Editing Hubble sequence (section)

==Classes of galaxies==

===Ellipticals===

[[Image:Abell S740, cropped to ESO 325-G004.jpg|thumb|The giant elliptical galaxy [[ESO 325-G004]].]]
{{see also|Elliptical galaxy}}
On the left (in the sense that the sequence is usually drawn) lie the [[elliptical galaxy|ellipticals]]. Elliptical galaxies have relatively smooth, featureless light distributions and appear as ellipses in photographic images. They are denoted by the letter E, followed by an integer {{mvar|n}} representing their degree of [[Flattening|ellipticity]] in the sky. By convention, {{mvar|n}} is ten times the ellipticity of the galaxy, rounded to the nearest integer, where the ellipticity is defined as {{nobr|  {{math|''e'' {{=}} 1 − {{sfrac| ''b'' | ''a''}} }}  }} for an ellipse with {{mvar|a}} the [[Semi-major and semi-minor axes|semi-major axis]] length and {{mvar|b}} the semi-minor axis length.<ref name="binney_merrifield">{{cite book |last1=Binney |first1=J. |author-link=James Binney |last2=Merrifield |first2=M. |year=1998 |title=Galactic Astronomy |publisher=Princeton University Press |location=Princeton, NJ |isbn=978-0-691-02565-0}}</ref> The ellipticity increases from left to right on the Hubble diagram, with near-circular (E0) galaxies situated on the very left of the diagram. It is important to note that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example, a flattened, discus-shaped galaxy can appear almost round if viewed face-on or highly elliptical if viewed edge-on). Observationally, the most flattened "elliptical" galaxies have ellipticities {{nobr| {{math|''e'' {{=}} 0.7 }} }} (denoted E7). However, from studying the light profiles and the ellipticity profiles, rather than just looking at the images, it was realised in the 1960s that the E5–E7 galaxies are probably misclassified lenticular galaxies with large-scale disks seen at various inclinations to our line-of-sight.<ref>{{cite journal |author=Liller, Martha |year=1966 |title=The distribution of intensity in elliptical galaxies of the Virgo cluster.&nbsp;II |journal=Astrophysical Journal |volume=146 |page=28 |doi=10.1086/148857 |bibcode=1966ApJ...146...28L }} <br /> {{cite journal |title=ADS abstract |journal=NASA / Astrophysics Data System |publisher=[[Harvard University]] |bibcode=1966ApJ...146...28L |last1=Liller |first1=Martha H. |date=1966 |volume=146 |page=28 |doi=10.1086/148857 }} </ref><ref>{{cite journal |author=Gorbachev, V.I. |year=1970 |title=The central part of the Coma cluster of galaxies |journal=Soviet Astronomy |volume=14 |page=182 |bibcode=1970SvA....14..182G }} <br /> {{cite journal |title=ADS abstract |journal=NASA / Astrophysics Data System |publisher=[[Harvard University]] |bibcode=1970SvA....14..182G |last1=Gorbachev |first1=V. I. |date=1970 |volume=14 |page=182 }}</ref> Observations of the kinematics of early-type galaxies further confirmed this.