Glossary
Galaxy Classification Schemes

For (primarily) historical reasons, the basic scheme is

• E: Elliptical Galaxies
  Classification Scheme:
  • En, where
    n=10(1-b/a) and b/a is the ratio of minor to major axes.
    Thus
    • E0 galaxy has b/a ~ 1 (and hence is round)
    • E7 galaxy has b/a ~ 0.3 (and is the most flattened galaxy generally classified as an Elliptical)
  • xEn, where
    • x=g represents a giant Elliptical
    • x=d represents a dwarf Elliptical
• S: Spiral Galaxies
  Classification Scheme:
  • S0, also commonly refered to as a Lenticular galaxy.
  • Syz,where
    • y is not included for a spiral without a bar
    • y=B when the spiral contains a bar
    and
    • z=a when the spiral arms at least dominant
    • z=d when the spiral arms at most dominant
  However there are intermediate objects (ie between "normal" spirals and "barred" spirals), thus there's also a common notational scheme, using
  • SA for unbarred spirals
  • SAB for transitional cases
  • (& then continuing onto SBz spirals as above)
  Finally there are the
  • dSph which are used to denote dwarf Spheriod galaxies
• Irr: Irregular Galaxies
  Classification Scheme:
  • IrrI
  • Im
  • IBm (barred)
  • Irr II
  • Irr0
  Finally there are the
  • dIrr: dwarf Irregular galaxies

• Sandage & Tammann (1987) A Revised Shapely Ames Catalog of Bright Galaxies
• da Vaucouleurs et al. (1995) Third Reference Catalogue of Bright Galaxies