The turbulent interstellar medium
The gas in galaxies is typically seen to be moving at very rapid, even supersonic velocities, providing clear evidence that the medium is highly turbulent. Looking more closely at gas clouds in our own Milky Way, astronomers have similarly demonstrated using a variety of different observations that the interstellar medium is turbulent. Turbulence is a key physical parameter in the star formation process because, like the thermal pressure of warm gas, it counters the collapse of clouds into stars from gravitational contraction. Despite its importance and ubiquity, however, turbulence is poorly understood. Even its origin is far from clear. Some scientists argue that turbulence results from star formation itself, as new stars and their associated supernovae drive winds that stir up the interstellar medium. Other astronomers counter that the influence of gravity alone is enough to induce supersonic motions in gas as it moves through and across a rotating galaxy.