The Hidden Lives of Galaxies - Identifying Galaxies
This classification sequence has become so widely used that the basic types, spiral, barred spiral, elliptical, irregular, and peculiar, are still used by astronomers today to classify galaxies according to their visible appearance. Spirals are denoted by "S", and barred spirals by "SB". Letters "a", "b", "c" denote how tightly the spiral arms are wound, with "a" being most tightly wound. The Andromeda Galaxy is an Sb. Elliptical galaxies are denoted by "E", with a number from 0-7 indicating how circular it appears (0 being most circular, 7 being more elongated). An example of this would be M87, which is an E0 galaxy. Irregulars, such as the Small Magellanic Cloud, are denoted by "Irr". Peculiar galaxies, such as Centaurus A, are denoted by "P".
To show how the various classes relate to each other, Hubble organized them into a diagram. A simplified version of Hubble’s Fork Diagram is shown below. Note that this diagram does not represent how galaxies form.
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Hubble’s Fork Diagram of Galaxy Classification
