  Our vision of galaxies has changed significantly since the era of large
galaxy surveys like the Sloan, which gave us extensive statistics with millions
of galaxies. The Hubble sequence classification described in Chapter 1 still
remains very widely used but has been enriched with broad categories based on
color that indicate the recent formation of stars: the red sequence of passive
galaxies, consisting solely of old stars, and the blue cloud of galaxies with
active star formation. Chapter 3 focused on galaxies with a dominant spheroid,
which are generally found on the red sequence. One of the key questions about
the evolution of galaxies that remain to be answered is to understand how a
galaxy can suddenly pass from one category to another. This is a
graduate-student level lecture, not a review article.
