The Formation of Star Clusters
Clouds in the summer sky provide clues about the organization of star populations
At the beginning of the universe, when the galaxies first appeared, there were no stars at all. The first stars began to form only after the hot, gaseous galaxies had time to cool. The densest of these early galaxies worked quickly, turning all of their gas into stars when the universe was still quite young. Others, like our own Milky Way, puttered along at a relatively modest rate of star formation. Indeed, our Galaxy has a lot of gas left, and is still forming stars today at a rate of about 10 per year. These stars are forged in what might be called stellar nurseries: vast clouds of dust and gas where tens, hundreds or even thousands of stars are born.
The nature of these gas clouds and precisely how they evolve to form the distribution of stars we see in the sky is one of the classic puzzles of astronomy. It has proved to be especially challenging because the formation of a star cluster is obscured by the same dust and gas that fuels the process. Moreover, the birth of a star like the sun takes place over the course of millions of years, many times longer than the age of our species. Such ephemeral beings as astronomers must deduce the mechanism of cluster-building from observations of present star groups in the Milky Way and neighboring galaxies.
As it happens, the star clusters we see in the sky are not all alike. Clusters in one part of our Galaxy (the thin disk, which includes the spiral arms) appear to differ from those in another part (the Galactic halo, a vast spherical area that surrounds the disk). Some star groups are densely packed in small volumes of space, whereas others are distributed over large expanses. Stars in some groups are gravitationally bound to each other, whereas stars in other groups are drifting apart. Some star groups are ancient, dating to the time when the galaxies first formed, yet others appear to be caught in the act of forming.
Because of this apparent diversity, astronomers have long assumed that the mechanisms involved in the formation of each type of cluster were unique. Certainly conditions do differ across the Galaxy, and there is reason to believe that the universe was a very different place when the galaxies first began to shine. But do these differences prohibit the existence of a common mechanism?
Our research suggests that all clusters, regardless of age and appearance, can be explained by the same basic mechanism: the structuring of star-forming gas into a hierarchical distribution of clouds by the action of turbulence and gravity. Here we describe some of the evidence for this process and address some of the previous objections raised against the idea of a universal mechanism.