Hubble classification

In astronomy, scheme for classifying galaxies according to their shapes, originally devised by US astronomer Edwin Hubble in the 1920s.

Elliptical galaxies are classed from type E0 to type E7, where the figure denotes the degree of ellipticity. An E0 galaxy appears circular to an observer, while an E7 is highly elliptical (this is based on the apparent shape; the true shape, distorted by foreshortening, may be quite different). Spiral galaxies are classed as type Sa, Sb, or Sc: Sa is a tightly wound spiral with a large central bulge, Sc is loosely wound with a small bulge, and Sb is in between. Intermediate types are denoted by Sab or Sbc. Barred spiral galaxies, which have a prominent bar across their centres, are similarly classed as type SBa, SBb, or SBc, with intermediates SBab or SBbc. Lenticular galaxies, which have no spiral arms, are classed as type S0. Irregular galaxies, type Irr, can be subdivided into Irr I, which resemble poorly formed spirals, and Irr II which cannot be classified because of disturbance.

The Hubble classification was once believed to reveal an evolutionary sequence (from ellipticals to spirals) but this is now known not to be the case. Our own Milky Way is a spiral galaxy, classified as type Sb or Sc, but may have a bar.