Formation of Open Clusters

New stars are formed in the disk of our galaxy when light pressure from surrounding luminous stars and shock waves compress a cloud of interstellar dust and gas. The shrinking cloud may condense around several sites to form new stars. Since these are formed from the same nebula under similar conditions, one can easily form a group of similar stellar neighbors from a nebula. The mutual gravitational attraction holds this group of stars together, and in this manner a new open cluster is formed.

Seeing through the Dust

The above IR image from the 8.2 meter telescope of the European Southern Observatory shows a very young open cluster in which the stars are still immersed in the dust from which they were born. This cluster, which is designated RCW38, is about 5000 light years away in the constellation Vela. When we see most open clusters, we don't see so much dust because the stars have had time to move away from the nebula in which they formed and to disperse the dust with radiation pressure. In this case the stars are so young that they are still hiding in the dust. In visible light they would be mostly obscured by the dust, but IR penetrates the dust much better and allows the open cluster to be imaged.

Cluster Formation in the Orion Nebula?

In the adjacent left Hubble Space Telescope image, one can see evidence for the formation of several stars in a portion of the Orion Nebula, and the resulting newly-born stars may form an open cluster. The four bright stars known as the Trapezium would be members of this new open cluster. Since the stars in an open cluster have presumably formed from the same initial nebula, we may expect that they are approximately the same age and have similar chemical compositions.