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Starbirth Reconsidered

The mostmassive stars in our galaxy formed in a process much like that which createdour Sun, and not by cannibalizing young, small stars as previously thought,according to a new study.

The starsin question are giants among giants, massive stars that weigh as much as 100small stars like the Sun, but because they are so rare and evolve so quickly,scientists were unsure how they achieved their colossal girths.

One populartheory was that they swallowed small immature stars called protostars in crowdedstellar nurseries, but astronomers recently caught a massive star in the act ofbeing born. Observations suggest it is developing through gravitationalcollapse, the same gradual process that built the Sun.

Using aradio telescope called the Submillimeter Array (SMA) in Hawaii, astronomersdetected a gaseous disk surrounding the massive protostar HW2, located 2,000light years away in the Cepheus constellation.

The diskcontains 1 to 8 times as much gas as the Sun and extends outward for more than30 billion miles, eight times the distance to Pluto. Earth and the otherplanets in our solar system are believed to have formed from such a disk 4.5billion years ago.

Astronomersalso detected jets of bipolar gas spewing out from both ends of the HW2'scircumstellar disk, a phenomenon previously observed only in the formation oflow-mass stars like the Sun.

"Merginglow-mass protostars woudn't form a circumstellar disk and a bipolar jet," saidSalvador Curiel, an astronomer at the National Autonomous University of Mexico(UNAM) and an author of the study.

If thecannibalizing theory of massive star formation were true, the spewing gas jetsand the circumstellar disk would be destroyed as additional stars wereswallowed, Curiel said.

The studyis detailed in the Sept. 1 issue of the journal Nature.

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