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{{Short description|Galaxy containing the Solar System}}
{{About|the galaxy|other uses}}
{{Pp-semi-indef|small=yes}}
{{Use mdy dates|date=March 2022}}
{{Infobox galaxy
| 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]]
| constellation name = [[Sagittarius (constellation)|Sagittarius]]
| ra = {{RA|17|45|40.03599}}<ref name="Petrov">{{Cite journal |last1=Petrov |first1=L. |last2=Kovalev |first2=Y. Y. |last3=Fomalont |first3=E. B. |last4=Gordon |first4=D. |year=2011 |title=The Very Long Baseline Array Galactic Plane Survey—VGaPS |journal=The Astronomical Journal |volume=142 |issue=2 |page=35 |arxiv=1101.1460 |bibcode=2011AJ....142...35P |doi=10.1088/0004-6256/142/2/35 |s2cid=121762178|issn=0004-6256 }}</ref>
| dec = {{DEC|−29|00|28.1699}}<ref name="Petrov" />
| dist_ly = {{convert|7.935|-|8.277|kpc|ly|sigfig=5|lk=on|abbr=on}}<ref name="ehtc_6">{{Cite journal |last1=Event Horizon Telescope Collaboration |last2=Akiyama |first2=Kazunori |last3=Alberdi |first3=Antxon |last4=Alef |first4=Walter |last5=Algaba |first5=Juan Carlos |last6=Anantua |first6=Richard |last7=Asada |first7=Keiichi |last8=Azulay |first8=Rebecca |last9=Bach |first9=Uwe |last10=Baczko |first10=Anne-Kathrin |last11=Ball |first11=David |last12=Baloković |first12=Mislav |last13=Barrett |first13=John |last14=Bauböck |first14=Michi |last15=Benson |first15=Bradford A. |display-authors=1 |year=2022 |title=First Sagittarius A* Event Horizon Telescope Results. VI. Testing the Black Hole Metric |journal=The Astrophysical Journal |volume=930 |issue=2 |pages=L17 |bibcode=2022ApJ...930L..17E |doi=10.3847/2041-8213/ac6756 |s2cid=248744741 |doi-access=free |last16=Bintley |first16=Dan |last17=Blackburn |first17=Lindy |last18=Blundell |first18=Raymond |last19=Bouman |first19=Katherine L. |last20=Bower |first20=Geoffrey C. |last21=Boyce |first21=Hope |last22=Bremer |first22=Michael |last23=Brinkerink |first23=Christiaan D. |last24=Brissenden |first24=Roger |last25=Britzen |first25=Silke |last26=Broderick |first26=Avery E. |last27=Broguiere |first27=Dominique |last28=Bronzwaer |first28=Thomas |last29=Bustamante |first29=Sandra |last30=Byun |first30=Do-Young|arxiv=2311.09484 }}</ref><ref name="idrani">{{Cite arXiv |eprint=2207.06034 |class=gr-qc |first1=Indrani |last1=Banerjee |first2=Subhadip |last2=Sau |title=Do shadows of SGR A* and M87* indicate black holes with a magnetic monopole charge? |last3=SenGupta |first3=Soumitra |year=2022}}</ref><ref name="gravity_2019">{{Cite journal |last1=Abuter |first1=R. |last2=Amorim |first2=A. |last3=Bauböck |first3=M. |last4=Berger |first4=J. P. |last5=Bonnet |first5=H. |last6=Brandner |first6=W. |last7=Clénet |first7=Y. |last8=Coudé Du Foresto |first8=V. |last9=De Zeeuw |first9=P. T. |last10=Dexter |first10=J. |last11=Duvert |first11=G. |last12=Eckart |first12=A. |last13=Eisenhauer |first13=F. |last14=Förster Schreiber |first14=N. M. |last15=Garcia |first15=P. |display-authors=1 |year=2019 |title=A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty |journal=Astronomy & Astrophysics |volume=625 |pages=L10 |arxiv=1904.05721 |bibcode=2019A&A...625L..10G |doi=10.1051/0004-6361/201935656 |s2cid=119190574 |last16=Gao |first16=F. |last17=Gendron |first17=E. |last18=Genzel |first18=R. |last19=Gerhard |first19=O. |last20=Gillessen |first20=S. |last21=Habibi |first21=M. |last22=Haubois |first22=X. |last23=Henning |first23=T. |last24=Hippler |first24=S. |last25=Horrobin |first25=M. |last26=Jiménez-Rosales |first26=A. |last27=Jocou |first27=L. |last28=Kervella |first28=P. |last29=Lacour |first29=S. |last30=Lapeyrère |first30=V.}}</ref>{{efn|name=distance}}
| type = Sb; Sbc; SB(rs)bc<ref name=ssr100_1_129 /><ref>{{Cite web |last1=Frommert |first1=Hartmut |last2=Kronberg |first2=Christine |date=August 26, 2005 |title=Classification of the Milky Way Galaxy |url=http://messier.seds.org/more/mw_type.html |url-status=live |archive-url=https://web.archive.org/web/20150531031937/http://messier.seds.org/more/mw_type.html |archive-date=May 31, 2015 |access-date=May 30, 2015 |website=SEDS}}</ref>
