Classification Of Galaxies
Hubble classified the galaxies he observed according to their shape. His scheme is still in use today. The basic regular shapes are elliptical and spiral. He classified galaxies with no regular shape as irregular galaxies. Galaxies that basically look like either elliptical or spiral galaxies but have some unusual feature are classified as peculiar galaxies. They are classified according to the closest match in the classification scheme then given the added designation peculiar (pec). Hubble initially thought that his classification scheme represented an evolutionary sequence for galaxies; they started as one type and gradually evolved into another type.
Modern astronomers have supplemented Hubble's original scheme with luminosity classes. The luminosity of a galaxy is its total energy output each second. Note that the luminosity refers to the intrinsic energy output of the galaxy corrected for the distance of the galaxy. Therefore a high luminosity but distant galaxy might appear fainter than a nearby low luminosity galaxy. The luminosity classes are the roman numerals I, II, III, IV, and V. The most luminous galaxies are class I, and the least luminous are V. As one might guess, the more luminous galaxies are generally larger in size and contain more stars.
How common are the various types of galaxies? In a given volume of space, about one third of all the galaxies (34%) are spirals, a little over half (54%) are irregulars, and the rest (12%) are ellipticals. However irregular and elliptical galaxies tend to be smaller and fainter on the average than spiral galaxies. They are therefore harder to find. Of the galaxies that we can observe the overwhelming majority (77%) are spirals and only 3% are irregular galaxies. The remaining 20% of observed galaxies are ellipticals.