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Star Clusters


Galactic (Open) Clusters

Open clusters of stars can be near or far, young or old, and diffuse or compact. Open clusters may contain from 100 to 1000 stars, all of which formed at nearly the same time. Bright blue stars frequently distinguish younger open clusters. M35, pictured in Figure 06-03a on the upper left, is relatively nearby at 2800 light years distant, relatively young at 150 million years old, and relatively
Open Clusters Pleiades diffuse, with about 2500 stars spread out over a volume 30 light years across. An older and more compact open cluster, NGC 2158, is visible in the same picture on the lower right. NGC 2158 is four times more distant that M35, over 10 times older (Figure 06-02a), and much more compact as it contains many more stars in roughly the same volume of space. NGC 2158's bright blue stars have self-destructed, leaving the cluster to be dominated by older and yellower stars. Figure 06-03b shows the open star cluster Pleiades,

Figure 06-03a Open Cluster, M35 & NGC 2158 [view large image]

Figure 06-03b Open Cluster, Pleiades [view large image]


also known as the Seven Sisters and Messier 45. It is a conspicuous object in the night sky with a prominent place in ancient mythology. The cluster contains thousands of stars, of which only a handful is commonly visible to the unaided eye. The stars in the
Pleiades are thought to have formed together around 100 million years ago, making them 1/50th the age of our sun, and they lie some 130 parsecs (425 light years) away. The blue haze around the hot stars is the reflection nebula produced by dust cloud behind the stars. For northern hemisphere viewers, the cluster is above and to the right of Orion the Hunter as one faces south, and it transits -- reaches its highest point in the sky -- around 4am in September, midnight in November, and 8pm in January.

See "Open Star Clusters" for more detail,

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