The Hubble sequence classifies types of galaxies based on their morphology. Edwin Hubble was created in 1925. The diagram below represents cons nearby galaxies, called the Hubble sequence: 3% are elliptical, lenticular 15%, 72% spirals, 10% irregular. Elliptical galaxies are denoted by the letter E, followed by a number n representing their perceived degrees of eccentricity. Spiral galaxies are regularly represented in the upper branch and are denoted by the letter S while the lower branch is occupied by the barred spiral galaxy, denoted by the letter SB. The lowercase letter following the S is based on the appearance of spiral structure. Sa and SBa galaxies arm tightened very diffuse and their nucleus is large and bright. Sb and characterize SBb galaxies under arms tightened with a smaller bulb. Sc and SBc galaxies to characterize arms outstretched and cowardly with a bulb even less important. The lenticular galaxies are denoted S0. These galaxies are composed of a bright central bulge and an extended structure shaped disk that surrounds the bulb. Irregular galaxies have no particular structure. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Hubble defines two classes of irregular galaxies. Irr characterizes asymmetric galaxies without central bulge or without spiral. In the extended classification, the Irr are placed at the end of the branch of spiral galaxies.
Image: Image formed by the photographs of galaxies made with Hubble Space Telescope and the Sloan Digital Sky Survey. They were placed in order of classification of the Hubble sequence (E: elliptical galaxies, S0: Lenticular galaxies; Sa_b_c_d: spiral galaxies, Pec: irregular galaxies). The diagram shows the nearby galaxies, called the Hubble sequence: 3% are elliptical, lenticular 15%, 72% spirals, 10% irregular. Credits: HST. SDSS. GEPI