Starbirth Reconsidered

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

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

One popular theory was that they swallowed small immature stars called protostars in crowded stellar nurseries, but astronomers recently caught a massive star in the act of being born. Observations suggest it is developing through gravitational collapse, the same gradual process that built the Sun.

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

The disk contains 1 to 8 times as much gas as the Sun and extends outward for more than 30 billion miles, eight times the distance to Pluto. Earth and the other planets in our solar system are believed to have formed from such a disk 4.5 billion years ago.

Astronomers also detected jets of bipolar gas spewing out from both ends of the HW2's circumstellar disk, a phenomenon previously observed only in the formation of low-mass stars like the Sun.

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

If the cannibalizing theory of massive star formation were true, the spewing gas jets and the circumstellar disk would be destroyed as additional stars were swallowed, Curiel said.

The study is detailed in the Sept. 1 issue of the journal Nature.