Galaxy morphological classification is a system used byastronomers to dividegalaxies 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 theHubble sequence, devised byEdwin Hubble and later expanded byGérard de Vaucouleurs andAllan Sandage. However, galaxy classification and morphology are now largely done using computational methods and physical morphology.
Spiral galaxyUGC 12591 is classified as an S0/Sa galaxy.[1]
The Hubble sequence is a morphological classification scheme forgalaxies invented byEdwin Hubble in 1926.[2][3]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 onphotographic plates):[4]
Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter "E", followed by an integern representing their degree of ellipticity on the sky.[5] The specific ellipticity rating depends on ratio of the major (a) to minor axes (b), thus:[6]
Spiral galaxies consist of a flattened disk, withstars forming a (usually two-armed)spiral structure, and a central concentration of stars known as thebulge, 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. Thesebarred spirals are given the symbol "SB".
Lenticular galaxies (designated S0) also consist of a bright centralbulge surrounded by an extended, disk-like structure but, unlikespiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity.[7]
The Hubble sequence throughout the universe's history[8]
These broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such asirregular galaxies, which have no obvious regular structure (either disk-like or ellipsoidal).[4]
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 on the right. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongs meet the “handle”.[9]
To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and inamateur astronomy.[10]Nonetheless, in June 2019,citizen scientists throughGalaxy Zoo reported that theusual Hubble classification, particularly concerning the relationship between spiral arms and galactic nucleus inspiral galaxies, may need reassessment.[11][12]
Hubble – de Vaucouleurs Galaxy Morphology DiagramNGC 6782: a spiral galaxy (type SB(r)0/a) with three rings of different radii, as well as a bar.NGC 7793: a spiral galaxy of type SA(s)d.TheLarge Magellanic Cloud: a type SBm galaxy.
The de Vaucouleurs system for classifying galaxies is a widely used extension to theHubble sequence, first described byGérard de Vaucouleurs in 1959.[13] De Vaucouleurs argued that Hubble's two-dimensional classification ofspiral galaxies—based on the tightness of the spiral arms and the presence or absence of a bar—did not adequately describe the full range of observed galaxy morphologies. In particular, he argued thatrings andlenses are important structural components of spiral galaxies.[14]
The de Vaucouleurs system retains Hubble's basic division of galaxies intoellipticals,lenticulars,spirals andirregulars. To complement Hubble's scheme, de Vaucouleurs introduced a more elaborate classification system for spiral galaxies, based on three morphological characteristics:[15]
Bars. Galaxies are divided on the basis of the presence or absence of a nuclear bar. De Vaucouleurs introduced the notation SA to denote spiral galaxies without bars, complementing Hubble’s use of SB for barred spirals. He also allowed for an intermediate class, denoted SAB, containing weakly barred spirals.[16] Lenticular galaxies are also classified as unbarred (SA0) or barred (SB0), with the notation S0 reserved for those galaxies for which it is impossible to tell if a bar is present or not (usually because they are edge-on to the line-of-sight).
Rings. Galaxies are divided into those possessing ring-like structures (denoted ‘(r)’) and those without rings (denoted ‘(s)’). So-called ‘transition’ galaxies are given the symbol (rs).[16]
Spiral arms. As in Hubble’s original scheme, spiral galaxies are assigned to a class based primarily on the tightness of their spiral arms. The de Vaucouleurs scheme extends the arms of Hubble’s tuning fork to include several additional spiral classes:
Sd (SBd) – diffuse, broken arms made up of individual stellar clusters and nebulae; very faint central bulge
Sm (SBm) – irregular in appearance; no bulge component
Im – highly irregular galaxy
Most galaxies in these three classes were classified as Irr I in Hubble’s original scheme. In addition, the Sd class contains some galaxies from Hubble’s Sc class. Galaxies in the classes Sm and Im are termed the“Magellanic” spirals and irregulars, respectively, after theMagellanic Clouds. TheLarge Magellanic Cloud is of type SBm, while theSmall Magellanic Cloud is an irregular (Im).
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 denoted SAB(r)c.
Visually, the de Vaucouleurs system can be represented as athree-dimensional version of Hubble's tuning fork, with stage (spiralness) on thex-axis, family (barredness) on they-axis, and variety (ringedness) on thez-axis.[17]
De Vaucouleurs also assigned numerical values to each class of galaxy in his scheme. Values of the numerical Hubble stageT run from −6 to +10, with negative numbers corresponding to early-type galaxies (ellipticals and lenticulars) and positive numbers to late types (spirals and irregulars).[18] Thus, as a rough rule, lower values ofT correspond to a larger fraction of the stellar mass contained in a spheroid/bulge relative to the disk. The approximate 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.[19]
Elliptical galaxies are divided into three 'stages': compact ellipticals (cE), normal ellipticals (E) and late types (E+). Lenticulars are similarly subdivided into early (S−), intermediate (S0) and late (S+) types. Irregular galaxies can be of type magellanic irregulars (T = 10) or 'compact' (T = 11).
The Yerkes scheme was created by American astronomerWilliam Wilson Morgan. Together withPhilip Keenan, Morgan also developed the MK system for the classification of stars through their spectra. The Yerkes scheme uses the spectra of stars in the galaxy; the shape, real and apparent; and the degree of the central concentration to classify galaxies.[21]
Spectral Type
Explanation
a
Prominent A stars
af
Prominent A–F stars
f
Prominent F stars
fg
Prominent F–G stars
g
Prominent G stars
gk
Prominent G–K stars
k
Prominent K stars
Galactic Shape
Explanation
B
Barred spiral
D
Rotational symmetry without pronounced spiral or elliptical structure
^Hubble, E. P. (1926). "Extra-galactic nebulae".Contributions from the Mount Wilson Observatory / Carnegie Institution of Washington.324:1–49.Bibcode:1926CMWCI.324....1H.
