Hot and cold. A computer model (left) shows veins of cold gas clouds (blue) within a warm bath of gas (red), similar to a stellar nursery in the Large Magellanic Cloud (right).

Hot Stars Churn the Milky Way

Stars in the night sky might look tranquil, but the biggest stars emit enough intense light to roil the Milky Way's gas, according to a new model. Nests of giant stars sear their neighborhoods with ultraviolet (UV) radiation, warming and diffusing nearby gas. The results might help explain the relative infertility of our galaxy, because clumps of gas must be cold and quiet for gravity to assemble new stars.

The vast space among stars, called the interstellar medium, is never dull. Regions of fast, warm gas reach about 10,000 kelvin, but they coexist with veins and knots of sluggish gas that plunge to 50 kelvin or below. Stars are born within these chilly nurseries as the gas collapses into dense balls. The sources of energy that sustain this mixture of hot and cold clouds aren't firmly identified, but the leading suspects have involved physical motions: shock waves from supernovas, winds blowing from young stars, and the rotation of the galaxy itself.

New work suggests that pulses of heat may be as important. Astrophysicists Alexei Kritsuk and Michael Norman of the University of California, San Diego, used a supercomputer to simulate the effects of occasional blasts of intense UV light from young stars. The hottest stars arise in tight clusters and flame out after brilliant lifetimes of about 10 million years--a blip on the cosmic calendar. The model shows that violent heating alternated with calm periods creates an agitated foam of cold clumps embedded within warm gas. The results resemble the networks of hot and cold clouds seen in space. "This may not be the dominant mechanism, but you can't get away from it," Norman says. Cycles of heating from the most massive stars might evaporate some clouds of gas and suppress further star birth, he notes.

The model is an interesting contribution, says astrophysicist David Hollenbach of NASA's Ames Research Center in Mountain View, California. "The UV field of the galaxy is one of the strongest energy sources there is." And depending on how it's channeled, he says, it should stir up a lot of turbulence. More work is needed to rigorously test the simulations against observations of the interstellar medium, Hollenbach adds.

Related sites
UC San Diego news announcement, with images and movie
Previous study on UV pulses in interstellar medium
A primer on star formation