M3 (NGC5272) M3 is one of the finest globular clusters. It was originally discovered by French astronomer Charles Messier in 1764 and was included in his catalog of nebulous objects that could be mistaken for comets. It was not until some 20 years later that William Herschel resolved individual stars and correctly classified M3 as a star cluster.
M3 is located in the constellation Canes Venatici, and so is best seen in the spring. Under a very dark sky, M3 could be glimpsed visually, as its visual magnitude is 6.2. It is an easy target for any telescope or binoculars.
M3 is located in the halo of our Milky Way galaxy at a distance of approximately 33,900 light years. This website offers a diagram showing the approximate location of all the Messier globular clusters:
http://www.galaxies3d.org/starcl-overview.htm . M3 has a diameter of about 180-220 light years and contains an estimated 500,000 stars.
Globular clusters are typically composed of very old stars, which is also the case with M3. Age estimates for the cluster vary from 8 billion to over 11 billion years old. By comparison, our Sun is a relatively youthful 4.6 billion years old. M3 contains a large number of Blue Straggler Stars, which appear to be much younger than the predominant stars in the cluster. These stars are thought to have undergone changes due to gravitational interactions with other stars, getting their cooler outer layers stripped away in close encounters in the dense central region of the cluster.
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