Hubble classification of galaxies. Illustration showing the scheme of classifying galaxies by their shapes as proposed by astronomer Edwin Hubble in 1925. At upper left are elliptical galaxies, classified in eight groups from E0 (spherical) to E7 by increasing elongation. There are two groups of spirals. The upper track shows pure spirals from Sa (tightly wound arms) through Sb to Sc. A similar classification applies to barred spirals on the lower track, from SBa (tightly wound arms) through SBb to SBc. Though subject to constant modification, this system is still used as the basis of describing galaxy morphology.