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| name = Milky Way
| image = File:ESO-VLT-Laser-phot-33a-07.jpg
| caption = {{longitem|The [[Galactic Center]] as seen from [[Earth]]'s night sky (featuring the telescope's [[laser guide star]]). Listed below is Galactic Center's information.|style=padding: 4px 0;}}
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
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<!--NOTE 2: Before editing, please do not state that the Milky Way has a large diameter (100 kly or more). This matter is currently under discussion, and any change in the figures should be consulted first at the talk page, with references being cited so that it can be subjected to a consensus. Thanks.-->
The '''Milky Way'''<!--Start of the "context" note-->{{efn|name=context|1=Some authors use the term ''Milky Way'' to refer exclusively to the band of light that the galaxy forms in the night sky, while the galaxy receives the full name '''Milky Way Galaxy'''. See for example
The Milky Way is a [[barred spiral galaxy]] with a [[Galaxy#Isophotal diameter|D<sub>25</sub> isophotal diameter]] estimated at {{convert|26.8|+/-|1.1|kpc|ly|-2|abbr=off|lk=on}},<ref name="Goodwin" /> but only about 1,000 light-years thick at the spiral arms (more at the bulge). Recent simulations suggest that a [[dark matter]] area, also containing some visible stars, may extend up to a diameter of almost 2 million light-years (613 kpc).<ref name=croswell2020 /><ref name="dearson2020" /> The Milky Way has several [[List of Milky Way's satellite galaxies|satellite galaxies]] and is part of the [[Local Group]] of galaxies,
It is estimated to contain 100–400 billion stars<ref>{{Cite web |title=Milky Way |url=http://www.bbc.co.uk/science/space/universe/key_places/milky_way |archive-url=https://web.archive.org/web/20120302071454/http://www.bbc.co.uk/science/space/universe/key_places/milky_way |archive-date=March 2, 2012 |publisher=[[BBC]]}}</ref><ref>{{Cite web |title=How Many Stars in the Milky Way? |url=http://asd.gsfc.nasa.gov/blueshift/index.php/2015/07/22/how-many-stars-in-the-milky-way/ |url-status=live |archive-url=https://web.archive.org/web/20160125140109/http://asd.gsfc.nasa.gov/blueshift/index.php/2015/07/22/how-many-stars-in-the-milky-way/ |archive-date=January 25, 2016 |website=NASA Blueshift}}</ref> and at least that number of [[
[[Galileo Galilei]] first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the [[Universe]].<ref>{{Cite news |title=Milky Way Galaxy: Facts About Our Galactic Home |url=http://www.space.com/19915-milky-way-galaxy.html |archive-url=https://web.archive.org/web/20170321063305/http://www.space.com/19915-milky-way-galaxy.html |archive-date=March 21, 2017 |access-date=April 8, 2017 |work=Space.com}}</ref> Following the 1920 [[Great Debate (astronomy)|Great Debate]] between the astronomers [[Harlow Shapley]] and [[Heber Doust Curtis]],<ref name="shapley_curtis" /> observations by [[Edwin Hubble]] in 1923 showed that the Milky Way is just one of many galaxies.
