Edwin Hubble introduced a classification scheme for galaxies that is referred to today as the "Hubble Tuning Fork Diagram." This scheme provides for subcategories of both elliptical and spiral galaxies, and introduces two new primary types of galaxies, lenticular and irregular.
Hubble realized that elliptical galaxies could be classified by how round or flat they look. He classified spiral galaxies by how large and bright their central regions are and how tightly their arms are wound. Hubble also noticed that some spiral galaxies have a bright line, or bar, running through their central regions, and called these barred spiral galaxies. A transition type between the elliptical and spiral galaxies, with a central bulge and a disk but no spiral arms, are known as lenticular galaxies. The final classification, irregular galaxies, are neither spiral nor elliptical, and can have any number of shapes.
Elliptical galaxies are designated as type E, with a subclassification based on a measure of the ellipticity of their appearance. In Hubble's classification scheme, an E0 galaxy is very round and an E7 galaxy is very elongated. The number after the E can be numerically determined by the ratio of the galaxy's major axis (a) to minor axis (b):
Galaxies with higher ellipticities have higher numbers.
Galaxies with spiral arms, type S, are sub-classified as type "a" through "d". Type Sa galaxies have their arms wound very tightly and have large central bulges. Type Sd galaxies have very loosely wound arms and have small central bulges. Because these classifications are subjective,astronomers often use intermediate designations of Sab, Sbc, or Scd. A galaxy that is obviously a spiral, but has some distinct peculiarity can have a "p" suffixed to its classification. A spiral galaxy that is encircled by a filamentary ring of brightness can have a "r" suffixed to its classification.
Barred spirals are designated by SB, with divisions into SBa through SBd defined the same as for normal spirals. Note that the capitalization of the B for a barred spiral is important: an SBc galaxy is not the same as an Sbc galaxy.
Lenticular galaxies are designated as S0. Galaxies in this transition classification can be recognized as having bars, where they are catalogued as SB0.
Irregular galaxies, designated by Irr, form the catch-all, otherwise category.