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See N. J. Mackintosh, Conditioning and Associative Learning (1983).
associationA loose group of young stars of similar spectral type. OB associations are groups of massive and highly luminous main-sequence stars of spectral types O and B. They occur in regions rich in gas and dust in the spiral arms of the Galaxy. They have dimensions ranging from a few parsecs to several hundred parsecs. Often an open cluster is found near the center of an association, e.g. the Zeta Persei association surrounds h and Chi Persei. R associations are groups of bright young stars of slightly lower mass (3–10 solar masses) that illuminate reflection nebulae. T associations are groups of T Tauri stars, i.e. young stars of about the Sun's mass. Most contain less than 30 stars though some contain as many as 400. R and T associations are often found in the vicinity of young open clusters.
Associations are generally too sparsely populated to be gravitationally bound systems and there is strong evidence that they represent the aftereffects of comparatively recent multiple star births. In some cases the stars appear to be expanding from a common center: by extrapolating back their present velocities an estimate of the age of the system can be derived. For instance, the association II Persei shows an expansion age of slightly over one million years.
- any group sharing a common purpose or interest. See also GEMEINSCHAFT AND GESELLSCHAFT.
- (STATISTICS) the degree to which two VARIABLES are related. See CORRELATION.