View source for Galaxy morphological classification

{{short description|System for categorizing galaxies based on appearance}}
[[File:Hubble Tuning Fork diagram.svg|thumb|upright=1.3|Tuning-fork-style diagram of the [[Hubble sequence]]]]

'''Galaxy morphological classification''' is a system used by [[astronomer]]s to divide [[galaxy|galaxies]] 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]], 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 ==
{{main|Hubble sequence}}
[[File:UGC12591 Hubble 4000.jpg|thumb|Spiral galaxy [[UGC 12591]] is classified as an S0/Sa galaxy.<ref>{{cite web|title=A remarkable galactic hybrid|url=https://www.spacetelescope.org/images/potw1709a/|website=www.spacetelescope.org|access-date=27 February 2017}}</ref>]]

The Hubble sequence is a morphological classification scheme for [[galaxies]] invented by [[Edwin Hubble]] in 1926.<ref name="hubble26a">{{cite journal |last=Hubble |first=E. P. |author-link=Edwin Hubble |date=1926 |title=Extra-galactic nebulae |journal=Contributions from the Mount Wilson Observatory / Carnegie Institution of Washington |volume=324 |pages=1–49|bibcode = 1926CMWCI.324....1H }}</ref><ref>{{cite book |last=Hubble |first=E. P. |author-link=Edwin Hubble |title=The Realm of the Nebulae |url=https://archive.org/details/in.ernet.dli.2015.212163 |date=1936 |publisher=[[Yale University Press]] |location=New Haven |lccn=36018182}}</ref>
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 plate]]s):<ref name="spacetelescope">{{Cite web|url=https://www.spacetelescope.org/images/heic9902o/|title=The Hubble tuning fork – classification of galaxies|last=|website=www.spacetelescope.org|language=en|access-date=2019-02-06}}</ref>
* [[Elliptical galaxy|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.<ref>{{Cite book|title=Galactic Astronomy|last=Binney|first=James|publisher=Princeton University Press|year=1998|isbn=978-0-691-02565-0|location=Princeton}}</ref> The specific ellipticity rating depends on ratio of the major (a) to minor axes (b), thus:<ref>{{cite web
 | title=Elliptical Galaxy
 | website=COSMOS – The SAO Encyclopedia of Astronomy
 | url=https://astronomy.swin.edu.au/cosmos/E/elliptical+galaxy
 | access-date=2020-09-19 }}</ref>
:: <math>E = 10 \times \left( 1-\frac{b}{a} \right)</math>
* [[Spiral galaxy|Spiral galaxies]] consist of a flattened disk, with [[star]]s forming a (usually two-armed) [[spiral]] structure, and a central concentration of stars known as the [[bulge (astronomy)|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 spiral galaxy|barred spirals]] are given the symbol "SB".
* [[Lenticular galaxy|Lenticular galaxies]] (designated S0) also consist of a bright central [[bulge (astronomy)|bulge]] surrounded by an extended, disk-like structure but, unlike [[spiral galaxy|spiral galaxies]], the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity.<ref>{{Cite web|url=http://cas.sdss.org/dr2/en/proj/basic/galaxies/lenticular.asp|title=Lenticular Galaxies|website=cas.sdss.org|access-date=2019-02-06}}</ref>

[[File:The Hubble Sequence throughout the Universe's history.jpg|thumb|300px|The Hubble sequence throughout the universe's history<ref>{{cite news|title=Hubble explores the origins of modern galaxies|url=http://www.spacetelescope.org/news/heic1315/|access-date=20 August 2013|newspaper=ESA/Hubble Press Release}}</ref>]]

These broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as [[irregular galaxy|irregular galaxies]], which have no obvious regular structure (either disk-like or ellipsoidal).<ref name="spacetelescope"/>

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”.<ref>{{Cite web|url=http://www.jb.man.ac.uk/distance/exploring/course/strobel/galaxy/galaxya.htm|title=Galaxies|website=www.jb.man.ac.uk|access-date=2019-02-06}}</ref>

