Standard guidebooks will tell you the Great Hercules Cluster, Messier 13, ranks as the finest globular cluster north of the celestial equator. But sooner or later… maybe tonight… you’ll discover the dazzling globular cluster M5 in the constellation Serpens, the Serpent. A little brighter than its counterpart in Hercules, this tight collection of 500,000 stars sparkles in the eyepiece of a small telescope like an electric arc.
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This cluster is a splendid object in a large telescope, with a tight sparkling core and innumerable tiny stars spraying throughout the halo in all manners of lines and arcs. A 4″ to 6″ telescope gives a lovely view as well, with fewer resolved stars but with an “electric spark” appearing to emanate from the core of the cluster. Most observers agree the cluster is perceptibly non-circular, or at least non-symmetric, with the core appearing somewhat brighter to the north.
You will find the cluster about 7.5º southwest of alpha Serpentis, the brightest star in Serpens. See the map above.
Charles Messier, who included the cluster in his famous list, could not resolve the cluster in his tiny telescope and believed the cluster was a round nebula. William Herschel had a better telescope and became one of the first to enjoy the cluster’s true nature.
Like all globular clusters, M5 is one of the elder statesmen of the Milky Way with an age of some 13 billion years. M5 is one of the oldest globular clusters and, at a diameter of 130 light years, one of the largest . The cluster is 24,000 light years away.
When you get M5 in your sights, don’t forget to look for a bright double star just 1/3 of a degree south and within the same low-power field of view. The star is 5 Serpentis, also known as Struve 1930. The primary star shines at magnitude 5 and its 10th-magnitude companion lies about 11” to the northeast. The difference in brightness makes this a good challenge of your observing skills. A small scope can split the stars at 70-80x or more, but the brighter star tends to overwhelm the fainter.