2.5. Late-type Galaxies

In Figure 1 spirals are subdivided into barred and ordinary spirals - not ``normal spirals'', as they are sometimes termed. In fact around ~ 65% of spirals have recognizable bars or bar-like features in the B-band, according to de Vaucouleurs (1963). Along the tines of the tuning fork the spirals are further subdivided into classes ranging from early to late-type (Sa, Sb, etc) based on three criteria: (a) the dominance of the bulge; (b) the degree of winding of the arms; and (c) degree of resolution of the arms <sup>(1)</sup> A major weakness of the Hubble system is that the variety of observed spiral structure is poorly encompassed by the system [52]. Important classes are missing, such as the flocculent spirals [39], and aenemic spirals [83] (these latter systems are seen only in clusters, and exhibit faint, ``ghostlike'' spiral structure, suggesting they may be related to S0 galaxies). In fact this deficiency led van den Bergh [82] to propose the important luminosity class extension to the Hubble system, in which arm morphology/surface brightness is used to rank spirals. This system roughly tracks galaxy luminosity, exploiting the fact that most luminous spirals have well-developed, long spiral arms (grand design spiral structure), while lower luminosity systems tend to exhibit poorly developed, disconnected arms.

¹ It is rather unsatisfactory to have the early-late spiral sequence based on three criteria, some of which may be contradictory, rather than on quantitative measures. For example, how does one classify a spiral with a smallish bulge and tight spiral structure (a combination that is not unknown). Is this designated as an early type (based on arm appearance) or late-type (based on bulge prominence), or perhaps classed as peculiar? The answer is that in these case one must look carefully through the Hubble Atlas for similar objects and determine what Sandage chooses to call such a galaxy - the system is subjective and ultimately defined by reference to archetypes.