Wednesday, September 21, 2011

Globular Cluster Photo Series (Part 11): M5

Today for your consideration I have a picture of the globular cluster Messier 5 in Serpens Caput, the Head of the Snake. With a diameter of about 165 light-years this is one of the larger clusters of the Milky Way, and its gravitational pull dominates a humongous volume of over 34 and a half million cubic light-years!

Messier 5 in Serpens Caput.
M5's apparent diameter of 23.0 arcminutes is also on the larger end of the scale for Milky Way globulars. This is despite its great distance of about 24,500 light years from Earth. The number of stars it contains is unknown exactly; it is estimated to be above 100,000, and possibly as high as 500,000 which would put it above the more famous M13 in Hercules. 

Being as bright and large as it is, it is perhaps not too surprising that M5 was discovered by the astronomer Gottfried Kirch 62 years before Charles Messier would rediscover it 1764 and make it famous. It contains 105 variable stars, a very large number for a globular cluster. These are what help pin its distance down so exactly.

One interesting thing I noticed about this cluster in comparison to the last cluster I showcased, M56, is the relative dearth of foreground stars in this frame. That's because seeing M56 requires looking through the disk of the Milky Way, while M5 is found by looking outside of it. It really does make quite a big difference!

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