OB association

An OB association is a stellar association, a loose group of stars of similar spectral class including O-type and early B-type stars, i.e., OB stars. It is necessarily a young association because such stars do not last long, and such inclusion of O and B stars is a means of identifying relatively young associations. After the OB stars are all gone, remaining stars will constitute a typical stellar association. An example OB association is the Scorpius-Centaurus Association, which is the nearest to us, about 380-470 light-years, thus of interest for observation.