5.2. The S Systems (Plates VI-XVI)

This group is arranged in approximate order of increasing relative intensity of the nuclear region. Those illustrated in Plate VI have little central concentration of light and rather indefinite spiral structure; the fS systems illustrated in Plates IX, X, and XI have extremely well-defined spiral arms, together with a nuclear region of intermediate intensity. By the time the kS systems are reached (Plates XIV, XV, and XVI), the nuclear region has become outstanding in relative luminosity and the spiral arms themselves less well marked. M 31 (Plate XVI) seems to resemble most closely such kS systems as NGC 2841 and 4698.