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Star Clusters A star cluster is a group of stars held together by gravity. Star clusters consist of many more stars than individual star systems, from tens to millions of stars. Stars in a cluster all formed at the about the same time; hence have roughly the same age. They are also at approximately the same distance from the Earth. This greatly simplifies analysis of the stars in the cluster. For example, their relative apparent magnitudes are equal to their relative absolute magnitudes. Thus, studies of star clusters have provided astronomers with valuable insights into stellar evolution.

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Open Clusters

Open clusters are groups of up to several-hundred stars, loosely held together by gravity. They are called open because their stars are relatively far apart and their shapes are irregular. The diameter of an open cluster is typically less than 100 light years. The stars in an open cluster are mainly younger stars, and the cluster is often associated with nebulosity - a cloud of gas and dust - from which the stars formed. Over time, the stars in an open cluster will disperse as the gravity of the Milky Way pulls the stars in different directions. (Our Sun was formed in, and was once a member of, an open cluster.) The Pleiades (M 45) open cluster, imaged by Robert Gendler. Most of the open clusters we see are located in our Milky Way galaxy; hence we see them concentrated along the plane of the Milky Way in the sky. Well-known examples include the Pleiades or “Seven Sisters” (M 45) in the constellation Taurus, and the Jewel Box cluster (NGC 4755) near the Southern Cross. The brightest open clusters, like the PIeiades, are easily visible to the naked eye.

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Globular Clusters

A globular cluster consists of up to several million stars tightly bound by gravity into a region only a few tens of light years across. The stars are extremely concentrated - if our Sun were located at the center of a globular cluster, the night sky would be filled with thousands of stars rivalling the full moon in brightness. The stars in a globular cluster are typically much older than those in an open cluster. Globular clusters are dominated by red giants, white dwarves, and other examples of late stellar evolution. Most globulars orbit our Milky Way galaxy at tens of thousands of light years distance, and are (roughly speaking) evenly distributed around the center of the Milky Way. Since the center of the Milky Way lies in the direction of the constellation Sagittarius, we see the distribution of globulars concentrated toward Sagittarius in the sky. Globular cluster M 13 in Hercules. (Jim Misti.) The best-known examples of globular clusters are probably the Great Hercules Cluster (M 13), and Omega Centauri (NGC 5139) in the southern hemisphere. A few of the brightest globular clusters (like M 13) are barely visible to the naked eye on dark nights; binoculars will show them as a faint, fuzzy glow. Backyard telescopes can resolve globulars into their thousands of individual stars, and with large observatory telescopes, astronomers can observe globular clusters orbiting other galaxies such as the Andromeda Galaxy (M 31) as well.