| mass = {{val|1.15|e=12|u=}}<ref name="SA-20190308" /><ref name="ARX-20190208" /><ref name="Kafle2012" />
| stars = {{nowrap|100–400 billion ({{Val|1|-|4|e=11}})<ref name=frommert_kronberg2005 /><ref name=wethington2010 />}}
| size = {{convert|26.8|±|1.1|kpc|ly|-2|abbr=on|lk=on|disp=x|<br /><small>(|)</small>}}<br />{{small|(''diameter; [[Galaxy#Isophotal diameter|D<sub>25</sub> isophote]]'')}}<ref name="Goodwin" />{{efn|name=Disk|This is the diameter measured using the D<sub>25</sub> standard. It has been recently suggested that there is a presence of disk stars beyond this diameter, although it is not clear how much of this influences the surface brightness profile.<ref name="Lopez2018" />
}}
| h1_scale_length_pc = {{convert|70|kpc|ly|sigfig=3|abbr=on|lk=on|disp=x|<br /><small>(|)</small>}}<ref name=kalberla>{{cite journal|bibcode=2009ARA&A..47...27K}}</ref>
| embed = {{Infobox| decat = yes | child = yes
| titlestyle = background: #ddd;
| headerstyle = background: #ddd;
| belowstyle = background: #ddd;
| labelstyle = background: inherit;
| label1 = Thickness of [[thin disk]] | data1 = {{Convert|220|-|450|pc|ly|abbr=on|lk=off|sigfig=3}}<ref name="bland">{{Cite journal |last1=Bland-Hawthorn |first1=Joss |last2=Gerhard |first2=Ortwin |year=2016 |title=The Galaxy in Context: Structural, Kinematic, and Integrated Properties |journal=Annual Review of Astronomy and Astrophysics |volume=54 |pages=529–596 |arxiv=1602.07702 |bibcode=2016ARA&A..54..529B |doi=10.1146/annurev-astro-081915-023441 |s2cid=53649594}}</ref>
| label2 = Thickness of [[thick disk]] | data2 = {{Convert|2.6|±|0.5|kpc|ly|abbr=on|lk=off|sigfig=2}}<ref name="bland" />
| label3 = Angular momentum | data3= ~{{val|1|e=67|u=J s}}<ref>{{Cite web |last=Karachentsev |first=Igor |title=Double Galaxies § 7.1 |url=https://ned.ipac.caltech.edu/level5/Sept02/Keel/Keel7.html |url-status=live |archive-url=https://web.archive.org/web/20160304054350/https://ned.ipac.caltech.edu/level5/Sept02/Keel/Keel7.html |archive-date=March 4, 2016 |access-date=April 5, 2015 |website=ned.ipac.caltech.edu |publisher=Izdatel'stvo Nauka}}</ref>
| label4 = Sun's [[Galactic year|Galactic rotation period]] | data4 = 212 Myr<ref name="newview">{{Cite web |date=April 1, 2020 |title=A New Map of the Milky Way |url=https://www.scientificamerican.com/article/a-new-map-of-the-milky-way/ |url-status=live |archive-url=https://web.archive.org/web/20210427183810/https://www.scientificamerican.com/article/a-new-map-of-the-milky-way/ |archive-date=April 27, 2021 |access-date=August 10, 2022 |website=[[Scientific American]]}}</ref>
| label5 = [[Density wave theory|Spiral pattern rotation period]] | data5 = 220–360 Myr<ref name=rotation_pattern_speeds />
| label6 = [[Barred spiral galaxy|Bar pattern]] rotation period | data6 = 160–180 Myr<ref name="structure">{{Cite journal |last1=Shen |first1=Juntai |last2=Zheng |first2=Xing-Wu |year=2020 |title=The bar and spiral arms in the Milky Way: Structure and kinematics |journal=Research in Astronomy and Astrophysics |volume=20 |issue=10 |page=159 |arxiv=2012.10130 |bibcode=2020RAA....20..159S |doi=10.1088/1674-4527/20/10/159 |s2cid=229005996}}</ref>
| label7 = Speed relative to [[Cosmic microwave background|CMB]] [[rest frame]] | data7 = {{val|552.2|5.5|u=km/s}}<ref name=COBE1993 />
| label8 = Escape velocity at Sun's position | data8 = {{val|550|u=km/s}}<ref name="Kafle2014" />
| label9 = Dark matter density at Sun's position | data9 = {{val|0.0088|0.0024|0.0018}} {{solar mass|link=yes}}<nowiki/>pc{{sup|−3}} ({{val|0.35|0.08|0.07}} GeV cm<sup>−3</sup>)<ref name="Kafle2014" />
}}
| half_light_radius_pc = {{val|5.75|0.38|ul=kpc}}<ref>{{Citation |last1=Lian |first1=Jianhui |title=The size of the Milky Way galaxy |date=2024-06-28 |arxiv=2406.05604 |last2=Zasowski |first2=Gail |last3=Chen |first3=Bingqiu |last4=Imig |first4=Julie |last5=Wang |first5=Tao |last6=Boardman |first6=Nicholas |last7=Liu |first7=Xiaowei}}</ref>
}}
<!--NOTE: Please do not change the lead sentence(s) without consulting the discussion page first. The lead sentence has been discussed and there is consensus that this is the best one for now. Thanks.-->
