How Massive Stars Form: Simple Solution Found
Snapshots of the simulation at 17.5, 25.0, 34.0, 41.7, and 55.9 thousand years (A–E). Plus signs indicate the projected positions of stars.
Credit: Krumholz, et al.

The existence of massive stars ? up to a whopping 120 times the mass of the sun ? has long perplexed astronomers. The big question was how these stellar behemoths reached their enormous sizes without blowing off all the gas that feeds them.

A new computer simulation of star formation has found a surprisingly simple solution to how these stars might get around this problem.

The new findings, detailed in the Jan. 16 issue of the journal Science, also explain why these giants tend to occur in binary or multiple star systems.

"We didn?t' set out to solve that question, so it was a nice side benefit of the study," said study leader Mark Krumholz of the University of California, Santa Cruz.

Balancing forces

When a star begins to form, two opposing forces are at play. One is the pull of gravity creating by the rotating gas cloud from which the star is born. Gravity pulls the gaseous material in, feeding the protostar.

The other force, called radiation pressure, is generated by the growing star itself. This pressure is the force exerted by electromagnetic radiation on the surfaces it strikes. For ordinary light, this force is nearly negligible, but it becomes significant in the interior of stars because of the intensity of their radiation.

For massive stars, radiation pressure is the dominant outward-flowing force counteracting gravity's inward pull to prevent the further collapse of the star. Previous studies had suggested that radiation pressure would blow away a star's gas cloud before the star could grow much larger than 20 times the mass of the sun.

"When you apply the radiation pressure from a massive star to the dusty interstellar gas around it, which is much more opaque than the star's internal gas, it should explode the gas cloud," Krumholz explained.

Yet plenty of these massive stars have been spotted by astronomers (though they are rarer than small stars).

Surprise solution

Krumholz and his colleagues solved the dilemma with a three-dimensional computer simulation of the collapse of a giant interstellar gas cloud to form a massive star. Their research was funded by the National Science Foundation, NASA and the U.S. Department of Energy.

As the dusty gas collapsed, onto the star's growing core, instabilities developed that resulted in channels where radiation blew out through the cloud into interstellar space, while gas continued falling inward through other channels.

"You can see fingers of gas falling in and radiation leaking out between those fingers of gas," Krumholz said. "This shows that you don't need any exotic mechanisms; massive stars can form through accretion processes just like low-mass stars."

The disk of the collapsing gas also did something unexpected: it clumped to form a series of small secondary stars, most of which collided into the primary star, but some of which came to be stars in their own right and formed a multiple star system.

"I think now we can consider the mystery of how massive stars are able to form to be solved," Krumholz said.