<ref>{{cite journal |author1=Graham |author2=Alister W. |author3=Colless, Matthew M. |author4=Busarello, Giovanni |author5=Zaggia, Simone |author6=Longo, Giuseppe |year=1998 |title=Extended stellar kinematics of elliptical galaxies in the Fornax cluster |journal=Astronomy and Astrophysics Supplement |volume=133 |issue=3 |pages=325–336 |doi=10.1051/aas:1998325 |arxiv=astro-ph/9806331 |bibcode=1998A&AS..133..325G  |s2cid=15164368 }} <br /> {{cite journal |title=ADS abstract |journal=NASA / Astrophysics Data System |publisher=[[Harvard University]] |bibcode=1998A&AS..133..325G |last1=Graham |first1=A. W. |last2=Colless |first2=M. M. |last3=Busarello |first3=G. |last4=Zaggia |first4=S. |last5=Longo |first5=G. |date=1998 |volume=133 |page=325 |doi=10.1051/aas:1998325 |arxiv=astro-ph/9806331 |s2cid=15164368 }}</ref><ref>{{cite journal |author=Emsellem, Eric |display-authors=etal |year=2011 |title=The ATLAS<sup>3D</sup> project - III. A census of the stellar angular momentum within the effective radius of early-type galaxies: unveiling the distribution of fast and slow rotators |journal=Monthly Notices of the Royal Astronomical Society |volume=414 |issue=2 |pages=888–912 |doi=10.1111/j.1365-2966.2011.18496.x |doi-access=free |arxiv=1102.4444 |bibcode=2011MNRAS.414..888E |s2cid=2707378 }} <br /> {{cite journal |title=ADS abstract |journal=NASA / Astrophysics Data System |publisher=[[Harvard University]] |bibcode=2011MNRAS.414..888E |last1=Emsellem |first1=Eric |last2=Cappellari |first2=Michele |last3=Krajnović |first3=Davor |last4=Alatalo |first4=Katherine |last5=Blitz |first5=Leo |last6=Bois |first6=Maxime |last7=Bournaud |first7=Frédéric |last8=((Bureau)) |first8=Martin |last9=Davies |first9=Roger L. |last10=Davis |first10=Timothy A. |last11=De Zeeuw |first11=P. T. |last12=Khochfar |first12=Sadegh |last13=Kuntschner |first13=Harald |last14=Lablanche |first14=Pierre-Yves |last15=McDermid |first15=Richard M. |last16=Morganti |first16=Raffaella |last17=Naab |first17=Thorsten |last18=Oosterloo |first18=Tom |last19=Sarzi |first19=Marc |last20=Scott |first20=Nicholas |last21=Serra |first21=Paolo |last22=Van De Ven |first22=Glenn |last23=Weijmans |first23=Anne-Marie |last24=Young |first24=Lisa M. |date=2011 |volume=414 |issue=2 |page=888 |doi=10.1111/j.1365-2966.2011.18496.x |doi-access=free |arxiv=1102.4444 |s2cid=2707378 }}</ref><ref>{{cite journal |author=Krajnović, Davor |display-authors=etal |year=2013 |title=The ATLAS<sup>3D</sup> project - XVII. Linking photometric and kinematic signatures of stellar discs in early-type galaxies |journal=Monthly Notices of the Royal Astronomical Society |volume=432 |issue=3 |pages=1768–1795 |doi=10.1093/mnras/sts315 |doi-access=free |arxiv=1210.8167 |bibcode=2013MNRAS.432.1768K }} <br /> {{cite journal |title=ADS abstract |journal=NASA / Astrophysics Data System |publisher=[[Harvard University]] |bibcode=2013MNRAS.432.1768K |last1=Krajnović |first1=Davor |last2=Alatalo |first2=Katherine |last3=Blitz |first3=Leo |last4=Bois |first4=Maxime |last5=Bournaud |first5=Frédéric |last6=((Bureau)) |first6=Martin |last7=Cappellari |first7=Michele |last8=Davies |first8=Roger L. |last9=Davis |first9=Timothy A. |last10=De Zeeuw |first10=P. T. |last11=Duc |first11=Pierre-Alain |last12=Emsellem |first12=Eric |last13=Khochfar |first13=Sadegh |last14=Kuntschner |first14=Harald |last15=McDermid |first15=Richard M. |last16=Morganti |first16=Raffaella |last17=Naab |first17=Thorsten |last18=Oosterloo |first18=Tom |last19=Sarzi |first19=Marc |last20=Scott |first20=Nicholas |last21=Serra |first21=Paolo |last22=Weijmans |first22=Anne-Marie |last23=Young |first23=Lisa M. |date=2013 |volume=432 |issue=3 |page=1768 |doi=10.1093/mnras/sts315 |doi-access=free |arxiv=1210.8167 }}</ref>

