M 3 - A dense Globular Cluster
Messier 3 or NGC 5272 is a galactic globular cluster in the constellation Hunting Dogs in the northern hemisphere. It is about 34,000 light-years away from the solar system, has a diameter of about 125 light-years and is a very large globular cluster with over half a million stars. The cluster contains 212 variable stars (170 of them RR Lyrae stars), the largest number of this star class in our Galaxy.
Out of 186 stars one period could be determined. These type of variable star, just at the low- mass end of the so called "instability band" in the Hertzsprung-Russel Diagram are showing a high frequency radial oscillation that leads to a change in total brightness of the object. They have a period of about 1 day and a nicely observable change in magnitude, which can also be used to calculate distances using methods similar to Cepheids.
The object was discovered by Charles Messier on May 3, 1764 and listed by him in his catalog as the third object and is easily visible with binoculars under a moderately dark sky in spring.