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| © CAMBRIDGE UNIVERSITY PRESS 1998 |
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).
Sa-Sb-Sc
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E0-E7
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SBa-SBb-Sbc