M3

A loose Globular Cluster in Canes Venatici, Messier 3 lies near the southern border of that constellation, where it meets Boötes. It is located to the northwest of the bright star Arcturus, but for most observers the cluster is too faint to be seen by the naked eye. Nonetheless, it is easily resolvable by even a modest telescope, and is considered one of the northern sky's richest star clusters.

M3 lies outside the Galactic disc, a little under 34,000 light years (or about 10,000 parsecs) from the Solar System. As is typical for Globular Clusters it is ancient: about 8 billion years old. The entire cluster is about 90 light years in diameter, but its loose structure means that the core of the cluster is only some 5 light years across. M3 is noted for its high population of variable stars, especially of the pulsating RR Lyrae type. Variables of this type are cosmic 'standard candles' that help establish the distance to the cluster.