Open or
Galactic Clusters

(Section Not Complete)

Open cluster with associated nebula (This is M16---the Eagle Nebula)

Pleiades open cluster

Open and globular clusters in the Messier catalog

Open cluster in the Lagoon Nebula

Here is an image of the open cluster M50, which is 3000 light years from Earth and is about 20 light years across. Open clusters such as this one have some common characteristics:

  1. Open clusters are preferentially found in the plane of the galaxy (thus they are also called galactic clusters); in contrast, the globular clusters are concentrated in the halo of the galaxy and can be very far out of the plane.

  2. Open clusters tend to be more irregular in shape than the highly symmetric globular clusters.

  3. Open clusters typically contain of order 100 stars, as compared with as many as hundreds of thousands in globular clusters.
In images of open clusters such as this one, not all stars in the field of view are members of the cluster; some are stars between us and the cluster and some are stars beyond the cluster. One way to distinguish members of the cluster from stars that just happen to lie in the same direction is by measuring their proper motion on the celestial sphere. The stars in the cluster must all have approximately the same proper motion or the cluster would have dissipated over time, but stars not associated with the cluster would be expected to have more random motions.

Here is a synopsis of what the colors of stars in the cluster M50 can tell us about astrophysics.

Here is a set of links to images of open clusters.

In the center of 30 Doradus "lies a huge cluster of the largest, hottest, most massive stars known. R136 is composed of thousands of hot blue stars, some about 50 times more massive than our Sun. Although the ages of stars in R136 cause it to be best described as an open cluster, R136's density will likely make it a low mass globular cluster in a few billion years."

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