This false-colour image of the nearby galaxy IC342 shows the relationship between gas, stars and star formation. The blue features are clusters of stars, whereas the red in the outer region is the atomic hydrogen gas. Green indicates the molecular hydrogen gas, out of which new stars are forming, and yellow regions are where the atomic and molecular gas overlap.

This map of the molecular gas was obtained by Jean Turner of the University of California at Los Angeles, using ‘on the fly’ mapping at the 12-metre telescope of the National Radio Astronomy Observatory at Kitt Peak.

IC342 is a spiral galaxy, like the Milky Way, and the spiral structure is obvious in the distribution of the gas, which changes from predominantly molecular in the inner region, to predominantly atomic in the outer part. The gas forms a bar-like structure around the nucleus, which is believed to play a role in funnelling gas towards the centre, thereby supplying the fuel to form new stars. Curiously, the green ring around the nucleus, which indicates a lot of molecular gas, does not seem to be associated with the formation of new stars.

The relationship between the phases of gas and the formation of new stars is still not completely understood. Although it is apparent that in many places simply having lots of gas on hand is sufficient to produce stars, this is not universally true. As we produce higher-resolution maps of the distribution of gas in galaxies, we will eventually sort out the physics of what is really happening.