© CAMBRIDGE UNIVERSITY PRESS 1998

1.5 Summary

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 SO galaxies.
• 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.