Scientists Resolve Mystery Of How Massive Stars Form

New research by Lawrence Livermore National Laboratory, University of California, Santa Cruz and UC Berkeley has shown how a massive star can grow despite outward-flowing radiation pressure that exceeds the gravitational force pulling material inward. The study appears in the Jan. 15 online edition of Science Express.

Using 3-D radiation hydrodynamics simulations, the group, which includes Livermore’s Richard Klein, who also is an adjunct professor at UC Berkeley, and his LLNL postdoc Andrew Cunningham, unexpectedly discovered that these massive stars also tend to occur in binary or multiple star systems.

“Originally, we were just exploring the physics of massive star formation,” Klein said. “As we were looking at the physics, we found that gravitational instabilities cause companion stars to form around massive stars.”

Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a circumstance that has long been expected to prevent them from growing by accretion (the growth of a massive object by gravitationally attracting more matter).

“We didn’t set out to solve that question, so it was a nice side benefit of the study,” said Mark Krumholz, lead author and an assistant professor of astronomy and astrophysics at the UC Santa Cruz said. “The main finding is that radiation pressure does not limit the growth of massive stars.”

Earlier studies suggested that radiation pressure would blow away the raw materials of star formation before a star could grow much larger than about 20 times the mass of the sun. But astronomers have seen stars much more massive than that.

The team spent years developing complex computer codes for simulating the processes of star formation. Combined with advances in computer technology, their latest code (called ORION) enabled them to run a detailed 3-D simulation of the collapse of an enormous interstellar gas cloud to form a massive star.

“Logically, we thought the massive amounts of radiation pressure would stop the star in its tracks from growing any larger,” Klein said. “But instead, gravitational instabilities channeled gas onto the star system through disks and filaments, sort of like fingers, that self-shield against the radiation, while allowing the radiation to escape through optically thin bubbles.”

Radiation pressure is the force exerted by electromagnetic radiation on the surfaces it hits. The effect is negligible for ordinary light, but it becomes significant in the interiors of stars due to the intensity of the radiation. In massive stars, radiation pressure is the dominant force counteracting gravity to prevent the further collapse of the star.

The rotation of the gas cloud as it collapses leads to the formation of a disk of material feeding onto the growing “protostar.” The disk is gravitationally unstable, causing it to clump and form a series of small secondary stars, most of which end up colliding and merging with the central protostar. In the simulation, one secondary star became massive enough to break away and acquire its own disk, growing into a massive companion star. A third small star formed and was ejected into a wide orbit before falling back in and merging with the primary star.

When the researchers stopped the simulation, after allowing it to evolve for virtually 57,000 years of time, the two stars had masses of 41.5 and 29.2 times the mass of the sun and were circling each other in a fairly wide orbit.

This research was funded by the National Science Foundation, NASA, and the U.S. Department of Energy.