Since one of the criteria used by Hubble for his form classification consists of the degree of central concentration of light, it might be thought that the classical Hubble system would be satisfactory for our present purpose; however, the recent investigation of Humason, Mayall, and Sandage (2) shows that the correlation between Hubble's types and spectral class is not close for certain kinds of galaxies. For example, the spectral types of systems classified by Hubble as Sc and SBc range from A5 to G5. There are correspondingly large ranges for the spectra of Sa and Sb systems.
This lack of close agreement between the spectra and the types according to Hubble should not be construed as a criticism of the latter; Hubble's system was formulated according to certain requirements that he had in mind; it satisfied those requirements and was - and still is - a most useful classification system. Current ideas on stellar populations, which originated in the work of Baade, came later; the interpretation of integrated spectra in terms of these stellar populations came later still.
The purpose, therefore, of the present reclassification can be formulated as follows: to make a preliminary attempt to devise a form classification that shall be as closely correlated as possible with the stellar population of the inner, brighter parts of galaxies; the new classification is to be based on the results of the interpretation of composite spectra.
2
M. L. Humason, N. U. Mayall, and A. R. Sandage,
A.J., 61, 97, 1956.