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2. EARLY CLASSIFICATION OF GALAXIES

The most extensive all-sky surveys before the introduction of photography were made by the Herschels from about 1780 to 1860 by visual methods. Star clusters, galactic nebulae, and galaxies were catalogued, and moderately extensive descriptions were given. The General Catalogue of Nebulae, incorporating the observations of 5079 objects, of which 4630 were discovered by the Herschels, was published by Sir John Herschel in 1864 in the Philosophical Transactions. This forms the largest single base for the New General Catalogue (NGC) of Dreyer which, together with the two Index Catologues, incorporates most discoveries of nebulae to 1908. The descriptive symbols used by the Herschels are summarized by Dreyer in the Introduction to the NGC, and by Curtis (1933) in his view of galaxian research to 1934. Although the descriptions do not constitute a classification in the formal sense, they are still valuable as a supplement to the current systems, and have been used as recently as 1956 by de Vaucouleurs (1956) in his survey of southern galaxies.

The faintest structural features of galaxies could be detected only when photographic surveys came into general use about 1890. These features proved to be decisive in the classification problem because the presence or absence of spiral arms is what divides galaxies into the two major groups (E and S), and separates the spirals along a linear sequence by the character of the arm structure.

Wolf's (1908) system was a classification based on photographs taken at Heidelberg in which letters were used for various forms, and in which no distinction was made between galactic planetary nebulae and galaxies. Although the system is not now in general use, it was the first to use a linear sequence which proceeds from amorphous forms with no spiral patterns (type d to k), to fully developed spirals (types r to w). Extensive use of the Wolf classification was made as late as the 1940s. Because of this, and because it provides a more detailed description of the many variations of spiral patterns than does Hubble's, Wolf's sequence is shown in figure 1.

Figure 1

Figure 1. Wolf's 1908 descriptive classification system of galaxies. The top row of his original diagram, which showed galactic nebulae (mostly planetaries), is deleted here.

Although Hubble's system places galaxies in a physically significant linear sequence (or a series of such sequences as in de Vaucouleurs's extension) it is ``simple'' in that it gives no notational recognition to the great variety of spiral arms, as does Wolf`s. For this reason, Danver (1942) preferred the Wolf types and commented: ``As to the Hubble classes, these [appear to constitute] a division along a line of development. This is certainly a great advantage, but if a conception of the appearance of the object is desired, it is better to designate the types according to Wolf.'' It is quite possible that when the theory of spiral structure is more fully developed than at present, a new classification of the arm patterns alone may be needed, and the Wolf scheme might serve as a new point of departure. The modern work of of Vorontsov-Velyaminov (Section 8) may be a step in this direction.

Among the more complete discussions using the Wolf symbols are Reinmuth's (1926) study of the Herschel galaxies, Lundmark's (1927) summary of galaxian research to 1926, Holmberg's (1937) work on double galaxies, Reiz's (1941) study of the surface distribution of galaxies, and Danver's (1942) work on the forms of spiral arms. Danver gives a comparison of the Hubble types with those of Wolf, extending a similar discussion by Shapley and Ames (1932, pp. 68-69). (1)

Lundmark (1926, 1927) proposed a classification based on a major division of galaxies into three groups (amorphous ellipticals, true spirals, and Magellanic Cloud-type irregulars), further dividing the groups according to the concentration of light toward the center. The division into E and S classes is similar to that made by Hubble (1926), but the parameter which divides galaxies within the classes (concentration by Lundmark and the nature of the spiral arms by Hubble) is different. Curtis (1933, appendix 5G), summarizes Lundmark's system, and compares it with the Wolf symbols.

Shapley (1928) also proposed a classification based on concentration, but included the nonintrinsic properties of apparent magnitude and apparent flattening in his notation. The system was used at Harvard in the early stages of the survey of galaxian distribution to mpg = 17 (e.g., Harvard Annals, 88), but is not now in general use.


1 Hubble (1917) used Wolf's classification in his early work at Yerkes. Back

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