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The Nearest Star Cluster
By Jeff Kanipe
posted: 30 June 2005
07:57 am
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The Big Dipper, which lies below and to the left of Draco around 10 o'clock as you face the northwest horizon, is more than just a famous star pattern. Except for the first star in its cup and the last star in its handle, the remaining central five stars belong to a star cluster called the Ursa Major group. Unlike the tiny assemblages of stars you have to find with binoculars and telescopes, this star cluster is huge on the sky because it is relatively nearby - in fact it is the nearest star cluster to the sun, between 70 and 80 light-years away. The Ursa Major group wasn't recognized as a bonafide star cluster until 1869, when English astronomer Richard Anthony Proctor determined that the five central stars of the asterism shared the same motion through space.Other bright stars scattered throughout the sky also appear to share the same space motion as the central five of the Big Dipper. Most of these, however, are not true cluster members but comprise a "stream" of about 100 stars moving through space with a similar motion. The nearest and brightest of these lies only 8 light-years away in the winter sky, Sirius, the Dog Star. Two other well-known asterisms are also physical clusters, both in the winter constellation of Taurus the Bull: the V-shaped Hyades (with the exception of Aldebaran which lies nearer the sun) and the Pleiades. We see these clusters at progressively greater distances (about 150 and 350 light-years, respectively) so their patterns shrink accordingly. Next in line at 500 light-years is M44, the Beehive Cluster in Cancer the Crab, a faint smattering of dusty stars seen in the spring sky. They do not form a true asterism, but if they were as near as, say, the Hyades, we can only imagine what ancient astronomers might have called the pattern they made in the sky.
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