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3.4 Finer Subdivision Along the Sequence

Hubble's bin sizes among the spirals were large, and inspection of survey plates showed that they could usefully be narrowed. At various times, the Mount Wilson observers have used combination symbols S0, S0/a, S0a, S0ab, S0b, S0bc, S0c, S0c/Irr, and Irr to divide the linear sequence into nine groups instead of three along both the ordinary (3) and barred families.

Holmberg (1958) has used + and - symbols with the notation a, b-, b+, c-, c+, Irr I to divide the spirals into six groups.

De Vaucouleurs has used both notations in his revised classification by dividing the E's and S0's into early, intermediate, and late bins as E, E+, S0-, S00, S0+, and by using the mixed notation along the spiral sequences as a, ab, b, bc, c.


3 Nonbarred spirals were called ``normal'' by Hubble. De Vaucouleurs (1959a) has emphasized that barred and nonbarred spirals are equally normal because both appear with about the same frequency (de Vaucouleurs 1963b, table 5) if the transition types are neglected. He suggested use of the term ``ordinary'' rather than ``normal,'' and we follow this notation in the remainder of this chapter. Back.