Open Clusters
Open clusters, also known as galactic clusters, are relatively small groups of about
ten to several
thousand stars. The stars in an open cluster typically formed at about the same
place and time and remain held together for some time by their
mutual gravitational attraction.
Our galaxy and other similar spiral galaxies are rich in open clusters,
and there are
many relatively nearby examples. The best known open cluster is the
Pleiades (M45 in the Messier catalog), shown in the adjacent figure. We shall describe
the Pleiades in more detail shortly.
Here is a
link to clusters (both open and globular) that are Messier objects,
and here is a set of links to
images of open clusters.
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Technically Speaking: Why Study Open Clusters?
Open clusters are interesting to astronomers because they are "laboratories" in which a set of
stars that all have about the same age and chemical composition can be studied. Although the
stars in a cluster are of about the same age and composition, they are not identical
because they likely formed with a wide range of masses and
rotation rates. A comparison
of the observed types and numbers of stars with predictions for stars having the age of the
cluster can be used as a sensitive test of theoretical models for stellar evolution.
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