Type of Galaxies.

taken from the Hubble Atlas of Galaxies.

ELLIPTICALS. These galaxies display no bright blue stars like those in the spiral arms of Sb, Sc, Irr, SBb, and SBc galaxies. The images of these galaxies display complete roational symmetry. A computation shows that the true ellipticities of E galaxies are distributed nearly uniformally from E0 (round) to E7 (elongated), in the true classes. The points to note are: (1.) The smooth texture of the photographic image. (2.) The smooth gradient of surface brightness from the nucleus outward. (3.) The absence of dust. (4.) The lack of any suggestion of resolution into the bright blue stars or knots. SO Galaxies: Have symmetrical forms which are flatter than E7, but which show no spiral structure and no trace of bar structure. The characteristic features are a bright nucleus, a central lens surrounded by a faint and sometimes extensive envelope, and in the later stages of the section, circular absorption lanes that appear to form a transition between the E types and the true spirals. SPIRALS. Normal spiral galaxies emerge from the SO sequence. On this basis, the spiral sequence is divided into three subsections: Sa, Sb, and Sc. There is a smooth transition between the sections, and the boundaries are somewhat arbitrary. Sa Galaxies: The first true spirals, they show tightly wound spiral patterns of obscuration, and may or may not ave tight wound spiral arms of luminous matter. The arms are invariably smooth, with no no resolution into stars. Sb Galaxies: Have definte and pronounced spiral arms which are tightly wound. There is, however, considerable extension along the sequence from early Sb to late Sb, and classification at both extremes is difficult. The Sb spirals are separted into two major divisions....ones having large amphorus nuclear regions, and ones having small nuclear regions with the intermediate Sb spiral pattern. Sc Galaxies: Dominance of multiple spiral arms and small amorphous nuclear regions are the characteristic of Sc galaxies. These features and the high degree of resolution of the open, branched-arm system make Sc galaxies easy to recognize. The Sc galaxy exhibit great diversity in form. These groups in general represent development along the sequence of classification from early to late Sc. BARRED SPIRALS. The SBO class represents a transitional stage between elliptical galaxies and the true barred spiral....there are three subtypes: SBO1- The bar in this system is a broad indistinct region whose surface brightness is higher than that of the surrounding lens. It is inclined at a random angle to the major axis of the projected image. SBO2- Thsi galaxy's bar structure does not extend completely across the face of the underlying lens. The are two diametrically opposite regions of enhanced luminosity on the rim of the lens which, together with the nucleus, constitute the bar. SBO3- The bar of this subtype extends completely across the face of the lens, it is narrow, well defined and bright. SBa- Galaxies possesss a characteristic bar that is prominent and smooth in texture with no trace of resolution into knots or stars. No dust lanes are present as in the SBb(s). Spiral arms first appear in the SBa..they are usually closely coiled about the central lens and bar; they are often faint and inconspicous, and usually smooth in texture, although traces of partial resolution begin in the very late SBa's. The arms can either begin tangent to an internal ring [(r) subtype], ot can spring from the ends of the bar [(s) subtype]. This division into (r) and (s) groups is not prominent in the SBa, but is dominent feture of the SBb and SBc galaxies (ed note: Most Irr systems designated by Hubble have been revised to include Sd, Sdm, Sm and Im. These galaxies form an important extension. Most galaxies of this type are of low surface brightness and have lower luminosities than the earlier type spirals. Contrarily, a galaxy designated BCD (blue compact dwarf) is actually in close association with a ImIII systems [ie. The Im of the highest luminosity] and are probably their closest associates). SBb- These galaxies have a well defined bar structure which is smooth in texture with no hint of resolution into stars. There are two subgroups: The arms of the SBb(s) group spring from the end of the bar at right angles. Two straight dust lanes in the bar turn sharpely at the end of the bar and follow the the inside of the spiral arms. The spiral arms of the SBb(r) group, start tangent to an internal ring on which the bar terminates. No dust lanes are present in the bar. There are some transitions that exist between the (s) and (r) subgroups and between the SBb(s) and the Sb types. SBc- The characteristics of SBc galaxies are: (1.) The high degree of resolution of the bar and of the spiral srms and luminous lumps; (2.) The openness of the spiral arms. As in the SBb, the SBc can be separated into two subgroups (r) and (s), although the pure ringed type is not common. Most central rings are broken into several segments. (ed. note: Another classification was made by Sidney van den Bergh (ApJ., _131_, 215 [1960] and ApJ., _131_, 215,p. 558 [1960]) From"Galaxies and The Universe, Vol., 9 of Stars and Stellar Systems" (G. Kuiper, ed.).... He indicated that galaxies of the highest luminosity have the longest and most highly developed arm, whereas fainter systems have poorly developed arms. The result is equivelant to stating that the appearance of galaxies vary systematically as one proceeds from left to right within the scatter of redshift-apparent magnitude diagrams for field galaxies. By inspecting the character of the spiral arms alone, van den Bergh was able to divide Sb types into five luminosty classes [I, I-II, II, II-III, and III], which with subsequent calibration showed were of a value of ~0.5 mag. apart in [Mpg.] The Sc-Irr systems could be divided into eight half sections [I-V, with intermediates]. Both calibrations of the types begin at -20 Mpg [absolute], and proceed downward [brighter]) [ie. -20.0, to -19.4, etc.])