Galaxy morphological classification

Galaxy morphological classification is a method astronomers use to sort galaxies by how they look, including their shape, structure, and light distribution. While several classification systems exist, the most well-known is the Hubble sequence, created by Edwin Hubble and later refined by Gérard de Vaucouleurs and Allan Sandage. Today, galaxy classification and morphology are mostly handled by computers and based on physical properties.

The Hubble sequence, introduced in 1926, is often called the "Hubble tuning-fork" because of its shape. It originally sorted galaxies into three broad types based on their appearance on photographic plates. Elliptical galaxies, labeled "E" with a number for their ellipticity, have smooth, featureless light and look like ellipses. The ellipticity rating comes from the ratio of the major axis (a) to the minor axis (b), calculated as E = 10 × (1 − b/a). Spiral galaxies, given the symbol "S", have a flattened disk with a spiral structure (usually two-armed) and a central bulge that resembles an elliptical galaxy. About half of spirals also have a bar extending from the bulge; these are labeled "SB". Lenticular galaxies, designated S0, have a bright bulge and a disk, but the disk lacks visible spiral structure and has little active star formation. These main types can be expanded to include irregular galaxies, which have no regular disk or ellipsoidal shape. In the tuning-fork diagram, ellipticals sit on the left (with increasing ellipticity), lenticulars are in the middle where the handle meets the prongs, and unbarred and barred spirals form the two prongs on the right. The Hubble sequence remains the most common classification system in both professional and amateur astronomy. However, in June 2019, citizen scientists from Galaxy Zoo suggested that the usual Hubble classification, especially the link between spiral arms and the galactic nucleus, may need revision.

The de Vaucouleurs system, first described in 1959, extends the Hubble sequence. De Vaucouleurs felt that Hubble’s two-dimensional approach—based only on arm tightness and bar presence—missed important features like rings and lenses. His system keeps Hubble’s basic categories (ellipticals, lenticulars, spirals, irregulars) but adds a more detailed scheme for spirals, using three characteristics: family (barred, unbarred, or intermediate), variety (ringed, unringed, or intermediate), and stage (how tightly the arms are wound). These are combined in order, so a weakly barred spiral with loose arms and a ring is labeled SAB(r)c. Visually, the de Vaucouleurs system can be seen as a three-dimensional tuning fork, with stage on the x-axis, family on the y-axis, and variety on the z-axis.

De Vaucouleurs also assigned numerical Hubble stages, from −6 to +10. Negative numbers are early-type galaxies (ellipticals and lenticulars), and positive numbers are late types (spirals and irregulars). Lower T values mean a larger fraction of the galaxy’s stellar mass is in the spheroid or bulge relative to the disk. For local galaxies, the approximate relation between the spheroid-to-total stellar mass ratio (MB/MT) and T is MB/MT = (10 − T)²/256. Ellipticals are split into compact (cE), normal (E), and late (E+). Lenticulars are early (S−), intermediate (S0), and late (S+). Irregulars can be magellanic (T = 10) or compact (T = 11). These numerical stages allow for more quantitative studies.

The Yerkes (or Morgan) scheme, created by William Wilson Morgan, uses the spectra of stars in a galaxy, along with its real and apparent shape and the degree of central concentration, to classify it. For instance, the Andromeda Galaxy is classified as kS5.