Editing Galaxy morphological classification (section)

== 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>