The Cosmic Century

The galaxies

The Hubble sequence of galaxy types shown in Figure 5.5 gives some impression of the diversity of forms found among the galaxies. Hubble planned to publish an atlas of galaxies illustrating the different galaxy types but, although all the plates for this project were taken with the 60-inch and 100-inch telescopes by 1948, he died in 1953, before what became the Hubble Atlas of Galaxies was published. The project was completed by Allan Sandage, who was Hubble's last research assistant, and it was published in 1961 (Sandage, 1961b). The basic Hubble sequence was preserved, including the S0 galaxies, and the irregular galaxies were placed at the end of the sequence.

The morphological classification of large samples of galaxies was pursued by Antoinette (1921–1987) and Gérard de Vaucouleurs (1918–1995), who published a series of Reference Catalogues of Bright Galaxies, in which the Hubble classification was refined, the basic linear sequence being preserved (de Vaucouleurs et al., 1991). The distinction between the normal and barred spirals was maintained, but they showed that all intermediate types between pure barred spirals and normal spirals are also observed. What gave this morphological scheme physical significance was the fact that certain physical properties of galaxies are correlated with their position along the sequence.