== Etymology and mythology ==
{{Main
In the [[Babylonia]]n epic poem ''[[Enūma Eliš]]'', the Milky Way is created from the severed tail of the primeval salt water [[dragon]]ess [[Tiamat]], set in the sky by [[Marduk]], the Babylonian [[national god]], after slaying her.<ref>{{Cite book |last=Brown |first=William P. |url=https://books.google.com/books?id=zce500t8puUC&pg=PA25 |title=The Seven Pillars of Creation: The Bible, Science, and the Ecology of Wonder |date=2010 |publisher=Oxford University Press |isbn=978-0-19-973079-7 |location=Oxford, England |page=25 |access-date=April 24, 2019 |archive-url=https://web.archive.org/web/20230326142933/https://books.google.com/books?id=zce500t8puUC&pg=PA25 |archive-date=March 26, 2023 |url-status=live}}</ref><ref>{{Cite book |last=MacBeath |first=Alastair |url=https://books.google.com/books?id=uobfAAAAMAAJ |title=Tiamat's Brood: An Investigation Into the Dragons of Ancient Mesopotamia |date=1999 |publisher=Dragon's Head |isbn=978-0-9524387-5-5 |page=41 |access-date=April 24, 2019 |archive-url=https://web.archive.org/web/20230326142947/https://books.google.com/books?id=uobfAAAAMAAJ |archive-date=March 26, 2023 |url-status=live}}</ref> This story was once thought to have been based on an older [[Sumer]]ian version in which Tiamat is instead slain by [[Enlil]] of [[Nippur]],<ref>{{Cite book |last=James |first=E. O. |url=https://archive.org/details/theworshipoftheskygod/ |title=The Worship of the Sky-God: A Comparative Study in Semitic and Indo-European Religion |date=1963 |publisher=University of London Press |series=Jordan Lectures in Comparative Religion |location=London, England |pages=24, 27f}}</ref><ref name="Lambert1964">{{Cite journal |last=Lambert |first=W. G. |date=1964 |title=E. O. James: The worship of the Skygod: A comparative study in Semitic and Indo-European religion. (School of Oriental and African Studies, University of London. Jordan Lectures in Comparative Religion, vi.) viii, 175 pp. London: University of London, the Athlone Press, 1963. 25s. |journal=Bulletin of the School of Oriental and African Studies |location=London, England |publisher=University of London |volume=27 |issue=1 |pages=157–158 |doi=10.1017/S0041977X00100345}}</ref> but is now thought to be purely an invention of Babylonian propagandists with the intention to show Marduk as superior to the Sumerian deities.<ref name="Lambert1964" />
In [[Greek mythology]], [[Zeus]] places his son born by a mortal woman, the infant [[Heracles]], on [[Hera]]'s breast while she is asleep so the baby will drink [[Milk of Hera|her divine milk]] and become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way. In another Greek story, the abandoned Heracles is given by [[Athena]] to Hera for feeding, but Heracles' forcefulness causes Hera to rip him from her breast in pain.<ref>{{Cite web |title=Myths about the Milky Way |url=http://judy-volker.com/StarLore/Myths/MilkyWay1.html |url-status=live |archive-url=https://web.archive.org/web/20220701070331/http://judy-volker.com/StarLore/Myths/MilkyWay1.html |archive-date=July 1, 2022 |access-date=2022-03-21 |website=judy-volker.com}}</ref><ref name="Leeming">{{Cite book |last=Leeming |first=David Adams |url=https://books.google.com/books?id=YJawuz5Q1vEC&pg=PA44 |title=Mythology: The Voyage of the Hero |date=1998 |publisher=Oxford University Press |isbn=978-0-19-511957-2 |edition=Third |location=Oxford, England |page=44 |access-date=April 24, 2019 |archive-url=https://web.archive.org/web/20230326142931/https://books.google.com/books?id=YJawuz5Q1vEC&pg=PA44 |archive-date=March 26, 2023 |url-status=live}}</ref><ref name="Pache">{{Cite book |last=Pache |first=Corinne Ondine |chapter-url=https://books.google.com/books?id=lNV6-HsUppsC&pg=RA2-PA400 |title=Ancient Greece and Rome |date=2010 |publisher=Oxford University Press |isbn=978-0-19-538839-8 |editor-last=Gargarin |editor-first=Michael |volume=1: Academy-Bible |location=Oxford, England |page=400 |chapter=Hercules |access-date=April 24, 2019 |editor-last2=Fantham |editor-first2=Elaine |editor-link2=Elaine Fantham |archive-url=https://web.archive.org/web/20230326142932/https://books.google.com/books?id=lNV6-HsUppsC&pg=RA2-PA400 |archive-date=March 26, 2023 |url-status=live}}</ref>