To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in [[amateur astronomy]].<ref>{{Cite web|url=http://svn.ari.uni-heidelberg.de/svn/edu/trunk/aida_03_galaxies/en/en_aida_03_galaxies.pdf|title=THE HUBBLE SEQUENCE|last=Iafrate|first=G.|website=uni-heidelberg.de|access-date=2019-02-06}}</ref>
<!--
{| border="1" cellspacing="0" cellpadding="2"
|+ '''Known properties of galaxies''' 
|- style="background:#efefef;"
! Galaxy Type
! [[Mass]] ([[Solar mass|Solar Mass]]es) 
! [[Luminosity]] ([[Sun|Solar Luminosity]])
! Diameter ([[parsec|kpc]])
! Stellar Populations
! Percentage of Observed Galaxies
|-
| Spiral /<br/>Barred&nbsp;Spiral 
| 10<sup>9</sup> to 10<sup>11</sup>
| 10<sup>8</sup> to 10<sup>10</sup>
| 5–250
| disk: [[metal-rich|Population I]]<BR>halo:[[metal-poor|Population II]]
| 77% {{Citation needed|date=September 2007}}
|-
| Elliptical
| 10<sup>5</sup> to 10<sup>13</sup>
| 10<sup>5</sup> to 10<sup>11</sup>
| 1–205
| [[metal-poor|Population II]]
| 20%
|-
| Irregular
| 10<sup>8</sup> to 10<sup>10</sup>
| 10<sup>7</sup> to 10<sup>9</sup>
| 1–10
| [[metal-rich|Population I]]
| 3%
|}
-->
<!--
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 [[orbit]]s 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
|-
| [[Elliptical Galaxy M49|M49]] (NGC 4472)
| 12<sup>h</sup> 29.8<sup>m</sup> || 8° 00′ || E4
|-
| [[Elliptical Galaxy M59|M59]] (NGC 4621) 
| 12<sup>h</sup> 42.0<sup>m</sup> || 11° 39′ || E3
|-
| [[Elliptical Galaxy M60|M60]] (NGC 4649)
| 12<sup>h</sup> 43.7<sup>m</sup> || 11° 33' || E1
|-
| [[Lenticular Galaxy M84|M84]] (NGC 4374)
| 12<sup>h</sup> 25.1<sup>m</sup> || 12° 53′ || E1
|-
| [[Lenticular Galaxy M86|M86]] (NGC 4406)
| 12<sup>h</sup> 26.2<sup>m</sup> || 12° 57′ || E3
|-
| [[Elliptical Galaxy M89|M89]] (NGC 4552)
| 12<sup>h</sup> 35.7<sup>m</sup> || 12° 33′ || E0
|-
| [[Elliptical Galaxy M110|M110]] (NGC 205)
| 00<sup>h</sup> 40.4<sup>m</sup> || 41° 41′ || E6
|}
-->
Nonetheless, in June 2019, [[citizen scientist]]s through [[Galaxy Zoo]] reported that the [[Hubble sequence|usual Hubble classification]], particularly concerning [[spiral galaxies]], may not be supported, and may need updating.<ref name="EA-20190611">{{cite news |author=Royal Astronomical Society |title=Citizen scientists re-tune Hubble's galaxy classification |url=https://www.eurekalert.org/pub_releases/2019-06/ras-csr061119.php |date=11 June 2019 |work=[[EurekAlert!]] |access-date=11 June 2019 |author-link=Royal Astronomical Society }}</ref><ref name="MNRAS-20190430">{{cite journal |author=Masters, Karen L.|display-authors=et al. |title=Galaxy Zoo: unwinding the winding problem – observations of spiral bulge prominence and arm pitch angles suggest local spiral galaxies are winding |url=https://academic.oup.com/mnras/article-abstract/487/2/1808/5482087 |date=30 April 2019 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=487 |issue=2 |pages=1808–1820 |doi=10.1093/mnras/stz1153 |arxiv=1904.11436 |bibcode=2019MNRAS.487.1808M |access-date=12 June 2019 |doi-access=free }}</ref>

