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3.2. Secondary Parameters: the "Form Family"

The next stage has been the introduction of a secondary parameter, expressed in terms of "form families"; the first four of these are related to the classification of Hubble; they are:

S = Spirals.
B = Barred spirals.
E = Ellipticals.
I = Irregular systems.

To these have been added four additional "form families":

Ep =

Elliptical systems containing well-marked dust absorption.

D

= Systems showing rotational symmetry without pronounced spiral or elliptical structure. This category includes such well-known "dustless" systems as NGC 4111 and 4762; in addition, non-spirals containing symmetrically distributed absorption from dust clouds are also placed in this class.

L =

Systems of low surface brightness. This is the least precisely defined of all of the form families, and only a very few galaxies are assigned to this class in Table I. However, large numbers of galaxies of this category arc known to exist, and a later investigation will deal with their classification. The determination of the actual critical value of the surface brightness for inclusion in this class can best be postponed until a time when a large number of these objects have been classified. It should be noted that the L systems - together with those of class D - require precise investigation (photometric and spectroscopic) of their stellar populations before the form classes can be interpreted accurately in terms of stellar constitution. In this respect, they differ from galaxies of classes I, S, B, and E, where the spectral classification carried out by Morgan and Mayall is sufficient to draw general conclusions concerning the stellar populations.

N =

Systems having small, brilliant nuclei superposed on a considerably fainter background.

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