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Concept
Galaxy morphological classification
Summary
Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan Sandage. However, galaxy classification and morphology are now largely done using computational methods and physical morphology.
Hubble sequence
The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926.
It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented. Hubble's scheme divides galaxies into three broad classes based on their visual appearance (originally on photographic plates):
Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter "E", followed by an integer n representing their degree of ellipticity on the sky. The specific ellipticity rating depends on ratio of the major (a) to minor axes (b), thus:
Spiral galaxies consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge, which is similar in appearance to an elliptical galaxy. They are given the symbol "S". Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol "SB".
Lenticular galaxies (designated S0) also consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity.
These broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as irregular galaxies, which have no obvious regular structure (either disk-like or ellipsoidal).
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