Until now, astronomers haven't been able to offer a full explanation for why the Milky Way and other galaxies produce new stars at a relative snail's pace. While they have known for decades that high turbulence keeps huge clouds of hydrogen gas from condensing into stars, they haven't identified all the causes of the galactic perturbations.

In a report to be published in the November 20 issue of Astrophysical Journal Letters, researchers at the University of California, San Diego have discovered that a well-known, but overlooked source of heating -- regular outbursts of ultraviolet radiation from clusters of very large, bright stars -- may play a significant role in keeping the Milky Way's gas continually stirred up.

The image generated by a supercomputer simulation (top) bears similarities to a telescope image of a cloud of gas being heated by bright, massive stars in the Large Magellanic Cloud, a nearby galaxy. In the simulation image (top), cold, high-density gas is blue; warm low-density gas is red; and intermediate density and temperature gases are light blue, green and yellow. In the image of a the 30 Doradus nebula (bottom) warm, diffuse gas is green; cold, dense gas is brown; and regions with no gas are black. Credit: Alexei G. Kritsuk, Michael L. Norman (UCSD) NASA, N. Walborn (STScI), J. Maiz-Apellaniz (STScI), and R. Barba (La Plata Observatory, Argentina)

In a rapid-release online publication of the report, Alexei G. Kritsuk, a visiting researcher at UCSD, and Michael L. Norman, a professor of physics at UCSD and a senior fellow at UCSD's San Diego Supercomputer Center, detailed their findings, which were made with a sophisticated hydrodynamic computer simulation.

"The most massive and brightest stars in the spiral arms of our galaxy emit lots of ultraviolet radiation in regular cycles," said Norman. As clouds of cold, dense gas condense into very bright stars, the ultraviolet radiation they emit dissipates the remaining clouds of gas, which causes further star formation to slow considerably. "We think this radiation may act as an important regulation mechanism, providing a feedback effect from star-formation that may inhibit further star formation," he said.

Light with wavelengths shorter than the human eye can see is called ultraviolet (beyond violet) light, and its energy is readily transferred to gas clouds in galaxies. (The sun also produces ultraviolet light, which is absorbed by the ozone layer in Earth's atmosphere or reflected back into space.)

Astrophysicists agree that some form of energy must be supplied to gases in the so-called interstellar medium of galaxies to sustain the turbulence that astronomers have noted in beautiful images of the spiral arms of the Milky Way and nearby galaxies. Researchers have identified a variety of potential energy sources that in some situations could maintain the turbulence -- such as violent stellar explosions called supernovae, or blasts of supersonic "winds" given off by massive newborn stars. "Astronomers have been thinking about these mechanical effects for several decades, but not really considering the radiation effects, which travel at the speed of light and exert a heating effect much farther away than a supernova shock or a stellar wind," said Norman. "Ultraviolet radiation may not be the complete answer to all the galactic turbulence, but I'm confident that it plays some role in what astronomers call the interstellar medium."

In a research paper published online this year at http://lanl.arXiv.org/abs/astro-ph/0202196 a group of researchers led by Venezuelan astrophysicist Antonio Parravano reported that large, bright stars generate large, bursts of ultraviolet radiation on timescales of about 10 million years. Norman and Kritsuk used Parravano's findings as the starting point for their simulation studies.

"The ultraviolet radiation in a huge section of the Milky Way's disk goes up by a factor of two to 10 and then, falls, and then repeats like heartbeats," said Kritsuk. "Our numerical simulations show that this time-dependent heating of the interstellar medium would be a good driving force for the filaments and blobby sheets of gas actually observed by astronomers in the regions of the galaxy where background ultraviolet radiation is the main source of energy."

Kritsuk and Norman used supercomputers at the San Diego Supercomputer Center at UCSD and the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign. The machines are provided by the National Science Foundation to enable computationally intensive approaches to astrophysical research by scientists at dozens of colleges and universities in the United States.