FEATURE ARTICLE
The Formation of Star Clusters
Clouds in the summer sky provide clues about the organization of star populations
Bruce Elmegreen, Yuri Efremov
Conclusion
Young and old globular clusters form in essentially the same way as open clusters (such as the Pleiades) and OB associations (such as Orion)—in a hierarchical fashion dictated by turbulence in the gas within a galaxy. The key difference between globular clusters and open clusters is the ambient pressure in which they form. Whether a cluster was made 15 billion years ago or in a more modern universe, the basic processes of turbulence and star formation appear to have changed little.
The hierarchical structure of gas described here may be responsible for the spontaneous formation of star clusters in all turbulent environments and at all epochs. But not all star formation is so free of environmental stimulation. There is another mode of star birth that follows the direct compression of gas by an outside source. This happens if there are hot young stars or supernovae to move the gas around, leading to the formation of new gas clouds out of low-density material, or to fast collapse and star formation inside pre-existing clouds.
There are many examples of such "triggered" star formation in our Galaxy and other nearby galaxies, including the Large Magellanic Cloud, where an arc of bright stars, 15 million years old, was triggered by pressure from a 30 million- year-old OB association located at the center of its curvature. The old association has long since dispersed, with only a trace of intermediate-mass stars and one Cepheid variable remaining. A third generation of stars formed later, as a big ring of clusters and nebulae that currently surrounds the arc. Similar arcs and rings of star formation have been seen in other galaxies as well. Some recent radio maps of the Large Magellanic Cloud and other small galaxies reveal so many holes and rings that their gaseous disks resemble Swiss cheese! We are now studying these regions.
The possibility of triggered star formation in galactic spiral arms is also under consideration. Spiral density waves explain well the regular spiral structure of spiral galaxies, but there is still a question of whether young stars concentrate in the spiral arms because most of the gas is there or because the spiral compression leads to some additional triggering. We have known for some time that the formation of stars in spiral arms is characterized by giant concentrations, somewhat equally spaced along the arms. Many of these are the star complexes that we discussed earlier. Such large-scale structure in a spiral arm is too regular to be fractal, and it may not be related to turbulence at all. Instead, it is probably the result of a gravitational instability in the spiral compressed gas.
In most locations, triggered star formation coexists with spontaneous star formation. Usually the triggering mode forms clouds that are turbulent and fractal on the inside, even though they may be regular in some way on the outside. Cluster formation proceeds inside the clouds as we have described above. This gives complicated but picturesque images, making the study of star formation interesting and challenging for a long time to come.
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