11.1.2. Classification Systems
Normal galaxies have been classified under several schemes. The Hubble classification (Sandage, 1961), based on the morphological features visible on blue-sensitive plates, is perhaps the most widely used. This scheme, which divides spiral galaxies into three classes - Sa, Sb, Sc - depending on the openness of the spiral features, has been extended by Holmberg (1958) and the de Vaucouleurs (1964), and has much to recommend it, in that it would seem to represent a real separation in terms of the evolutionary parameters (e.g., fractional hydrogen content) of the galaxies. This is not to say that the series is an evolutionary sequence. Indeed, unless there can be considerable accretion of mass by a galaxy it would appear impossible for evolution to take place along the series from the irregular and Sc galaxies (which, on average have the highest fractional hydrogen content) to the more massive Sa spirals and ellipticals.
Equivalent classifications under these morphological systems are set out in the introduction to the de Vaucouleurs' (1964) Reference Catalogue of Bright Galaxies.
The Morgan (Yerkes) (1958, 1959) classification is a valuable complementary alternative to the Hubble classification. In this system the parameter a - k denotes increasing central concentration of light, while the luminosity is progressively from A- to K-type stars (Morgan and Mayall, 1957). Although the early-type (Sa) spirals tend to have a higher degree of central condensation and the Hubble type correlates well with the integrated color of galaxies (Holmberg, 1958), there is not a one-to-one correspondence between the Hubble and Morgan classifications. A further classification system for late-type spirals (Sc and Sb) is that of van den Bergh (1960a, 1960b), based on a strong correlation between the absolute luminosity and the degree of development of the spiral arms.
These classification systems are interrelated, but not exactly equivalent, and each is useful in discussing the properties of spiral galaxies.