Star Clusters

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    It seems as if stars are born into very large families, with hundreds, if not thousands of siblings. The stellar nurseries in which stars are created become clusters of one sort or another. There are, in fact, only two types of star clusters which we speak of; globular clusters and open clusters. Globular clusters are very old. Not an awful lot is going on in them any more. Open clusters, on the other hand, are relatively recent objects. The Sun was formed in an open cluster just a few billion years ago.

• Open Clusters
    Also known as galactic clusters, these groups of young stars are constantly forming within the disk of the Milky Way Galaxy.
• Globular Clusters
    These ancient objects were the first structures to form in our home galaxy. They lack the interesting heavy elements which make planets such as Earth possible.

   In late winter and in early spring, northern hemisphere observers are treated to two of the most easily visible star clusters in the heavens, the Pleiades and the Hyades.

   Both types of clusters can reveal some information by constructing a color magnitude diagram of the cluster. Like an H-R diagram, two characteristics of an entire group of stars in a cluster are plotted. Since you are plotting the apparent magnitude and not the absolute magnitude, the diagram will be offset. This offset is the distance modulus for the cluster. Of course, once you have the distance modulus, you can then calculate the distance to the cluster.

   Here is one laboratory exercise on star clusters which you can do to determine the distance to your assigned cluster.

   Here is a collection of course notes from professor Barbara Ryden of Ohio State that you may find useful.

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Updated April 31, 2009