3.1. The Terminology for the Fundamental Parameter

A method similar to that formerly employed to express stellar colors has been followed: irregular systems and spirals having little or no central concentration of light are denoted by population group "a"; this is based on the correlation found by Morgan and Mayall ⁽¹⁾ between the forms and spectra of these galaxies; in the violet spectral region, the luminosity is due principally to B-, A-, and F-type stars. At the opposite end of the fundamental sequence lie the giant ellipticals and spirals of most extreme central concentration of light; these are denoted by population group "k"; here, the spectra indicate that, on the average, the principal contribution to the luminosity of the inner, brightest part in the violet region comes from giant K stars. These two points define the extremes for the fundamental parameter.

By a process of simple inspection, other groups are interpolated between the two extremes; the single criterion of central concentration of luminosity is utilized exclusively. This operation therefore results in a one-dimensional classification. The intermediate categories are denoted in a similar manner: "af," "f," "fg," "g," "gk." In each case, there is an approximate average equivalent with the violet spectral type; however, the "population group" is determined entirely from the form.