The existence of galaxies external to our own ("Island Universes" as they were first called) was not considered an established fact until the early 1900s. The galactic nebulae (bright diffuse nebulae) and the so-called external nebulae were thought of as one.

Today over a million external galaxies have been counted and we are yet in the early stages of deep space exploration. The astronomer Hubble introduced a system of galaxy classification in 1925 that, with some revision, is still in general use. It recognizes three main classes of galaxies: (1) Elliptical shaped galaxies, (2) Spiral shaped galaxies, and (3) Barred spiral galaxies.

There are also a large group of galaxies that are classified as "Irregular" in shape. Among the Spirals there are three stages Sa, Sb and Sc and these are distinguished according to the relative size of the nuclear or central bulge (decreasing from Sa to Sc) and the relative strength of the arms (increasing from Sa to Sc). Elliptical galaxi6l have a smooth structure from a bright center out to indefinite edges and they differ only in ellipticity, from round (E0) to a 3:1 axis ratio (E7). Spiral galaxies show their typical spiral arms or whorls emerging either directly from a bright round nucleus (ordinary spirals) or at the ends of a diametrical bar (barred spirals). Irregular galaxies are either of the Magellanic Cloud type or chaotic, and difficult to classify in the Hubble method. This method was later revised to include the SO Or lenticular type of galaxy, which shares the smooth structure of the ellipticals, but has a definite, nucleus, occasional interstellar matter, and luminosity similar to the spirals. There are also two varieties of the barred spiral, the classical S-shaped spirals (s) and the ringed type (r) in which the arms start at the rim of an inner ring. Transition types exist between all the main types. There also appear to be large numbers of 'dwarf galaxies' that are small, have low surface brightness, and are difficult to detect. Dwarfs exist only as ellipticals or Magellanic irregulars. The beautiful spirals appear to only occur among the giant galaxies.

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Copyright (c) 1997-99 Michael Erlewine