STELLAR ASSOCIATION

a very large, loose grouping of stars that are of similar spectral type and relatively recent origin. Stellar associations are thought to be the birthplaces of most stars. The stars in stellar associations are grouped together much more loosely than they are in star clusters of the open and globular types. A star cluster's members are bound together by gravity into a relatively tight configuration, whereas an association simply consists of young stars that have not yet had time to move very far from a common site of formation. About 90 percent of all stars exist as members of associations. In the Milky Way Galaxy, the largest number of associations are found in the spiral arms of the galaxy and are located less than 10,000 light-years from the Sun. Stellar associations vary in size but tend to be large. Those near the Sun measure roughly 100 to 200 light-years in diameter, while those elsewhere in the galaxy typically extend about 700 light-years across. Stellar associations contain a relatively small number of stars (from about 10 to a few hundred in most cases), and so their total mass amounts to only several hundred or a few thousand solar masses. Stellar associations are generally classified into three types on the basis of their most prominent components: OB, R, and T associations. OB associations consist largely of very young, massive stars (about 10 to 50 solar masses) of spectral types O and B, which have an absolute luminosity about 100,000 times that of the Sun. In many cases, one or more small open star clusters lie near the centre of such an association. R associations consist of young, bright stars of intermediate mass (3 to 10 solar masses). Stars in this type of association are surrounded by patches of dust that reflect and absorb light from nebulae, and hence these associations are sometimes called reflection nebulae. T associations contain mostly T Tauri stars. These are comparatively cool, newly formed stars of low mass (3 or less solar masses) that are still in the process of contraction. Associations of this kind are thought to be the primary source of low-luminosity stars in the vicinity of the Sun. The stars in stellar associations are typically not more than 10 million years old. Certain stars consume their hydrogen fuel so rapidly that they exhaust it within a million years. The high luminosities of the stars in OB associations suggest that they have such short lifetimes, and according to current astrophysical theory, such stars had to have been created out of interstellar material a very short time ago. Some OB associations, moreover, give evidence of continuing star formation from interstellar clouds within them. A stellar association is thus, in effect, an extremely young group of stars, formed at essentially the same time in the same region of space from a single interstellar cloud. OB associations are places where the most massive stars have formed, while R associations are sites of the birth of intermediate-mass stars and T associations are the birth sites of low-mass stars. Ninety percent of all stars are believed to form in stellar associations, with the remaining 10 percent forming in clusters.