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1.1 The Hubble tuning fork diagram

The first galaxy classification system to have gained more-or-less universal acceptance was due to Hubble (1926) who arranged galaxies in his now famous ``tuning fork'' diagram


Along the normal and barred spiral tines of this diagram the position of a galaxy was determined by nuclear size and spiral arm tilt. The fact that the integrated colors and spectral types of galaxies exhibit a monotonic increase along the sequence E-Sa-Sb-Sc-Ir (Holmberg 1958, de Vaucouleurs 1959b, 1963) strongly suggests that the Hubble sequence is a manifestation of a deep linkage between the evolution and morphology of galaxies. In recognition of theoretical speculations that have long been forgotten, galaxies of types E and Sa are referred to as ``early'' and those of types Sc-Ir as ``late''. Over the past 70 years the Hubble classification system has proved to be enormously useful. However, one of the few major failings of the Hubble paradigm is that it has not yet been possible to incorporate S0 galaxies in an entirely satisfactory fashion. Early proposals to place the S0 type at the intersection between E, Sa and SBa galaxies (Hubble 1936, Sandage 1961, 1975) have not proved to be entirely convincing because S0 galaxies are typically less luminous than either E or Sa galaxies. It is, however, quite possible that there exists a sub-class of S0 galaxies that are truly intermediate between ellipticals and spirals. Furthermore it appears that galaxies may approach the S0 morphology from differing initial states by moving along quite different evolutionary paths (van den Bergh 1990). On balance, it now seems more likely that the apparent chasm between elliptical and spiral galaxies might, at least in part, be bridged by disk-like structures that exist within some ellipticals (Kormendy & Bender 1996).

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