|| © CAMBRIDGE UNIVERSITY PRESS 1998
The Hubble classification system provides an excellent framework for the
classification of galaxies in the field and in nearby small clusters.
However, it is less useful for the classification of galaxies in rich
clusters and for the classification of galaxies that are viewed at large
look-back times. Particular problems are:
- The evolutionary relationships between E, dE and dSph
galaxies are not yet understood.
- The Hubble system does not provide criteria for objectively
distinguishing between late-type spiral and irregular galaxies, nor does
it provide for an unambiguous and clear-cut way of distinguishing E and
- Elliptical galaxies are classified on the basis of
their apparent axial ratios.
- The Hubble/de Vaucouleurs/Sandage classification
systems do not
distinguish clearly between the effects of luminosity and of mean
population age within the sequence Sc-Sd-Sm.
- The original Hubble system ignores the dichotomy
between normal and barred irregulars.