Messier 3 is one of the finest examples of a globular cluster. It is a collection of around 500,000 stars that have condensed into a sphere due to gravity within the cluster. It is one of the brightest and largest clusters spotted to date. It is estimated to be at least eight billion years old and is one of around 150 globular clusters that orbit our galaxy. Globular clusters are some of the oldest objects in the universe.

This cluster is a spectacular sight in a telescope, hard to see with the naked eye, though it can be made out on a particularly dark night, but when in the eyepiece it is a real jewel of the night sky. It has been much studied as it contains an unusually large number of variable stars, with 274 identified so far, more than any other known globular cluster.

Although itself only 180 light years in diameter, due to its high mass (about 450,000 solar masses, 9 x 10³⁵ kg) and very large tidal radius, the cluster gravitationally dominates a sphere around 760 light years wide. On the other hand, despite its large diameter, half of its mass is contained within a sphere of radius only 22 light years, which is why the cluster looks so dense towards the centre and then sparser towards the edges.

M3 contains mostly old red stars, hence its yellowy colour, but it also contains a relatively large number of ‘Blue Stragglers’, blue main sequence stars which appear much younger than the majority of the cluster. These are formed by stellar interactions as they pass through the dense centre of the cluster.