Luminosity classes in Sb galaxies are also assigned on the basis of the degree of order in the arm pattern. But unlike Sc galaxies, there are no entirely chaotic Sb forms. Hence the luminosity classes do not extend beyond SbIII in the RSA and in van den Bergh's (1960c) original list. There are no dwarf Sb galaxies. The physical reason for this may be the presence of the large central spheroid in Sb's and its absence in Sc, Sd, and Sm galaxies. This stronger central mass concentration changes the nature of the variation of rotational velocity with radius and hence, presumably, the character of the spiral pattern. Clearly the central spheroid causes a more ordered spiral pattern. In this regard note that the arms in Sa galaxies are generally even more regular than in Sb systems.
The SbI galaxies shown here cover the small range of absolute magnitude -22m.6 < MBT0, i < -21m.8. The arms are thin, regular, and well separated. The interarm region is well defined in the multiple arm cases such as NGC 5985 and NGC 3992. There is, however, considerable branching of the arms, and in several cases two systems of arms (an inner and an outer set) exist. This is particularly true in NGC 6753 and NGC 210, where the inner set is tightly wound close to the central spheroid and the outer set is more open and slightly fragmented.
A particularly interesting galaxy is NGC 3347, with its two principal arms. Note the discontinuous break in the north arm at a bright knot. Notice also that the dust in this arm is not along the inner edge but threads the middle.
The inner arms of NGC 5985 are thin and well formed. On the west side, the outer structures are branched into a series of more radial twigs that clearly have a different pitch angle than the brighter main arm from which they radiate.