History

Along with Messier 13 in the constellation Hercules, Messier 3 is the most magnificent globular cluster in the northern starry sky. It was discovered by Charles Messier on 3 May 1764. He described it as a round nebula without a star, becoming darker towards the outside.

Physical Properties

What is remarkable about M 3 is its oval shape. The greatest star concentration is a little southwest of the center of the cluster. Similar to other large globular clusters, some outer stars seem to be arranged along a radial line, a phenomenon that was first noticed by the two Herschels and Lord Rosse. Rosse also found some dark areas in the central mass, similar to M 13. The presence of dark clouds is of great interest as it was generally assumed that globular clusters do not contain dust and gas. If one considers these dark nebulae as foreground objects, however, one could argue that such small dark nebulae could also be distributed all over the sky, but could not be detected due to the lack of a bright background object. However, M 3 lies in an area far away from the Milky Way, which is largely free of interstellar absorption. This suggests that the dark clouds actually belong to the globular cluster itself.

M 3 contains more than 45'000 stars from around 12 mag up to the limit of perceptibility. The number of variable stars is greater than in any other globular cluster observed. The period of most of these stars lasts about half a day, and in some cases the light changes so quickly that it doubles in ten minutes. Stars of this type, called cluster variables, appear to be a subclass of the known Cepheids. The typical example is the star RR Lyrae. Using the period-luminosity relationship, a distance of 31'000 light years could be determined. ^{[4, 100]}