Editing Hubble sequence (section)

===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]].