is classified as an S0/Sa galaxy. 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: :: E = 10 \times \left( 1-\frac{b}{a} \right) •
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). To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in
amateur astronomy. Nonetheless, in June 2019,
citizen scientists through
Galaxy Zoo reported that the
usual Hubble classification, particularly concerning the relationship between spiral arms and galactic nucleus in
spiral galaxies, may need reassessment. == De Vaucouleurs system == : a spiral galaxy (type SB(r)0/a) with three rings of different radii, as well as a bar : a spiral galaxy of type SA(s)d : a type SBm galaxy 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 adequately describe the full range of observed galaxy morphologies. In particular, he argued that
rings and
lenses are important structural components of spiral galaxies. The de Vaucouleurs system retains Hubble's basic division of galaxies into
ellipticals,
lenticulars,
spirals and
irregulars. To complement Hubble's scheme, de Vaucouleurs introduced a more elaborate 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 denoted 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 assigned 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 corresponding to early-type galaxies (ellipticals and lenticulars) and positive numbers to late types (spirals and irregulars). Thus, as a rough rule, lower values of
T 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. 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 use of numerical stages allows for more
quantitative studies of galaxy morphology. == Yerkes (or Morgan) scheme ==