Galaxy morphological classification

thumb|upright=1.3|Tuning-fork-style diagram of the [[Hubble sequence]]

Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance, shape, structure, and distribution of light. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan Sandage. However, galaxy classification and morphology are now largely done using computational methods and physical morphology.

Hubble sequence

thumb|Spiral galaxy [[UGC 12591 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:

:: <math>E = 10 \times \left( 1-\frac{b}{a} \right)</math>

• 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.

thumb|300px|The Hubble sequence throughout the universe's history

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.

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{| border="1" cellspacing="0" cellpadding="2"

|+ Known properties of galaxies

|- style="background:#efefef;"

! Galaxy Type

! Mass (Solar Masses)

! Luminosity (Solar Luminosity)

! Diameter (kpc)

! Stellar Populations

! Percentage of Observed Galaxies

|-

| Spiral /<br/>Barred&nbsp;Spiral

| 10⁹ to 10¹¹

| 10⁸ to 10¹⁰

| 5–250

| disk: Population I<BR>halo:Population II

| 77%

|-

| Elliptical

| 10⁵ to 10¹³

| 10⁵ to 10¹¹

| 1–205

| Population II

| 20%

|-

| Irregular

| 10⁸ to 10¹⁰

| 10⁷ to 10⁹

| 1–10

| Population I

| 3%

|}

-->

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More modern observations of galaxies have given us the following information about these types:

• Elliptical galaxies are generally fairly low in gas and dust, and are composed mostly of older stars.
• Spiral galaxies generally have plentiful supplies of gas and dust, and have a broad mix of older and younger stars.
• Irregular galaxies are fairly rich in gas, dust, and young stars.

From this, astronomers have constructed a theory of galaxy evolution which suggests that ellipticals are, in fact, the result of collisions between spiral and/or irregular galaxies, which strip out much of the gas and dust and randomize the orbits of the stars. See galaxy formation and evolution.

{| border="1" cellspacing="0" cellpadding="2"

|+ Elliptical galaxy examples

|- style="background:#efefef"

! Name !! Right Ascension !! Declination !! Hubble Type

|-

| M49 (NGC 4472)

| 12^h 29.8^m || 8° 00′ || E4

|-

| M59 (NGC 4621)

| 12^h 42.0^m || 11° 39′ || E3

|-

| M60 (NGC 4649)

| 12^h 43.7^m || 11° 33' || E1

|-

| M84 (NGC 4374)

| 12^h 25.1^m || 12° 53′ || E1

|-

| M86 (NGC 4406)

| 12^h 26.2^m || 12° 57′ || E3

|-

| M89 (NGC 4552)

| 12^h 35.7^m || 12° 33′ || E0

|-

| M110 (NGC 205)

| 00^h 40.4^m || 41° 41′ || E6

|}

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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 ==<!-- This section is linked from Andromeda Galaxy -->

thumb|250px|Hubble – de Vaucouleurs Galaxy Morphology Diagram

right|thumb|250px

thumb|[[NGC 6782: a spiral galaxy (type SB(r)0/a) with three rings of different radii, as well as a bar]]

thumb|[[NGC 7793: a spiral galaxy of type SA(s)d]]

thumb|The [[Large Magellanic Cloud: 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:

</math>,

where <math>R_{ij}</math> is the residual image after subtraction and <math>I_{ij}</math> is the original image.

In 1998, Sidney van den Bergh published a book on galaxy morphology in which he interrogated weaknesses in the Hubble sequence as a classification scheme. Subsequent work revealed further weaknesses. In 2003, Christopher Conselice formalized the CAS parameters, a set of quantitative morphological parameters which could be used to approximate the Hubble types of galaxy populations at low redshift. In 2004, a paper by Jennifer Lotz found that the Gini coefficient, when used in tandem with the second-order moment of the brightest 20% of the galaxy's flux (referred to as M₂₀) and the CAS parameters, could distinguish normal galaxies from mergers.

See also

• CAS parameters – Scheme of quantitative morphological indicators
• Gini coefficient – Measure of economic inequality also used as a morphological indicator

References