The Hubble Sequence (also called the Hubble 'tuning fork'). Published in 1936 by US astronomer Edwin Hubble (1889-1953), this diagram shows the classification of galaxies (here called nebulae) into different types. This system is still used today. Elliptical galaxies (left) have a subscript number to show increasing elongation. Shown here are the E0, E3 and E7 types. Spiral galaxies are at right, and are separated into normal spirals (upper right, Sa, Sb and Sc) and barred spirals (lower right, SBa, SBb and SBc). Barred spirals have a 'bar' structure in their nucleus, compared to the spherical nucleus of a normal spiral. The subscripts a, b, and c, mark increasingly open spirals. Published in The Realm of the Nebulae (1936).