M 13 - NGC 6205 - Great Hercules Cluster
First discovered by Edmond Halley in 1714, this impressive globular cluster lies some 25,100 light years away, which is relatively close. It contains, some say, more than a million stars, and estimated to have been formed 12 billion years ago. This is the largest in the northern hemisphere. In 1974 the Arecibo radio telescope transmitted a message towards M 13. The only problem with this was, of course, that the cluster will no longer be there when the message arrives in about 2,500 years!
M 5 - NGC 5904
This globular cluster is in the constellation Serpens. About 24,500 light-years distant from Earth, it is estimated to contain as many as 500,000 stars. Globular clusters are fairly common and in the Milky Way there are about 158 of them with about 10 to 20 more that are undiscovered.
M 3 - NGC 5273
Discovered by Charles Messier in 1764, this globular cluster in one of the three brightest in the northern sky. Containing about half a million stars, it is 33,900 light-years from us. It is said to be roughly 8 billion years old and contains mostly old (red) stars. This is just a Luminance integration, awaiting a chance for me to add R,G,B data to get some colour at some later date.