group of physically associated stars bound by gravitation and presumably having a common origin. Associations (loose collections of usually young stars), open (or galactic) clusters and globular clusters belong to this group.
A grouping of anywhere from a dozen to a million of stars which formed at the same time from the same cloud of interstellar gas. Stars in clusters are useful to aid our understanding of stellar evolution because they are all roughly the same age and chemical composition, and lie at roughly the same distance from the Earth.
cluster of stars held together by the mutual gravitational attraction; an open cluster is a set of a few hundreds or thousands of stars, with an irregular shape. A globular cluster is more compact, with a spheroidal shape, and it can contain up to hundreds of thousands of stars.
Star clusters are groups of stars which are gravitationally bound. Two distint types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters 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.