||The classification of galaxies according to their visual appearance into four basic
types suggested by E. Hubble: ellipticals (E), spirals (S), barred spirals (SB), and
irregulars (Ir). Later on a separate class of lenticulars (S0) was appended as an
intermediate type between ellipticals and spirals. The sequence starts with round
elliptical galaxies (E0). Flatter galaxies are arranged following a number which is
calculated from the ratio (a - b)/a, where a and b are the major and minor axes as
measured on the sky. Ellipticals are divided into eight categories (E0, E1, ..., E7).
Beyond E7 a clear disk is apparent in the lenticular (S0) galaxies. The sequence then
splits into two parallel branches of disk galaxies showing spiral structure: ordinary
spirals, S, and barred spirals, SB. The spiral and barred types are subdivided into
Sa, Sb, Sc, and SBa, SBb, SBc, respectively. Along the sequence from Sa to Sc, the
central bulge becomes smaller, while the spiral arms become more and more paramount.
The original, erroneous idea that such arrangement of the galaxies might represent
an evolutionary sequence led to the ellipticals being referred to as early-type galaxies,
and the spirals and Irr I irregulars as late-type galaxies.