The Classification and Morphology of Galaxies

Abstract

The discovery of galaxies as such goes back to 1924. As a result of observations he made at the 2.5 m telescope at Mount Wilson, Edwin Hubble demonstrated definitively that certain nebulae do not form part of our Galaxy but are ‘island universes’, independent conglomerations of stars, gas, and dust. The historical confusion between the nebulae of ionized gas in the Milky Way and the external galaxies derives from the use of general catalogues, such as Messier’s (1794), which contains 39 galaxies out of 109 nebular objects or clusters of Galactic stars, and especially the New General Catalogue (NGC)(Dreyer 1890), which contains 7840 objects, of which 3200 are galaxies, and the Index Catalogue (IC)(1895–1910), which contains 5836 objects, of which 2400 are galaxies. The Harvard catalogue (Shapley and Ames 1932) contains only the brightest galaxies, with apparent magnitudes m < 13 (1249 objects in total). The apparent magnitude is equal to −2.5 log (luminosity). The eye can only perceive stars up to a magnitude m = 6. Up to an apparent magnitude m = 17.5 there are 500 000 galaxies, and up to m = 23, 10⁹. The first step in getting to know the galaxies better is to describe the various types and classifications. We shall see throughout the book how important the classifications are, the different types of galaxy corresponding to different formation mechanisms and different environments.