Open (or Galactic) Clusters

Now that we see the type of environment in which stars are born, lets view some regions which have recently experienced stellar birth. We would expect to see a loose cluster of stars near their birthplace. As time passes, gravitational interactions will tend to cause these loose affiliations to dissipate.

A fine example of such a cluster, called an {\it open} or {\it galactic cluster} is the Pleiades (M45). Also known as the "Seven Sisters" (although you may see dozens of stars in the region through binoculars), this cluster is estimated to be about 20 million years old. Here's a photograph.

Many of the stars in the Pleiades are B type giants These stars lie at an estimated distance of 400 light years. Therefore the light from these stars left at about the beginning of the renaissance and the scientific revolution. This light left at about the same time that Galileo was making his discoveries with the first telescope! There are estimated to be between 200 and 500 stars in the cluster. The youth of this cluster is reflected in the fact that much of the gas from which the stars condensed is still visible in the cluster. The cloud of gas reflects the light of the stars which are embedded in it and is easily seen in photographs and in small telescopes.

[Display "Satellite!"]

Another example of an open cluster is M46 (NGC 2437) in the constellation Puppis. It lies in the plane of the Milky Way galaxy near this point. [Show location.] Here's a photograph.

The total population of stars in this region is around 500. The brighter members are blue giants of spectral class A0 and are about 100 times as bright as our Sun. However, at an estimated distance of about 5000 light years, this cluster is only visible through a telescope. You'll notice this small fuzzy spot. This is the planetary nebula NGC 2438 and is actually only about 3000 light years away, therefore it is not physically associated with the M46 cluster.

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