Llys Dôn (literally "The Court of [[Dôn]]") is the traditional Welsh name for the constellation [[Cassiopeia (constellation)|Cassiopeia]]. At least
In Western culture, the name "Milky Way" is derived from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term is a translation of the [[Classical Latin]] ''via lactea'', in turn derived from the [[Hellenistic Greek]] {{lang|grc|γαλαξίας}}, short for {{lang|grc|γαλαξίας κύκλος}} (''{{transliteration|grc|galaxías kýklos}}''), meaning "milky circle". The [[Ancient Greek]] {{lang|grc|γαλαξίας}} (''{{transliteration|grc|galaxias}}'') – from root {{lang|grc|γαλακτ}}-, {{lang|grc|γάλα}} ("milk") + {{lang|grc|-ίας}} (forming adjectives) – is also the root of "galaxy", the name for our, and later all such, collections of stars.<ref name=eo_galaxy /><ref name=jankowski2010 /><ref name=oxford />
The Milky Way, or "milk circle", was just one of 11 "circles" the Greeks identified in the sky, others being the [[zodiac]], the [[meridian (astronomy)|meridian]], the [[horizon]], the [[equator]], the [[tropics of Cancer and Capricorn]], the [[Arctic Circle]] and the [[Antarctic Circle]], and two [[colure]] circles passing through both poles.<ref name=eratosthenes1997 />
=== Common names ===
* "Birds' Path" is used in several [[Uralic languages|Uralic]] and [[Turkic languages]] and in the [[Baltic languages]]. Northern peoples observed that [[migratory birds]] follow the course of the galaxy<ref>^ Sauer, EGF (July 1971). "Celestial Rotation and Stellar Orientation in Migratory Warblers". Science 30: 459–461.</ref> while migrating at the Northern Hemisphere. The name "Birds' Path" (in Finnish, Estonian, Latvian, Lithuanian, Bashkir and Kazakh) has some variations in other languages, e.g. "Way of the grey (wild) goose" in Chuvash, Mari and Tatar and "Way of the Crane" in Erzya and Moksha.
*House river: The [[Kaurna people]] of the [[Adelaide Plains]] of South Australia called the Milky Way ''wodliparri'' in the [[Kaurna language]], meaning "house river".<ref>{{cite web | title=Reconciliation | website=Adelaide City Council | url=https://www.adelaide.edu.au/kwp/placenames/council/areas/map_kwp.html | access-date=26 February 2020 | archive-date=12 July 2019 | archive-url=https://web.archive.org/web/20190712144216/https://www.adelaide.edu.au/kwp/placenames/council/areas/map_kwp.html | url-status=dead }}</ref>
*Emu in the Sky: The [[Gamilaraay|Gomeroi people]] between [[New South Wales]] and [[Queensland]] called the Milky Way ''Dhinawan'', the giant [[Emu|Emu in the Sky]] that it stretches across the night sky.<ref>{{cite web |first=Rami |last=Mandow |title=Moonhack – Coding the Story of the Emu in the Sky |website=Space Australia |date=2021-05-03 |url=https://spaceaustralia.com/news/moonhack-coding-story-emu-sky |access-date=2022-06-05}}</ref>
*Milky Way: Many European languages have borrowed, directly or indirectly, the Greek name for the Milky Way, including English and Latin.
*Road to Santiago: the Milky Way was traditionally used as a guide by [[pilgrims]] traveling to the holy site at [[Santiago de Compostela|Compostela]], hence the use of "The Road to Santiago" as a name for the Milky Way.<ref name="Fiona">{{cite book
|last = Macleod
|first = Fiona
|author-link = William Sharp (writer)
|title = Where the forest murmurs
|page = Chapter 21: ''Milky Way''
|publisher = Duffield & Company
|year = 1911
|location = New York
|url = http://www.sundown.pair.com/SundownShores/Volume_VI/ForestMurmers/milky%20way.htm
|no-pp = true
|url-status = dead
|archive-url = https://web.archive.org/web/20070217215638/http://www.sundown.pair.com/SundownShores/Volume_VI/ForestMurmers/milky%20way.htm
|archive-date = 17 February 2007
}}</ref> Curiously, ''La Voje Ladee'' "The Milky Way" was also used to refer to the pilgrimage road.<ref>{{cite web|title= The Pilgrim's Way: El Camino de Santiago|url= http://dspace.dial.pipex.com/telegraph/04camino/04000001.htm|access-date= 2007-01-06|url-status= dead|archive-url= https://web.archive.org/web/20061217160109/http://dspace.dial.pipex.com/telegraph/04camino/04000001.htm|archive-date = 17 December 2006}}</ref>
*River Ganga of the Sky: this Sanskrit name ({{lang|sa|आकाशगंगा}} ''Ākāśagaṃgā'') is used in many Indian languages following a Hindu belief .
