Scientists have known for years that turbulence within the gaseous clouds found throughout the Milky Way stymies star formation. But some of the forces behind this disturbance remain unknown. Building on previous work that reported large stars generate huge bursts of radiation every 10 million years or so, Alexei G. Kritsuk and Michael L. Norman of the University of California at San Diego used supercomputer simulations to investigate the potential effects of this energy source. "We think this radiation may act as an important regulation mechanism, providing a feedback effect from star-formation that may inhibit further star formation," Norman says. The researchers found that ultraviolet radiation in a large section of the Milky Way galaxy first doubles and then increases by nearly 10 times before decreasing again. (The image shows ultraviolet heating in a cube of interstellar medium.) The pattern repeats like a heartbeat, the scientists report. According to Kritsuk, such time-dependent heating of the interstellar medium could be a driving force behind 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."