A stellar association is a loose grouping of young, hot stars, born more or less at the same time, with a total mass of 100 to 1,000 solar masses; it lacks enough collective gravity to hold itself together against the individual motions of its stars and is doomed to disperse in about 10 million years.
Stellar associations are concentrated along the spiral arms of our galaxy and come in several types. OB associations contain predominately high-mass stars of spectral types O to B2. R associations are characterized by medium-mass stars. T associations are made up numerous lower-mass T Tauri stars. All three types may be found together. The internationally-approved designation for associations is the name of the constellation followed by an Arabic numeral; for example, Perseus OB2.