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There are several methods being used in astronomy in defining the size of a galaxy, and each of them can yield different results with respect to one another. The most commonly employed method is the [[Galaxy#Isophotal diameter|D<sub>25</sub> standard]] – the [[isophote]] where the photometric brightness of a galaxy in the B-band (445 nm wavelength of light, in the blue part of the [[visible spectrum]]) reaches 25 mag/arcsec<sup>2</sup>.<ref name="Dimensions">{{Cite web |title=Dimensions of Galaxies |url=https://ned.ipac.caltech.edu/level5/PROPERTIES/dog.html |url-status=live |archive-url=https://web.archive.org/web/20220927170330/http://ned.ipac.caltech.edu/level5/PROPERTIES/dog.html |archive-date=September 27, 2022 |access-date=August 22, 2022 |website=ned.ipac.caltech.edu}}</ref> An estimate from 1997 by Goodwin and others compared the distribution of [[Cepheid variable]] stars in 17 other spiral galaxies to the ones in the Milky Way, and modelling the relationship to their surface brightnesses. This gave an [[isophotal diameter]] for the Milky Way at {{convert|26.8|±|1.1|kpc|ly|-2|abbr=off}}, by assuming that the galactic disc is well represented by an exponential disc and adopting a central surface brightness of the galaxy (μ<sub>0</sub>) of {{val|22.1|0.3}} ''B''-mag/arcsec<sup>−2</sup> and a disk scale length (''h'') of {{convert|5.0|±|0.5|kpc|ly|-2|abbr=on}}.<ref name="Average">{{Cite arXiv |eprint=astro-ph/9704216 |first1=S. P. |last1=Goodwin |first2=J. |last2=Gribbin |title=The Milky Way is just an average spiral |date=22 April 1997 |last3=Hendry |first3=M. A.}}</ref><ref name="Goodwin">{{Cite journal |last1=Goodwin |first1=S. P. |last2=Gribbin |first2=J. |last3=Hendry |first3=M. A. |date=August 1998 |title=The relative size of the Milky Way |journal=The Observatory |volume=118 |pages=201–208 |bibcode=1998Obs...118..201G}}</ref><ref name="Castro">{{Cite journal |last1=Castro-Rodríguez |first1=N. |last2=López-Corredoira |first2=M. |last3=Sánchez-Saavedra |first3=M. L. |last4=Battaner |first4=E. |year=2002 |title=Warps and correlations with intrinsic parameters of galaxies in the visible and radio |journal=Astronomy & Astrophysics |volume=391 |issue=2 |pages=519–530 |arxiv=astro-ph/0205553 |bibcode=2002A&A...391..519C |doi=10.1051/0004-6361:20020895 |s2cid=17813024}}</ref>
This is significantly smaller than the Andromeda Galaxy's isophotal diameter, and slightly below the mean isophotal sizes of the galaxies being at {{convert|28.3|kpc|ly|sigfig=2|abbr=on}}.<ref name="Goodwin" /> The paper concludes that the Milky Way and Andromeda Galaxy were not overly large spiral galaxies,
A 2015 paper discovered that there is a ring-like filament of stars called Triangulum–Andromeda Ring (TriAnd Ring) rippling above and below the relatively flat [[galactic plane]], which alongside [[Monoceros Ring]] were both suggested to be primarily the result of disk oscillations and wrapping around the Milky Way, at a diameter of at least {{convert|50|kpc|ly|sigfig=2|abbr=on}},<ref>{{Cite journal |last1=Newberg |first1=Heidi Jo |last2=Xu |first2=Yan |last3=Carlin |first3=Jeffrey L. |last4=Liu |first4=Chao |last5=Deng |first5=Licai |last6=Li |first6=Jing |last7=Schoenrich |first7=Ralph |last8=Yanny |first8=Brian |display-authors=1 |date=March 1, 2015 |title=Rings and Radial Waves in the Disk of the Milky Way |journal=[[The Astrophysical Journal]] |volume=801 |issue=2 |page=105 |arxiv=1503.00257 |bibcode=2015ApJ...801..105X |doi=10.1088/0004-637X/801/2/105 |s2cid=119124338}}</ref> which may be part of the Milky Way's outer disk itself, hence making the stellar disk larger by increasing to this size.<ref name="rpi2015" /> A more recent 2018 paper later somewhat ruled out this hypothesis, and supported a conclusion that the Monoceros Ring, [[A13 (stellar overdensity)|A13]] and TriAnd Ring were stellar overdensities rather kicked out from the main stellar disk, with the velocity dispersion of the RR Lyrae stars found to be higher and consistent with halo membership.<ref name="allyson">{{Cite journal |last1=Sheffield |first1=Allyson A. |last2=Price-Whelan |first2=Adrian M. |last3=Tzanidakis |first3=Anastasios |last4=Johnston |first4=Kathryn V. |last5=Laporte |first5=Chervin F. P. |last6=Sesar |first6=Branimir |year=2018 |title=A Disk Origin for the Monoceros Ring and A13 Stellar Overdensities |journal=The Astrophysical Journal |volume=854 |issue=1 |page=47 |arxiv=1801.01171 |bibcode=2018ApJ...854...47S |doi=10.3847/1538-4357/aaa4b6 |s2cid=118932403 |doi-access=free}}</ref>
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