*Silver River: this Chinese name "Silver River" ({{lang|zh-Hant|銀河}}) is used throughout East Asia, including Korea and Vietnam. In Japan and Korea, "Silver River" ({{langx|ja|銀河|ginga}}; {{korean|hangul=은하|rr=eunha}}) means galaxies in general.
*River of Heaven: The Japanese name for the Milky Way is the "River of Heaven" ({{lang|ja|天の川}}, ''Amanokawa''), as well as an alternative name in Chinese ({{zh|c=天河|p=Tiānhé}}).
*Straw Way:In West Asia, Central Asia and parts of the Balkans the name for the Milky Way is related to the word for [[straw]]. Today, Persians, Pakistanis, and Turks use it in addition to Arabs. It has been suggested that the term was spread by medieval [[Arab]]s who in turn borrowed it from Armenians.<ref>{{cite journal|last=Harutyunyan |first=Hayk |title=The Armenian name of the Milky Way|journal=ArAS News|volume=6|publisher=Armenian Astronomical Society (ArAS)|date=2003-08-29 |url=http://www.aras.am/ArasNews/arasnews06.html |access-date=2009-08-10 |url-status=dead |archive-url=https://web.archive.org/web/20060429194053/http://www.aras.am/ARASNEWS/arasnews06.html |archive-date=29 April 2006 }}</ref>
*Walsingham Way: In England the Milky Way was called the Walsingham Way in reference to the shrine of [[Our Lady of Walsingham]] which is in [[Norfolk]], England. It was understood to be either a guide to the pilgrims who flocked there, or a representation of the pilgrims themselves.<ref>{{cite web|last=Bogle|first=Joanna|title=A Pilgrimage to Walsingham, 'England's Nazareth'|url=http://www.ncregister.com/site/article/a-pilgrimage-to-walsingham-englands-nazareth|work=National Catholic Register|date=16 September 2011 |publisher=EWTN|access-date=2013-11-13}}</ref>
*Winter Street: Scandinavian peoples, such as Swedes, have called the galaxy Winter Street (''Vintergatan'') as the galaxy is most clearly visible during the winter at the northern hemisphere, especially at high latitudes where the [[midnight sun|glow of the Sun late at night]] can obscure it during the summer.
== Appearance ==
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=== Galactic quadrants ===
{{Main|Galactic quadrant}}
[[File:Galactic longitude.JPG|thumb
A galactic quadrant, or quadrant of the Milky Way, refers to one of four circular sectors in the division of the Milky Way. In astronomical practice, the delineation of the galactic quadrants is based upon the [[galactic coordinate system]], which places the [[Sun]] as the [[Polar coordinate system|origin of the mapping system]].<ref>{{Citation |last1=Blaauw |first1=A. |title=The new I. A. U. system of galactic coordinates (1958 revision) |date=1960 |journal=Monthly Notices of the Royal Astronomical Society |volume=121 |issue=2 |pages=123–131 |bibcode=1960MNRAS.121..123B |doi=10.1093/mnras/121.2.123 |display-authors=1 |last2=Gum |first2=C. S. |last3=Pawsey |first3=J. L. |last4=Westerhout |first4=G. |doi-access=free}}</ref>
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The nature of the Milky Way's bar is actively debated, with estimates for its half-length and orientation spanning from {{convert|1|to(-)|5|kpc|ly|-3|abbr=on}} and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.<ref name="vanhollebeke09" /><ref name="majaess10" /><ref name="Cabrera-Lavers08" /> Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.<ref name="nishiyama06" /> However, [[RR Lyrae variable|RR Lyrae-type]] stars do not trace a prominent Galactic bar.