Editing Elliptical galaxy (section)

==Sizes and shapes==
[[File:A Multi-Wavelength View of Radio Galaxy Hercules A.jpg|thumb|[[Hercules A]], a supergiant elliptical galaxy and also a [[radio galaxy]]. The radio lobes shown here in pink are over a million light-years across.]]
The [[List of largest galaxies|largest galaxies]] are supergiant ellipticals, or [[Type-cD galaxy|type-cD galaxies]]. Elliptical galaxies vary greatly in both size and mass with diameters ranging from 3,000 [[light year]]s to more than 700,000 light years, and masses from 10<sup>5</sup> to nearly 10<sup>13</sup> solar masses.<ref>{{cite book|last1=Fraknoi|first1=Andrew|last2=Morrison|first2=David|last3=Wolf|first3=Sidney C.|title=Open Stax Astronomy|date=13 January 2017|url=https://cnx.org/contents/LnN76Opl@10.13:1G3DS4T6@3/Properties-of-Galaxies|access-date=2 February 2017}}</ref> This range is much broader for this galaxy type than for any other. The smallest, the [[Dwarf elliptical galaxy|dwarf elliptical galaxies]], may be no larger than a typical [[globular cluster]], but contain a considerable amount of [[dark matter]] not present in clusters. Most of these small galaxies may not be related to other ellipticals.[[File:MACSJ1423.8+2404.jpg|thumb|The brilliant central object is the supergiant elliptical galaxy SDSS J142347.87+240442.4, the dominant member of the [[galaxy cluster]] MACS J1423.8+2404. It has a diameter of 380,000 light-years.<ref name=NED_SDSSJ1423>{{Cite web|url=https://ned.ipac.caltech.edu/cgi-bin/objsearch?objname=SDSS+J142347.87+240442.4&extend=no&hconst=67.74&omegam=0.3089&omegav=0.6911&corr_z=1&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES|title=Your NED Search Results|website=ned.ipac.caltech.edu|accessdate=8 May 2023}}</ref> Note the [[gravitational lensing]].]]

The [[Galaxy morphological classification|Hubble classification]] of elliptical galaxies contains an integer that describes how elongated the galaxy image is.
The classification is determined by the ratio of the major (''a'') to the minor (''b'') axes of the galaxy's [[Contour line#Other phenomena|isophotes]]:

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

Thus for a spherical galaxy with ''a'' equal to ''b'', the number
is 0, and the Hubble type is E0. While the limit in the literature is about E7, it has been known since 1966<ref name="adsabs.harvard.edu" /> that the E4 to E7 galaxies are misclassified lenticular galaxies with disks inclined at different angles to our line of sight. This has been confirmed through spectral observations revealing the rotation of their stellar disks.<ref>Graham, A.W. et al. (1998), [http://adsabs.harvard.edu/abs/1998A%26AS..133..325G Extended stellar kinematics of elliptical galaxies in the Fornax cluster]</ref><ref>Emsellem, E. (2011), [http://adsabs.harvard.edu/abs/2011MNRAS.414..888E 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]</ref> Hubble recognized that his shape classification depends both on the intrinsic shape of the galaxy, as well as the angle with which the galaxy is observed. Hence, some galaxies with Hubble type E0 are actually elongated.

It is sometimes said that there are two physical types of ellipticals: the giant ellipticals with slightly "boxy"-shaped isophotes, whose shapes result from random motion which is greater in some directions than in others (anisotropic random motion); and the "disky" normal and [[dwarf elliptical]]s, which contain disks.<ref>Pedraz, S. et al. (2002), [http://adsabs.harvard.edu/abs/2002MNRAS.332L..59P Evidence of fast rotation in dwarf elliptical galaxies]</ref><ref>Toloba, E. et al. (2015), [http://adsabs.harvard.edu/abs/2015ApJ...799..172T Stellar Kinematics and Structural Properties of Virgo Cluster Dwarf Early-type Galaxies from the SMAKCED Project. III. Angular Momentum and Constraints on Formation Scenarios]</ref> This is, however, an abuse of the nomenclature, as there are two types of early-type galaxy, those with disks and those without. Given the existence of ES galaxies with intermediate-scale disks, it is reasonable to expect that there is a continuity from E to ES, and onto the S0 galaxies with their large-scale stellar disks that dominate the light at large radii.

[[Dwarf spheroidal galaxy|Dwarf spheroidal galaxies]] appear to be a distinct class: their properties are more similar to those of irregulars and late spiral-type galaxies.

At the large end of the elliptical spectrum, there is further division, beyond Hubble's classification. Beyond ''gE'' giant ellipticals, lies [[Type-D galaxy|D-galaxies]] and [[Type-cD galaxy|cD-galaxies]]. These are similar to their smaller brethren, but more diffuse, with large haloes that may as much belong to the galaxy cluster within which they reside than the centrally-located giant galaxy.

[[File:A galactic mega-merger.jpg|thumb|[[NGC 3597]] is the product of a collision between two galaxies. It is evolving into a giant elliptical galaxy.]]