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A stellar association is a loose group of stars with the same age, with metallicities, spectral types, and velocities suggesting a common origin. They are presumed to be the result of a recent molecular cloud's star formation, essentially what is left after an open cluster has spread out and mixed somewhat with surrounding stars. Associations that have been identified are generally in nearby portions of the Milky Way and are somewhat young, before the stars have scattered much, and generally form a moving group, a group of stars near each other in position and peculiar velocity, which can be identified through astrometry over time, e.g., with a similar proper motion. The peculiar velocity is generally a necessary identification factor because two or more associations often overlap within the celestial sphere, and even within the same volume of space. Across the sky, an association can be spread over hundreds of square degrees.
It is thought that most stars are in such associations. Associations are often typed by their stellar class (such as OB associations, with both O-type stars and B-type stars), or by other characteristics, such as R associations (i.e., illuminate a reflection nebula) or T associations (i.e., including T-Tauri stars).