== De Vaucouleurs system ==<!-- This section is linked from [[Andromeda Galaxy]] -->
[[File:Hubble - de Vaucouleurs Galaxy Morphology Diagram.png|thumb|250px|Hubble – de Vaucouleurs Galaxy Morphology Diagram]]
[[Image:Galaxy morphology.jpg|right|thumb|250px]]
[[Image:NGC 6782 I HST2002.jpg|thumb|[[NGC 6782]]: a spiral galaxy (type SB(r)0/a) with three rings of different radii, as well as a bar.]]
[[Image:NGC 7793SpitzerFull.jpg|thumb|[[NGC 7793]]: a spiral galaxy of type SA(s)d.]]
[[Image:Large.mc.arp.750pix.jpg|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.<ref>{{cite book |last=De Vaucouleurs |first=G.  |title=Astrophysik IV: Sternsysteme / Astrophysics IV: Stellar Systems |chapter=Classification and Morphology of External Galaxies |series=Handbuch der Physik / Encyclopedia of Physics |author-link=Gérard de Vaucouleurs|date=1959 |volume=53 |pages=275–310|doi=10.1007/978-3-642-45932-0_7 |bibcode = 1959HDP....53..275D |isbn=978-3-642-45934-4 }}</ref> De Vaucouleurs argued that Hubble's two-dimensional classification of [[spiral galaxy|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 [[torus|rings]] and [[lens (geometry)|lenses]] are important structural components of spiral galaxies.<ref name="binney_merrifield">{{cite book |last=Binney |first=J. |author-link=James Binney |author2=Merrifield, M. |title=Galactic Astronomy |date=1998 |publisher=[[Princeton University Press]] |location=Princeton |isbn=978-0-691-02565-0}}</ref>

The de Vaucouleurs system retains Hubble's basic division of galaxies into [[elliptical galaxy|ellipticals]], [[lenticular galaxy|lenticulars]], [[spiral galaxy|spirals]] and [[irregular galaxy|irregulars]]. To complement Hubble's scheme, de Vaucouleurs introduced a more elaborate classification system for spiral galaxies, based on three morphological characteristics:<ref>{{Cite web|url=https://www.britannica.com/science/galaxy|title=Galaxy – Types of galaxies|website=Encyclopedia Britannica|language=en|access-date=2019-02-06}}</ref>

{{unordered list
<!-- this list is coded to allow proper formatting of the final item in which the first-level paragraph continues after the sublist -->
|1= 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.<ref name="Class">{{cite journal|last=de Vaucouleurs|first=Gérard|title=Revised Classification of 1500 Bright Galaxies|journal=Astrophysical Journal Supplement|date=April 1963|volume=8|page=31|doi=10.1086/190084|bibcode = 1963ApJS....8...31D }}</ref>  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).

|2= 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).<ref name="Class"/>

|3= 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:
{{unordered list
  |1= Sd (SBd) – diffuse, broken arms made up of individual stellar clusters and nebulae; very faint central bulge
  |2= Sm (SBm) – irregular in appearance; no bulge component
  |3= 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 spiral|“Magellanic” spirals]] and irregulars, respectively, after the [[Magellanic Clouds]]. The [[Large Magellanic Cloud]] is of type SBm, while the [[Small 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 a [http://www.astr.ua.edu/keel/galaxies/classify.html 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.<ref name="deVaucouleurs1994">{{cite journal |last=De Vaucouleurs |first=G. |author-link=Gérard de Vaucouleurs |date=1994 |title=Global Physical Parameters of Galaxies |url=https://www.stsci.edu/stsci/meetings/galaxy-morphology/proceedings/devaucouleurs.ps |format=[[PostScript]] |journal=Proceedings of "Quantifying Galaxy Morphology at High Redshift", A Workshop Held at the Space Telescope Science Institute, Baltimore MD, April 27-29 1994 |archive-url=https://web.archive.org/web/20180817141814/http://www.stsci.edu/institute/conference/galaxy-morphology/devaucouleurs.ps |archive-date=2018-08-17 |access-date=2008-01-02}}</ref>

===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).<ref>{{Cite web|url=https://ned.ipac.caltech.edu/level5/Tinsley/Tinsley1.html|title=Qualitative and Quantitative Classifications of Galaxies|website=ned.ipac.caltech.edu|access-date=2019-02-06}}</ref> 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<sub>B</sub>/M<sub>T</sub>) and the Hubble stage is M<sub>B</sub>/M<sub>T</sub>=(10−T)<sup>2</sup>/256 based on local galaxies.<ref>{{Cite journal|title=Simulation of the Cosmic Evolution of Atomic and Molecular Hydrogen in Galaxies|journal = The Astrophysical Journal|volume = 698|issue = 2|at=Equation (18)|arxiv = 0904.2221|doi = 10.1088/0004-637X/698/2/1467|year = 2009|last1 = Obreschkow|first1 = D.|last2 = Croton|first2 = D.|last3 = De Lucia|first3 = G.|last4 = Khochfar|first4 = S.|last5 = Rawlings|first5 = S.|bibcode = 2009ApJ...698.1467O| s2cid=204925243 }}</ref>

Elliptical galaxies are divided into three 'stages': compact ellipticals (cE), normal ellipticals (E) and late types (E<sup>+</sup>). Lenticulars are similarly subdivided into early (S<sup>−</sup>), intermediate (S<sup>0</sup>) and late (S<sup>+</sup>) types. Irregular galaxies can be of type magellanic irregulars (''T'' = 10) or 'compact' (''T'' = 11).