Examples of elliptical galaxies: [[Messier 49|M49]], [[Messier 59|M59]], [[Messier 60|M60]], [[Messier 87|M87]], [[NGC 4125]].

===Lenticulars===
[[Image:Ngc5866 hst big rotated.jpg|thumb|The [[NGC 5866|Spindle Galaxy]] (NGC 5866), a lenticular galaxy with a prominent [[dust lane]] in the [[Draco (constellation)|constellation of Draco]].]]
{{see also|Lenticular galaxy}}
At the centre of the Hubble tuning fork, where the two spiral-galaxy branches and the elliptical branch join, lies an intermediate class of galaxies known as [[lenticular galaxy|lenticulars]] and given the symbol S0. These galaxies consist of a bright central [[bulge (astronomy)|bulge]], similar in appearance to an [[elliptical galaxy]], surrounded by an extended, [[galaxy disk|disk]]-like structure. 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.

When simply looking at a galaxy's image, lenticular galaxies with relatively face-on disks are difficult to distinguish from ellipticals of type E0–E3, making the classification of many such galaxies uncertain. When viewed edge-on, the disk becomes more apparent and prominent dust-lanes are sometimes visible in [[absorption (electromagnetic radiation)|absorption]] at optical wavelengths.

At the time of the initial publication of Hubble's galaxy classification scheme, the existence of lenticular galaxies was purely hypothetical. Hubble believed that they were necessary as an intermediate stage between the highly flattened "ellipticals" and spirals. Later [[observational astronomy|observations]] (by Hubble himself, among others) showed Hubble's belief to be correct and the S0 class was included in the definitive exposition of the Hubble sequence by [[Allan Sandage]].<ref name="sandage">{{cite conference |first = A. |last = Sandage |author-link = Allan Sandage |year = 1975 |title = Classification and stellar content of galaxies obtained from direct photography |book-title = Galaxies and the Universe |editor1 = Sandage, A. |editor2 = Sandage, M. |editor3 = Kristian, J. |url = http://nedwww.ipac.caltech.edu/level5/Sandage/frames.html |access-date = 2007-11-20}}</ref> Missing from the Hubble sequence are the early-type galaxies with intermediate-scale disks, in between the E0 and S0 types, Martha Liller denoted them '''ES''' galaxies in 1966.

Lenticular and spiral galaxies, taken together, are often referred to as [[disk galaxy|disk galaxies]].  The bulge-to-disk flux ratio in lenticular galaxies can take on a range of values, just as it does for each of the spiral galaxy morphological types (Sa, Sb, etc.).<ref>{{cite journal |last=Graham |first=A. |author2=Worley, C. |date=August 2008 |title=Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size-luminosity relations |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=388 |issue=4 |pages=1708–1728 |bibcode=2008MNRAS.388.1708G |doi=10.1111/j.1365-2966.2008.13506.x |doi-access=free |arxiv = 0805.3565 |s2cid=14406950 }}</ref>

Examples of lenticular galaxies: [[Messier 85|M85]], [[Messier 86|M86]], [[NGC 1316]], [[NGC 2787]], [[NGC 5866]], [[Centaurus A]].

===Spirals===
[[Image:M101 hires STScI-PRC2006-10a.jpg|thumb|The [[Pinwheel Galaxy]] (Messier 101/NGC 5457): a spiral galaxy classified as type Scd on the Hubble sequence]]
[[Image:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg|thumb|right|The barred spiral galaxy [[NGC 1300]]: a type SBbc]]
{{see also|Spiral galaxy}}
On the right of the Hubble sequence diagram are two parallel branches encompassing the [[spiral galaxy|spiral galaxies]]. A spiral galaxy consists of a flattened [[galaxy disk|disk]], with [[star]]s forming a (usually two-armed) [[spiral]] structure, and a central concentration of stars known as the [[bulge (astronomy)|bulge]]. Roughly half of all spirals are also observed to have a bar-like structure, with the bar extending from the central bulge, and the arms begin at the ends of the bar. In the tuning-fork diagram, the regular spirals occupy the upper branch and are denoted by the letter S, while the lower branch contains the barred spirals, given the symbol SB. Both type of spirals are further subdivided according to the detailed appearance of their spiral structures. Membership of one of these subdivisions is indicated by adding a lower-case letter to the morphological type, as follows:
* Sa (SBa) – tightly wound, smooth arms; large, bright central bulge
* Sb (SBb) – less tightly wound spiral arms than Sa (SBa); somewhat fainter bulge
* Sc (SBc) – loosely wound spiral arms, clearly resolved into individual stellar clusters and nebulae; smaller, fainter bulge