<ref name="majaess10" /><ref name="alcock98" /><ref name="kunder08" /> The bar may be surrounded by a ring called the "5 kpc ring" that contains a large fraction of the molecular hydrogen present in the Milky Way, as well as most of the Milky Way's [[star formation]] activity. Viewed from the [[Andromeda Galaxy]], it would be the brightest feature of the Milky Way.<ref name="fn14" /> X-ray emission from the core is aligned with the massive stars surrounding the central bar<ref name="wang13">{{Cite journal |last1=Wang |first1=Q.D. |last2=Nowak |first2=M.A. |last3=Markoff |first3=S.B. |last4=Baganoff |first4=F.K. |last5=Nayakshin |first5=S. |last6=Yuan |first6=F. |last7=Cuadra |first7=J. |last8=Davis |first8=J. |last9=Dexter |first9=J. |last10=Fabian |first10=A.C. |last11=Grosso |first11=N. |last12=Haggard |first12=D. |last13=Houck |first13=J. |last14=Ji |first14=L. |last15=Li |first15=Z. |year=2013 |title=Dissecting X-ray-Emitting Gas Around the Center of Our Galaxy |journal=Science |volume=341 |issue=6149 |pages=981–983 |arxiv=1307.5845 |bibcode=2013Sci...341..981W |doi=10.1126/science.1240755 |pmid=23990554 |s2cid=206550019 |last16=Neilsen |first16=J. |last17=Porquet |first17=D. |last18=Ripple |first18=F. |last19=Shcherbakov |first19=R.V.}}</ref> and the [[Galactic ridge]].<ref name="nature318_267">{{Cite journal |last1=Bhat, C. L. |last2=Kifune, T. |last3=Wolfendale, A. W. |date=November 21, 1985 |title=A cosmic-ray explanation of the galactic ridge of cosmic X-rays |journal=Nature |volume=318 |issue=6043 |pages=267–269 |bibcode=1985Natur.318..267B |doi=10.1038/318267a0 |s2cid=4262045}}</ref>
In June 2023, astronomers led by [[Naoko Kurahashi Neilson]] reported using a new cascade neutrino technique<ref>{{Cite journal |last=Wright |first=Katherine |date=2023 |title=Milky Way Viewed through Neutrinos |journal=Physics |publisher=Physics 16, 115 (29 June 2023) |volume=16 |page=115 |bibcode=2023PhyOJ..16..115W |doi=10.1103/Physics.16.115 |quote=Kurahashi Neilson first came up with the idea to use cascade neutrinos to map the Milky Way in 2015. |doi-access=free}}</ref> to detect, for the first time, the release of [[neutrino]]s from the [[galactic plane]] of the Milky Way [[galaxy]], creating the first neutrino view of the Milky Way.<ref name="NYT-20230629">{{Cite news |last=Chang |first=Kenneth |date=29 June 2023 |title=Neutrinos Build a Ghostly Map of the Milky Way – Astronomers for the first time detected neutrinos that originated within our local galaxy using a new technique. |work=[[The New York Times]] |url=https://www.nytimes.com/2023/06/29/science/neutrinos-milky-way-map.html |url-status=live |access-date=30 June 2023 |archive-url=https://archive.today/20230629182106/https://www.nytimes.com/2023/06/29/science/neutrinos-milky-way-map.html |archive-date=29 June 2023}}</ref><ref name="SCI-20230629">{{Cite journal |last=IceCube Collaboration |date=29 June 2023 |title=Observation of high-energy neutrinos from the Galactic plane |url=https://www.science.org/doi/10.1126/science.adc9818 |url-status=live |journal=[[Science (journal)|Science]] |volume=380 |issue=6652 |pages=1338–1343 |arxiv=2307.04427 |doi=10.1126/science.adc9818 |pmid=37384687 |bibcode=2023Sci...380.1338I |s2cid=259287623 |archive-url=https://archive.today/20230630042539/https://www.science.org/doi/10.1126/science.adc9818 |archive-date=30 June 2023 |access-date=30 June 2023}}</ref>
==== Gamma rays and x-rays ====
