughout the Milky Way, the researchers say.

Details of the simulation will be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.