{| class="wikitable" style="text-align: center; margin: 1em auto;"
|+ Numerical Hubble stage
!Hubble stage ''T''
| −6
| −5
| −4
| −3
| −2
| −1
| 0
| 1
| 2
| 3
| 4
| 5
| 6
| 7
| 8
| 9
|9-10
| 10
| 11
|12
|13
|-
!de Vaucouleurs class<ref name="deVaucouleurs1994"/>
| cE
| E
| E<sup>+</sup>
| S0<sup>−</sup>
| S0<sup>0</sup>
| S0<sup>+</sup>
| S0/a
| Sa
| Sab
| Sb
| Sbc
| Sc
| Scd
| Sd
| Sdm
| Sm
|Ir
| Im
| Irp
|d Trans
|dSph
|-
!approximate Hubble class<ref>{{cite book |last=Binney |first=J. |author2=Merrifield, M. |title=Galactic Astronomy |date=1998 |publisher=Princeton University Press |location=Princeton |isbn=978-0-691-02565-0}}</ref>
| colspan="3" align="center" | E
| colspan="3" align="center" | S0
| S0/a
| Sa
| Sa-b
| Sb
| Sb-c
| colspan="3" align="center" | Sc
| Sc-Irr
| colspan="3" align="center" | Irr I
|Irr Il
|d E
|dSph
|-
|}
The use of numerical stages allows for more [[Quantitative research|quantitative]] studies of galaxy morphology.

== Yerkes (or Morgan) scheme ==
The Yerkes scheme was created by American astronomer [[William Wilson Morgan]].  Together with [[Philip Childs Keenan|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.<ref>{{Cite web|url=https://ned.ipac.caltech.edu/level5/CLASSIFICATION/yc.html|title=The Yerkes Classification|website=ned.ipac.caltech.edu|access-date=2019-02-06}}</ref>

{| class="wikitable"
!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
|}

{| class="wikitable"
!Galactic Shape
!Explanation
|-
| B || Barred spiral
|-
| D || Rotational symmetry without pronounced spiral or elliptical structure
|-
| E || Elliptical
|-
| Ep || Elliptical with dust absorption
|-
| I || Irregular
|-
| L || Low surface brightness
|-
| N || Small bright nucleus
|-
| S || Spiral
|}

{| class="wikitable"
! Inclination !! Explanation
|-
| 1 || Galaxy is "face-on"
|-
| 2 || 
|-
| 3 || 
|-
| 4 || 
|-
| 5 || 
|-
| 6 || 
|-
| 7 || Galaxy is "edge-on"
|}

Thus, for example, the [[Andromeda Galaxy]] is classified as kS5.<ref>{{Cite web|url=http://www.daviddarling.info/encyclopedia/G/galaxy_classification.html|title=galaxy classification|last=Darling|first=David|website=www.daviddarling.info|access-date=2019-02-06}}</ref>

== See also ==
* {{annotated link|Morphological Catalogue of Galaxies}}
* {{annotated link|Galaxy color–magnitude diagram}}
* {{annotated link|Galaxy Zoo}}
* {{annotated link|William Wilson Morgan}}
* {{annotated link|Fritz Zwicky}}

== References ==
<references/>

== External links ==
{{Commons category|Galaxy morphological classification}}
* [http://www.astr.ua.edu/keel/galaxies/classify.html Galaxies and the Universe] – an introduction to galaxy classification
* [http://www.ipac.caltech.edu/2mass/gallery/galmorph/ Near-Infrared Galaxy Morphology Atlas], T.H. Jarrett
* [https://archive.today/20121205083205/http://sings.stsci.edu/Publications/sings_poster.html The Spitzer Infrared Nearby Galaxies Survey (SINGS) Hubble Tuning-Fork], [http://sings.stsci.edu/ SINGS] Spitzer Space Telescope Legacy Science Project
* Go to [http://www.galaxyzoo.org GalaxyZoo.org] to try your hand at classifying galaxies as part of an Oxford University open community project

{{Galaxy}}
{{Portal bar|Stars|Outer space}}
[[Category:Galaxy morphological types| ]]
[[Category:Astronomical classification systems]]
[[Category:Extragalactic astronomy]]
[[Category:Edwin Hubble]]