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In November 2018, astronomers reported the discovery of one of the oldest stars in the universe. About 13.5 billion-years-old, [[2MASS J18082002-5104378 B]] is a tiny ultra metal-poor (UMP) star made almost entirely of materials released from the [[Big Bang]], and is possibly one of the first stars. The discovery of the star in the Milky Way [[Galaxy]] suggests that the galaxy may be at least 3 billion years older than previously thought.<ref name="EA-20181105">{{Cite news |last=Johns Hopkins University |author-link=Johns Hopkins University |date=November 5, 2018 |title=Johns Hopkins scientist finds elusive star with origins close to Big Bang |work=[[EurekAlert!]] |url=https://www.eurekalert.org/pub_releases/2018-11/jhu-jhs110518.php |url-status=live |access-date=November 5, 2018 |archive-url=https://web.archive.org/web/20181106132235/https://www.eurekalert.org/pub_releases/2018-11/jhu-jhs110518.php |archive-date=November 6, 2018}}</ref><ref name="JHU-20181105">{{Cite news |last=Rosen |first=Jill |date=November 5, 2018 |title=Johns Hopkins scientist finds elusive star with origins close to Big Bang – The newly discovered star's composition indicates that, in a cosmic family tree, it could be as little as one generation removed from the Big Bang |publisher=[[Johns Hopkins University]] |url=https://hub.jhu.edu/2018/11/05/scientists-find-star-with-big-bang-origins/ |url-status=live |access-date=November 5, 2018 |archive-url=https://web.archive.org/web/20181106021959/https://hub.jhu.edu/2018/11/05/scientists-find-star-with-big-bang-origins/ |archive-date=November 6, 2018}}</ref><ref name="ApJ-20181105">{{Cite journal |last1=Schlaufman |first1=Kevin C. |last2=Thompson |first2=Ian B. |last3=Casey |first3=Andrew R. |date=November 5, 2018 |title=An Ultra Metal-poor Star Near the Hydrogen-burning Limit |journal=[[The Astrophysical Journal]] |volume=867 |page=98 |arxiv=1811.00549 |bibcode=2018ApJ...867...98S |doi=10.3847/1538-4357/aadd97 |s2cid=54511945 |doi-access=free |number=2}}</ref>
Several individual stars have been found in the Milky Way's halo with measured ages very close to the 13.80-billion-year [[age of the Universe]]. In 2007, a star in the galactic halo, [[HE 1523-0901]], was estimated to be about 13.2 billion years old. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way.<ref name="frebel" /> This estimate was made using the UV-Visual Echelle Spectrograph of the [[Very Large Telescope]] to [[Measurement|measure]] the relative strengths of [[spectral line]]s caused by the presence of [[thorium]] and other [[Chemical element|elements]] created by the [[R-process]]. The line strengths yield abundances of different elemental [[isotope]]s, from which an estimate of the age of the star can be derived using [[nucleocosmochronology]].<ref name="frebel" /> Another star, [[HD 140283]],
According to observations utilizing [[adaptive optics]] to correct for Earth's atmospheric distortion, stars in the galaxy's bulge date to about 12.8 billion years old.<ref>{{Cite web |last=Specktor |first=Brandon |date=March 23, 2019 |title=Astronomers Find Fossils of Early Universe Stuffed in Milky Way's Bulge |url=https://
The age of stars in the galactic [[thin disk]] has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the [[galactic halo]] and the thin disk.<ref name="del_Peloso" /> Recent analysis of the chemical signatures of thousands of stars suggests that stellar formation might have dropped by an order of magnitude at the time of disk formation, 10 to 8 billion years ago, when interstellar gas was too hot to form new stars at the same rate as before.<ref>Skibba, Ramon (2016), "Milky Way retired early from star making" (New Scientist, March 5, 2016), p.9</ref>
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{{Main|Local Group}}
