Galaxy morphological classification is a system used by

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...

s to divide

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

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 The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a t ...

, devised by

Edwin Hubble Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an Americans, American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. Hubble proved that many objects ...

and later expanded by

Gérard de Vaucouleurs Gérard Henri de Vaucouleurs (25 April 1918 – 7 October 1995) was a French astronomer. Life and career Born in Paris, he had an early interest in amateur astronomy and received his undergraduate degree in 1939 at the Sorbonne in that city. ...

and

Allan Sandage Allan Rex Sandage (June 18, 1926 – November 13, 2010) was an American astronomer. He was Staff Member Emeritus with the Carnegie Observatories in Pasadena, California. He determined the first reasonably accurate values for the Hubble con ...

. 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

invented by

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

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s): *

Elliptical galaxies An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Real ...

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 \right)

Spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s forming a (usually two-armed)

spiral In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:bulge __NOTOC__ Bulge may refer to: Astronomy and geography *Bulge (astronomy), a tightly packed group of stars at the center of a spiral galaxy *Equatorial bulge, a bulge around the equator of a planet due to rotation * Tharsis bulge, vast volcanic pl ...

, which is similar in appearance to an

elliptical galaxy An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The R ...

. 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 A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. L ...

(designated S0) also consist of a bright central

bulge __NOTOC__ Bulge may refer to: Astronomy and geography *Bulge (astronomy), a tightly packed group of stars at the center of a spiral galaxy *Equatorial bulge, a bulge around the equator of a planet due to rotation * Tharsis bulge, vast volcanic pl ...

surrounded by an extended, disk-like structure but, unlike

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae'' These broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as

irregular galaxies An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, ...

, 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 Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur astronomers m ...

. Nonetheless, in June 2019,

citizen scientist Citizen science (CS) (similar to community science, crowd science, crowd-sourced science, civic science, participatory monitoring, or volunteer monitoring) is scientific research conducted with participation from the public (who are sometimes re ...

s through

Galaxy Zoo Galaxy Zoo is a crowdsourced astronomy project which invites people to assist in the morphological classification of large numbers of galaxies. It is an example of citizen science as it enlists the help of members of the public to help in scien ...

reported that the usual Hubble classification, particularly concerning

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''

De Vaucouleurs system

Hubble - de Vaucouleurs Galaxy Morphology Diagram

The de Vaucouleurs system for classifying galaxies is a widely used extension to the

, first described by

in 1959. De Vaucouleurs argued that Hubble's two-dimensional classification of

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''rings Ring may refer to: * Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry * To make a sound with a bell, and the sound made by a bell :(hence) to initiate a telephone connection Arts, entertainment and media Film and ...

and

lenses A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

are important structural components of spiral galaxies. The de Vaucouleurs system retains Hubble's basic division of galaxies into ellipticals, lenticulars,

spirals In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:irregulars Irregular military is any non-standard military component that is distinct from a country's national armed forces. Being defined by exclusion, there is significant variance in what comes under the term. It can refer to the type of military orga ...

. 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
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 (M_B/M_T) and the Hubble stage is M_B/M_T=(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 Quantitative may refer to: * Quantitative research, scientific investigation of quantitative properties * Quantitative analysis (disambiguation) * Quantitative verse, a metrical system in poetry * Statistics, also known as quantitative analysis ...

studies of galaxy morphology.

Yerkes (or Morgan) scheme

The Yerkes scheme was created by American astronomer

William Wilson Morgan William Wilson Morgan (January 3, 1906 – June 21, 1994) was an American astronomer and astrophysicist. The principal theme in Morgan's work was stellar and galaxy classification. He is also known for helping prove the existence of spiral arms i ...

. Together with Philip 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. Thus, for example, the

Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gala ...

is classified as kS5.

References

External links