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, figured out by Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan Sandage. However, galaxy classification and morphology are now mostly 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 like an everyday expression as the “Hubble tuning-fork” because of the shape in which it is usually represented. Hubble's scheme divides galaxies into three broad classes based on their visual appearance (originally on photographic plates): Elliptical galaxies have smooth, blank light distributions and appear as ovals in images. They are represented 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), this way:
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 (named 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 amount.
These broad classes can be extended to enable finer honors of appearance and to include other types of galaxies, such as irregular galaxies, which have no obvious regular structure (either disk-like or ellipsoidal).
The Hubble sequence is often represented in the form of a two-pronged fork, with the ellipticals on the left (with the degree of ellipticity increasing from left to right) and the barred and unbarred spirals forming the two parallel prongs of the fork. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongs meet the “handle”.
To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.
Anyway, in June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, especially concerning spiral galaxies, may not be supported, and may need updating.
De Vaucouleurs system
The de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959. De Vaucouleurs argued that Hubble's two-dimensional classification of spiral galaxies—based on the tightness of the spiral arms and the presence or absence of a bar—did not acceptably describe the full range of watched galaxy morphologies. Especially, he argued that rings and lenses are important structural components of spiral galaxies.
The de Vaucouleurs system keeps Hubble's basic division of galaxies into ellipticals, lenticulars, spirals and irregulars. To make Hubble's scheme complete, de Vaucouleurs introduced a more detailed classification system for spiral galaxies, based on three morphological characteristics:
The different elements of the classification scheme are combined — in the order in which they are listed — to give the complete classification of a galaxy. For example, a weakly barred spiral galaxy with loosely wound arms and a ring is represented SAB(r)c.
Visually, the de Vaucouleurs system can be represented as a three-dimensional version of Hubble's tuning fork, with stage (spiralness) on the x-axis, family (barredness) on the y-axis, and variety (ringedness) on the z-axis.
Numerical Hubble stage
De Vaucouleurs also selected numerical values to each class of galaxy in his scheme. Values of the numerical Hubble stage T run from −6 to +10, with negative numbers going along with early-type galaxies (ellipticals and lenticulars) and positive numbers to late types (spirals and irregulars). So, as a rough rule, lower values of T go along with a larger fraction of the stellar mass contained in a spheroid/bulge relative to the disk. The close guess mapping between the spheroid-to-total stellar mass ratio (MB/MT) and the Hubble stage is MB/MT=(10−T)2/256 based on local galaxies.