Hubble originally described three classes of spiral galaxy. This was extended by [[Gérard de Vaucouleurs]]<ref name="devaucouleurs">{{cite book |last1=de Vaucouleurs |first1=G. |author-link1=Gérard de Vaucouleurs |last2=Oemler |first2=Augustus Jr. |last3=Butcher |first3=Harvey R. |last4=Gunn |first4=James E. |chapter=Classification and Morphology of External Galaxies |date=1959 |title=Astrophysik IV: Sternsysteme / Astrophysics IV: Stellar Systems |series=Handbuch der Physik |volume=53 |pages=275–310 |bibcode=1959HDP....53..275D |doi=10.1007/978-3-642-45932-0_7 |isbn=978-3-642-45934-4 }}</ref> to include a fourth class:
* Sd (SBd) – very loosely wound, fragmentary arms; most of the luminosity is in the arms and not the bulge

Although strictly part of the [[Galaxy morphological classification#De Vaucouleurs system|de Vaucouleurs system]] of classification, the Sd class is often included in the Hubble sequence. The basic spiral types can be extended to enable finer distinctions of appearance. For example, spiral galaxies whose appearance is intermediate between two of the above classes are often identified by appending two lower-case letters to the main galaxy type (for example, Sbc for a galaxy that is intermediate between an Sb and an Sc).

Our own [[Milky Way]] is generally classed as Sc or SBc,<ref>{{cite journal |first=Paul |last=Hodge |year=1983 |title=The Hubble type of the Milky Way galaxy |journal=Publications of the Astronomical Society of the Pacific|volume=95 |pages=721–723 |doi=10.1086/131243 |bibcode=1983PASP...95..721H |s2cid=121754114 |doi-access=free }}</ref> making it a barred spiral with well-defined arms.

Examples of regular spiral galaxies: ([[Andromeda_Galaxy#Structure|visually]]) [[Andromeda Galaxy|M31]] (Andromeda Galaxy), [[Messier 74|M74]], [[Messier 81|M81]], [[Sombrero Galaxy|M104]] (Sombrero Galaxy), [[Whirlpool Galaxy|M51a]] (Whirlpool Galaxy), [[NGC 300]], [[NGC 772]].

Examples of barred spiral galaxies: [[Messier 91|M91]], [[Messier 95|M95]], [[NGC 1097]], [[NGC 1300]], [[NGC1672]], [[NGC 2536]], [[NGC 2903]].

===Irregulars===
[[Image:large.mc.arp.750pix.jpg|thumb|The [[Large Magellanic Cloud]] (LMC) – a dwarf [[irregular galaxy]]]]
{{see also|Irregular galaxy}}
Galaxies that do not fit into the Hubble sequence, because they have no regular structure (either disk-like or ellipsoidal), are termed [[irregular galaxy|irregular galaxies]]. Hubble defined two classes of irregular galaxy:<ref name="longair">{{cite book |last=Longair |first=M.S. |author-link=Malcolm Longair |title=Galaxy Formation |date=1998 |publisher=Springer |location=New York |isbn=3-540-63785-0}}</ref>
* Irr I galaxies have asymmetric profiles and lack a central bulge or obvious spiral structure; instead they contain many individual clusters of young stars
* Irr II galaxies have smoother, asymmetric appearances and are not clearly resolved into individual stars or stellar clusters
In his extension to the Hubble sequence, de Vaucouleurs called the Irr I galaxies 'Magellanic irregulars', after the [[Magellanic Clouds]] – two satellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure<ref>{{cite journal |last1=de&nbsp;Vaucouleurs |first1=G. |author-link1=Gérard de Vaucouleurs |date=1955 |title=Studies of Magellanic Clouds. I. Dimensions and structure of the Large Cloud |journal=[[The Astronomical Journal]] |volume=160 |pages=126–140 |bibcode=1955AJ.....60..126D |doi=10.1086/107173 |last2=Oemler |first2=Augustus Jr. |last3=Butcher |first3=Harvey R. |last4=Gunn |first4=James E.}}</ref> in the [[Large Magellanic Cloud]] led de Vaucouleurs to further divide the irregular galaxies into those that, like the LMC, show some evidence for spiral structure (these are given the symbol Sm) and those that have no obvious structure, such as the [[Small Magellanic Cloud]] (denoted Im). In the extended Hubble sequence, the Magellanic irregulars are usually placed at the end of the spiral branch of the Hubble tuning fork.

Examples of irregular galaxies: [[Messier 82|M82]], [[NGC 1427A]], [[Large Magellanic Cloud]], [[Small Magellanic Cloud]].