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Object Name: Messier 5
Alternative Designations: NGC 5904
Object Type: Class V Globular Cluster
Right Ascension: 15 : 18.6 (h:m)
Declination: +02 : 05 (deg:m)
Distance: 24.5 (kly)
Visual Brightness: 5.6 (mag)
Apparent Dimension: 23.0 (arc min)
Locating Messier 5: Finding Messier Object 5 in binoculars is much like finding M3 – the key is the Arcturus star and the secondary star hop is Antares. You’ll find M5 about 1/3 the distance between Alpha Bootes and Alpha Scorpii. For finderscopes, place Arcturus in the center and look for bright 109 and 110 Virginis to the southwest. To the east you’ll see a small triangle of stars – aim there. In binoculars M5 is bright and easy, even under urban skies, but difficult to resolve because it is so dense. Small telescopes will also have difficulty resolving this globular cluster, but will begin to pick out edge stars and notice that its shape is not quite round. Larger aperture telescopes will easily begin resolution and notice that nearby 5 Serpentis is also a double star!
What You Are Looking At: M5 is believed to be one of the oldest globular clusters with a calculated age of 13 billion years, and it contains 105 known variable stars – as well as a dwarf nova. The brightest and most easily observed variable star in M5 changes from magnitude 10.6 to 12.1 in a period of just under 26.5 days – Variable 42. Keep a watching for it. Oddly enough, two millisecond pulsars were also discovered in 1997 by S. B. Anderson et al over a five year period of observations – another first! At a distance of 24,500 light-years and stretching across 165 light-years of space, this magnificent object so dominates its territory that it would gather in any stars straying within 400 light-years of its tidal influence!
History: While Gottfried Kirch and his wife Maria were watching a comet on May 5, 1702, they stumbled across a huge, bright object that they considered a “nebulous star.” Forty-two years later, it was found again by Messier who labeled it as M5: “The night of May 23 to 24, 1764, I have discovered a beautiful nebula in the constellation of Serpens, near the star of sixth magnitude; the fifth according to the catalog of Flamsteed. That nebula doesn’t contain any star; it is round, and could have a diameter of 3 arc minutes; one can see it very well, under a good sky, with an ordinary [non-achromatic] refractor of one foot [FL]. I have observed that nebula in the Meridian, and I have compared it to the star Alpha Serpentis. Its position was right ascension 226d 39′ 4″, and its declination 2d 57′ 16″ north. On March 11, 1769, at about four o’clock in the morning, I have reviewed that nebula with a good Gregorian telescope of 30 pouces, which magnified 104 times, and I have ensured that it doesn’t contain any star.”
Well, thank heaven for William Herschel! Some 27 years later he counted up to 200 resolvable stars in this globular cluster and reported “”With a magnifying power of 250, it is all resolved into stars: they are very close, and the appearance is beautiful. With 600, perfectly resolved. There is a considerable star not far from the middle; another not far from one side, but out of the cluster; another pretty bright one; a great number of small ones. Here we have a case where the penetrating power of 20 fell short, when 29 resolved the nebula completely. This object requires also great magnifying power to shew the stars of it well; but that power had before been tried, in the 7-feet, as far as 460, without success, and could only give an indication of its being composed of stars; whereas the lower magnifying power of 250, with a greater penetrating power, in the 10-feet instrument, resolved the whole nebula into stars. I counted about 200 of them. The middle of it is so compressed that it is impossible to distinguish the stars.”
Enjoy your observations and keep watching for Variable 42!
B&W image thanks to Palomar Observatory, courtesy of Caltech, and Hubble Image courtesy of NASA.