Hubble classification

Hubble classification. A method devised by Edwin Hubble (1889-1953) of classifying galaxies according to their shape (see illustration). The scheme classifies elliptical galaxies on a scale from E0 for a circular disc, through E1, E2, and so on, to E7 in order of increasing elongation. Spirals are designated as Sa, Sb or Sc in order of increasing openness of the arms and decreasing size of the nuclear bulge in relation to the overall size of the galaxy. There is a parallel sequence for barred spirals, which are designated SBa, SBb or SBc. Galaxies that are neither elliptical nor spiral in form are designated Ir for "irregular". Hubble suggested in 1925 that a transitional type, S0, was the "missing link" in an evolutionary chain from E0 through to the open spirals Sc and SBc. This progression is no longer accepted as an evolutionary sequence, but Hubble's classification continues to be widely used as a simple way of describing the shapes of galaxies.