Galaxy morphological classification
The de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959. De Vaucouleurs argued that Hubble's two-dimensional classification of spiral galaxies—based on the tightness of the spiral arms and the presence or absence of a bar—did not acceptably describe the full range of watched galaxy morphologies. Especially, he argued that rings and lenses are important structural components of spiral galaxies.