Star cluster

Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, a more loosely clustered group of stars, generally contain less than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.

Star clusters visible to the naked eye include Pleiades, Hyades and the Beehive Cluster.

Once an open cluster has become gravitationally unbound, the constituent stars will continue to move on similar paths through space. The group is then known as a stellar association, or a moving group. Most of the stars in the Big Dipper are members of a former open cluster, the Ursa Major Moving Group, and have similar proper motions. Other stars across the sky, including Alphecca and Zeta Trianguli Australis, are related to this group. The Sun lies at the edge of this stream of stars at the moment, but isn't a member as is shown by its different galactic orbit, age, and chemical composition.