Note 10 — PART I. CLASSIFICATION OF NEBULAE

“Early” and “late,” in spite of their temporal connotations, appear to be the most convenient adjectives available for describing relative positions in the sequence. This sequence of structural forms is an observed phenomenon. As will be shown later in the discussion, it exhibits a smooth progression in nuclear luminosity, surface brightness, degree of flattening, major diameters, resolution, and complexity. An antithetical pair of adjectives denoting relative positions in the sequence is desirable for many reasons, but none of the progressive characteristics are well adapted for the purpose. Terms which apply to series in general are available, however, and of these “early” and “late” are the most suitable. They can be assumed to express a progression from simple to complex forms.

An accepted precedent for this usage is found in the series of stellar spectral types. There also the progression is assumed to be from the simple to the complex, and in view of the great convenience of the terms “early” and “late,” the temporal connotations, after a full consideration of their possible consequences, have been deliberately disregarded.