ABSTRACT
Yerkes classification scheme of galaxies A classification scheme of galaxies, conceived by W.W. Morgan and based on the central concentration of light, which closely correlates to the stellar population of a galaxy. The concentration class of a galaxy is indicated with the same letter used for stellar spectral types, but written in lower case (i.e., k for ellipticals and a for Sc spirals whose spectrum is dominated by K giants and A stars, respectively). A second parameter describes the “form family” of galaxies: S indicates a spiral; B a barred spiral; E an elliptical; I an irregular; R a galaxy having rotational symmetry without prominent spiral arms (an S0 galaxy according to the Hubble scheme); and D indicates an elliptical galaxy with an extended envelope. A third symbol (a number from 0 to 7) describes the apparent flattening of a galaxy. For example, M31 is classified as kS5. Albeit no longer widely in use, the Yerkes scheme is still used to denote supergiant elliptical galaxies often found at the center of clusters and groups of galaxies (cD galaxies).
