Figure 23-11: Hubble’s Tuning Fork Diagram Edwin Hubble’s classification of regular galaxies is shown in his tuning fork diagram. An elliptical galaxy is classified by how flattened it appears. A spiral or barred spiral galaxy is classified by the texture of its spiral arms and the size of its central bulge. A lenticular galaxy, is intermediate between ellipticals and spirals. Irregular galaxies do not fit into this simple classification scheme. The ordering of galaxies does not represent an evolutionary sequence.