Rotation, Expansion and Duplicity of Be Stars

In recent years there is a general tendency to divide Be stars into different groups. Bidelman's (1976) division on supergiants, rapidly rotating single stars, interacting binaries, early type nebular variables and quasi (or young) planetary nebulae is often quoted. Another example is the classification by Lesh (1968) or a very recent classification by Jaschek et al (1980), limited a priori to “normal” Be stars i.e. stars of luminosity classes V to III. I do not think such an approach is recommendable. There is still no general agreement as to the nature of the Be phenomenon and all such classifications must be more or less descriptive. The possibility that the hydrogen emission observed in the spectra of apparently very different objects has always the same physical cause, and that the differences between these objects are caused only by continuously varying physical and/or geometrical parameters should seriously be considered, along with all other concepts. At the moment, I find it more promising to study Be stars in context of all early type emission objects, including supergiants, P Cyg stars, Beta Cep objects, WR stars, symbiotic stars and novae. Such a viewpoint is not new – it was expressed already by Struve (1942).