Abstract
Three types of associations are presently recognized. These are OB, R, and T, and represent, respectively, concentrations of O and B type stars, reflection nebulae, and T Tauri stars, in certain regions of the sky. OB and T associations are identified on objective prism plates; R associations may be found using direct plates such as those of the Palomar Sky Survey. All associations are intimately connected with what appear optically as dark clouds and are now detected as sources of molecular line emission and known as molecular clouds. Often, all three types of associations are found within the same cloud complex (eg, Mon OB1). However, there are also examples of T associations (Taurus) and R associations (Mon R2) which are not connected with recognized OB associations.
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