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1.3 Modifications of the Hubble Scheme

Most modifications of the Hubble classification system (e.g., de Vaucouleurs 1959a, van den Bergh 1960 a,b,c, Elmegreen & Elmegreen 1982) have added features to the original Hubble scheme. However, Morgan (1958, 1959a) devised a classification system that was even simpler than Hubble's original scheme. He classified galaxies mainly on the basis of their central concentration of light. This contrasts with the Hubble system which is based on both the central concentration of light and on the morphology of their spiral structure. Abraham et al. (1994) have shown that a one-dimensional classification system, such as that introduced by Morgan, is particularly well suited to classification of digital images by computers. Furthermore, Morgan's ``Yerkes'' system avoids the conflicts that occasionally arise between the classifications based on arm morphology and those relying on central concentration of light. An example of an object where such a conflict arises is the galaxy NGC 4866 which is assigned to type Sa, even though it has a rather small core. Abraham et al. (1994) also find that Morgan's one-parameter classification system can be used to provide an adequate description of the morphologies of galaxies in very rich clusters. In such clusters the Hubble system provides little resolution because almost all galaxies are of types E and S0.