New study resolves mystery of how massive stars form

Jan 15, 2009
Volume renderings of the density field in a region of the simulation at 55,000 years of evolution. The left panel shows a polar view, and the right panel shows an equatorial view. The fingers feeding the equatorial disk are clearly visible.

(PhysOrg.com) -- Theorists have long wondered how massive stars--up to 120 times the mass of the Sun--can form without blowing away the clouds of gas and dust that feed their growth. But the problem turns out to be less mysterious than it once seemed. A study published this week by Science shows how the growth of a massive star can proceed despite outward-flowing radiation pressure that exceeds the gravitational force pulling material inward.

The new findings also explain why massive stars tend to occur in binary or multiple star systems, said lead author Mark Krumholz, an assistant professor of astronomy and astrophysics at the University of California, Santa Cruz. The formation of companion stars emerged unexpectedly from the sophisticated computer simulations the researchers used to explore the physics of massive star formation.

"We didn't set out to solve that question, so it was a nice side benefit of the study," Krumholz said. "The main finding is that radiation pressure does not limit the growth of massive stars."

Computer simulation of the formation of a massive star yielded these snapshots showing stages in the process over time. Panels on the left represent a polar view (the axis of rotation is perpendicular to the plane of the image), and panels on the right represent an equatorial view. Plus signs indicate projected positions of stars. Colors represent density. Images by Krumholz et al.

Radiation pressure is the force exerted by electromagnetic radiation on the surfaces it strikes. This 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.

"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 said. 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. Yet astronomers observe stars much more massive than that.

Krumholz and his coauthors at UC Berkeley and Lawrence Livermore National Laboratory have spent years developing complex computer codes for simulating the processes of star formation. Combined with advances in computer technology, their latest software (called ORION) enabled them to run a detailed three-dimensional simulation of the collapse of an enormous interstellar gas cloud to form a massive star. The project required months of computing time at the San Diego Supercomputer Center.

The simulation showed that as the dusty gas collapses onto the growing core of a massive star, with radiation pressure pushing outward and gravity pulling material in, instabilities develop that result in channels where radiation blows out through the cloud into interstellar space, while gas continues 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 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, however, causing it to clump and form a series of small secondary stars, most of which end up colliding 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 57,000 years of simulated 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.

"What formed in the simulation is a common configuration for massive stars," Krumholz said. "I think we can now consider the mystery of how massive stars are able to form to be solved. The age of supercomputers and the ability to simulate the process in three dimensions made the solution possible."

The paper describing these results is being published by Science on the Science Express web site on January 15, 2009.

Provided by University of California - Santa Cruz

Explore further: Far from home: Wayward cluster is both tiny and distant

add to favorites email to friend print save as pdf

Related Stories

What is a Wolf-Rayet star?

Feb 06, 2015

Wolf-Rayet stars represent a final burst of activity before a huge star begins to die. These stars, which are at least 20 times more massive than the Sun, "live fast and die hard", according to NASA.

Two low-cost Canadian nanosatellites launched today

Jun 19, 2014

Two nanosatellites were launched from Yasny, Russia, at 15:11:11 Eastern Daylight Time today by Anthony Moffat, of the University of Montreal and the Centre for Research in Astrophysics of Quebec, and the ...

Could alien life cope with a hotter, brighter star?

Mar 24, 2014

The stars in the night sky shine in myriad hues and brightnesses—piercing blues, clean whites, smoldering crimsons. Every star has a different mass, the basic characteristic that determines its size, lifespan, ...

Recommended for you

Far from home: Wayward cluster is both tiny and distant

Mar 03, 2015

Like the lost little puppy that wanders too far from home, astronomers have found an unusually small and distant group of stars that seems oddly out of place. The cluster, made of only a handful of stars, ...

An old-looking galaxy in a young universe

Mar 02, 2015

A team of astronomers, led by Darach Watson, from the University of Copenhagen used the Very Large Telescope's X-shooter instrument along with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe ...

Giant methane storms on Uranus

Mar 02, 2015

Most of the times we have looked at Uranus, it has seemed to be a relatively calm place. Well, yes its atmosphere is the coldest place in the solar system. But, when we picture the seventh planet in our ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.