Star cluster

A star cluster is a group of stars, that share an origin, formed at roughly the same time and held together by self-gravitation. Two main types of star clusters can be distinguished: globular clusters, tight groups of ten thousand to millions of old stars which are gravitationally bound; and open clusters, less tight groups of stars, generally containing fewer than a few hundred members. If a star cluster is obscured in visible light due to extinction, but visible in infrared light, it is classified as an infrared cluster.

As they move through their galaxy, over time, open clusters become disrupted by the gravitational influence of giant molecular clouds, so that the clusters observed are often young. Even though no longer gravitationally bound, they will continue to move in broadly the same direction through space and are then known as stellar associations, sometimes referred to as moving groups. Globular clusters, with more members and more mass, remain intact for far longer and the globular clusters observed are usually billions of years old.

Star clusters within the Milky Way that are visible to the naked eye, include the Pleiades and Hyades open clusters, and the globular cluster 47 Tucanae.

Open cluster

Open clusters are confined to the galactic plane, and are almost always found within spiral arms. They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as Messier 67 (the closest and most observed old open cluster) for example. They form in H II regions such as the Orion Nebula.