In the late 18th century, Charles Messier was busy hunting for comets in the night sky, and noticed several “nebulous” objects. After initially mistaking them for the comets he was seeking, he began to compile a list of these objects so other astronomers would not make the same mistake. Known as the Messier Catalog, this list consists of 100 objects, consisting of distant galaxies, nebulae, and star clusters.
Among the many famous objects in this catalog is the M5 globular star cluster (aka. NGC 5904). Located in the galactic halo within the Serpens Constellation, this cluster of stars is almost as old as the Universe itself (13 billion years)! Though very distant from Earth and hard to spot, it is a favorite amongst amateur astronomers who swear by its beauty.
At 13 billion years of age, M5 is believed to be one of the oldest globular clusters in our galaxy (over twice the age of our Solar System). Located 24,500 light years from Earth, it is home to more than 100,000 stars, with some estimates saying it has as many as 500,000. Of these, 105 variable stars call M5 home, as does a dwarf nova.
The cluster is also one of the largest known, measuring 165 light-years in diameter and exerting a gravitational influence over a radius of 200 light-years. The brightest and most easily observed variable star in M5 – Cepheid Variable 42 – changes from magnitude 10.6 to 12.1 in a period of just under 26.5 days. Amateur astronomers are encouraged to keep an eye out for it. Interestingly enough, M5 is also home to two millisecond pulsars, which were also discovered in 1997 by S. B. Anderson et al. over a five year period of observations.
Gottfried Kirch and his wife Maria were the first to make a recorded observation of M5 on May 5th, 1702. While searching from comets, they stumbled across a huge, bright object that they considered to be a “nebulous star.” On May 23rd, 1764, Charles Messier found it independently and labeled it as M5. As he recorded of it at the time:
“Beautiful Nebula discovered between the Balance [Libra] & the Serpent [Serpens], near the star in the Serpent, of 6th magnitude, which is the 5th according to the Catalog of Flamsteed [5 Ser]: it doesn’t contain any star; it is round, & one sees it very well, in a fine [clear dark] sky, with an ordinary refractor of 1-foot.”
In 1771, while compiling the first edition of the Messier Catalog, he described the object and his observations of it in more detail:
“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.”
Enter William Herschel, who once again saved the day by seeing this object for what it was. In 1791, he was able to resolve the individual stars and counted up to 200 in this globular cluster. As he wrote of the cluster at the time:
“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.”
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.
Under ideal viewing conditions (i.e. where light pollution is not an issue), M5 can be spotted with the naked eye. It will appear as a faint point of light, located about 5° to the south-east (or 30 minutes to the east) of Alpha Serpentis (aka. Unukalhai). Using binoculars, spotting M5 is easy since it will appear quite bright, even under urban skies. However, it will be 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.
For your convenience, here are the quick facts for Messier 5:
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)
Enjoy your observations, and keep watching for Variable 42!
We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, M1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.
Be to sure to check out our complete Messier Catalog.
For more information, check out the SEDS Messier Database.