Two of the molecular clouds studied by Kainulainen and colleagues: The Pipe Nebula (left) and the Rho Ophiuchi cloud (right) in the Milky Way. In the background, an ordinary image of the Milky Way; each inset map shows to what extent the light of background stars is dimmed as it passes through the cloud in question. These maps form the basis of the 3-D reconstruction of cloud structure from which the astronomers derived their "recipe for star formation."