Source: arXiv:0911.4480v2 [astro-ph.CO]
In 1936 Edwin Hubble invented a morphological classification scheme for galaxies that is now known as Hubble sequence. It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented.
Hubble sequence. Source: Wikimedia Commons. Credit:Ville Koistinen
Hubble’s scheme divides galaxies into 3 broad classes based on their visual appearance: Elliptical (E), lenticular(S0) and spiral (S). Spiral galaxies can also present a central bar and be classified as spiral barred (SB). All galaxies that don't fit on these classes are called irregular like the Magellanic Clouds.
Two astronomers, Andrew J. Benson and Nick Devereux, have now used the alform semi-analytic model of galaxy formation is used to explore the mechanisms primarily responsible for the three types of galaxies seen in the local universe: bulge, bulge+disk and disk, identified with the visual morphological types E, S0/a-Sbc, and Sc-Scd, respectively.(read more)