The Milky Way and the [[Andromeda Galaxy]] are a [[binary system (astronomy)|binary system]] of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the [[Local Group]], surrounded by a Local Void, itself being part of the [[Local Sheet]]<ref name="apj676_1_184">{{Cite journal |last1=Tully |first1=R. Brent |last2=Shaya |first2=Edward J. |last3=Karachentsev |first3=Igor D. |last4=Courtois |first4=Hélène M. |author-link4=Hélène Courtois |last5=Kocevski |first5=Dale D. |last6=Rizzi |first6=Luca |last7=Peel |first7=Alan |date=March 2008 |title=Our Peculiar Motion Away from the Local Void |journal=The Astrophysical Journal |volume=676 |issue=1 |pages=184–205 |arxiv=0705.4139 |bibcode=2008ApJ...676..184T |doi=10.1086/527428 |s2cid=14738309}}</ref> and in turn the [[Virgo Supercluster]]. Surrounding the Virgo Supercluster are a number of [[Void (astronomy)|voids]], devoid of many galaxies, the Microscopium Void to the "north", the Sculptor Void to the "left", the [[Boötes Void]] to the "right" and the Canes-Major Void to the "south". These voids change shape over time, creating filamentous structures of galaxies. The Virgo Supercluster, for instance, is being drawn towards the [[Great Attractor]],<ref>{{cite web |last=Hadhazy |first=Adam |date=2016-11-03 |url=https://www.discovermagazine.com/the-sciences/why-nothing-really-matters |title=Why Nothing Really Matters |website=Discover Magazine |access-date=2022-04-24 |archive-date=April 24, 2022 |archive-url=https://web.archive.org/web/20220424224846/https://www.discovermagazine.com/the-sciences/why-nothing-really-matters |url-status=live }}</ref> which in turn forms part of a greater structure, called [[Laniakea Supercluster|Laniakea]].<ref>{{cite journal |title=The Laniakea supercluster of galaxies |author=R. Brent Tully |author2=Helene Courtois |author3=Yehuda Hoffman |author4=Daniel Pomarède |date=September 2, 2014 |publication-date=September 4, 2014 |journal=Nature |volume=513 |number=7516 |pages=71–73 |bibcode=2014Natur.513...71T |arxiv=1409.0880 |doi=10.1038/nature13674 |pmid=25186900|s2cid=205240232 }}</ref>
Two smaller galaxies and a number of [[dwarf galaxy|dwarf galaxies]] in the Local Group orbit the Milky Way. The largest of these is the [[Large Magellanic Cloud]] with a diameter of 32,200 light-years.<ref name=RC3>{{cite book | year=1991 | title=Third Reference Catalogue of Bright Galaxies | last1=De Vaucouleurs | first1=Gerard | last2=De Vaucouleurs | first2=Antoinette | last3=Corwin | first3=Herold G. | last4=Buta | first4=Ronald J. | last5=Paturel | first5=Georges | last6=Fouque | first6=Pascal | doi = 10.1007/978-1-4757-4363-0 | bibcode=1991rc3..book.....D | isbn=978-1-4757-4365-4 }}</ref> It has a close companion, the [[Small Magellanic Cloud]]. The [[Magellanic Stream]] is a stream of neutral [[hydrogen]] gas extending from these two small galaxies across 100° of the sky. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way.<ref>{{Cite journal |last1=Putman |first1=M. E. |last2=Staveley-Smith |first2=L. |last3=Freeman |first3=K. C. |last4=Gibson |first4=B. K. |last5=Barnes |first5=D. G. |title=The Magellanic Stream, High-Velocity Clouds, and the Sculptor Group |doi=10.1086/344477 |journal=The Astrophysical Journal |volume=586 |issue=1 |pages=170–194 |year=2003 |arxiv=astro-ph/0209127 |bibcode=2003ApJ...586..170P|s2cid=6911875 }}</ref> Some of the [[Milky Way's satellite galaxies|dwarf galaxies orbiting the Milky Way]] are [[Canis Major Dwarf]] (the closest), [[Sagittarius Dwarf Elliptical Galaxy]], [[Ursa Minor Dwarf]], [[Sculptor Dwarf]], [[Sextans Dwarf]], [[Fornax Dwarf]], and [[Leo I Dwarf]].<ref name=Koposov2015/>
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