<!--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''' or '''Milky Way Galaxy'''<!--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 Laustsen et al.,<ref>{{Cite book |last1=Laustsen |first1=Svend |title=Exploring the Southern Sky: a Pictorial Atlas from the European Southern Observatory (ESO) |last2=Madsen |first2=Claus |last3=West |first3=Richard M. |date=1987 |publisher=Springer |isbn=978-3-642-61588-7 |___location=Berlin, Heidelberg |page=119 |oclc=851764943}}</ref> Pasachoff,<ref>{{Cite book |last=Pasachoff |first=Jay M. |title=Astronomy: From the Earth to the Universe |date=1994 |publisher=Harcourt School |isbn=978-0-03-001667-7 |page=500 |author-link=Jay Pasachoff}}</ref> Jones,<ref>{{Cite book |last=Jones |first=Barrie William |title=The Search for Life Continued: Planets Around Other Stars |date=2008 |publisher=Springer |isbn=978-0-387-76559-4 |___location=Berlin |page=89 |oclc=288474262}}</ref> van der Kruit,<ref>{{Cite book |last=Kruit |first=Pieter C. van der |title=Jan Hendrik Oort: Master of the Galactic System |date=2019 |publisher=Springer |isbn=978-3-030-17801-7 |___location=Cham, Switzerland |pages=65,717 |oclc=1110483488}}</ref> and Hodge et al.<ref>{{Cite web |last=Hodge |first=Paul W. |author-link=Paul W. Hodge |display-authors=etal |date=2020-10-13 |title=Milky Way Galaxy |url=https://www.britannica.com/place/Milky-Way-Galaxy |url-status=live |archive-url=https://web.archive.org/web/20220119174143/https://www.britannica.com/place/Milky-Way-Galaxy |archive-date=January 19, 2022 |access-date=April 24, 2022 |website=[[Encyclopædia Britannica]]}}</ref>}}<!--End of the "context" note--> is the [[galaxy]] that includes the [[Solar System]], with the name describing the [[#Appearance|galaxy's appearance]] from [[Earth]]: a hazy band of light seen in the [[night sky]] formed from stars in other arms of the galaxy, which are so far away that they cannot be individually distinguished by the [[naked eye]].
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, forming part of the [[Virgo Supercluster]] which is itself a component of the [[Laniakea Supercluster]].<ref>{{Cite web |title=Laniakea: Our home supercluster | date=September 3, 2014 |url=https://www.youtube.com/watch?v=rENyyRwxpHo |url-status=live |archive-url=https://web.archive.org/web/20140904162040/http://www.youtube.com/watch?v=rENyyRwxpHo |archive-date=September 4, 2014 |publisher=YouTube}}</ref><ref>{{Cite journal |last1=Tully |first1=R. Brent |last2=Courtois |first2=Hélène |last3=Hoffman |first3=Yehuda |last4=Pomarède |first4=Daniel |display-authors=1 |date=September 4, 2014 |title=The Laniakea supercluster of galaxies |journal=Nature |volume=513 |issue=7516 |pages=71–73 |arxiv=1409.0880 |bibcode=2014Natur.513...71T |doi=10.1038/nature13674 |pmid=25186900 |s2cid=205240232}}</ref>
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 [[planet]]s.<ref name="Nature-20120111" /><ref name="Space-20130102">{{cite news |author=<!--Not stated--> |title=100 Billion Alien Planets Fill Our Milky Way Galaxy: Study |url=http://www.space.com/19103-milky-way-100-billion-planets.html |date=January 2, 2013 |work=[[Space.com]] |access-date=January 3, 2013 |archive-url=https://web.archive.org/web/20130103060601/http://www.space.com/19103-milky-way-100-billion-planets.html |archive-date=January 3, 2013}}</ref> The Solar System is located at a radius of about 27,000 light-years (8.3 kpc) from the [[Galactic Center]],<ref name="Gillessen2016" /> on the inner edge of the [[Orion Arm]], one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a [[Bulge (astronomy)|bulge]] and one or more bars that radiate from the bulge. The Galactic Center is an intense radio source known as [[Sagittarius A*]], a [[supermassive black hole]] of 4.100 (± 0.034) million [[solar mass]]es.<ref name="NYT-20220131">{{Cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=January 31, 2022 |title=An Electrifying View of the Heart of the Milky Way – A new radio-wave image of the center of our galaxy reveals all the forms of frenzy that a hundred million or so stars can get up to. |work=[[The New York Times]] |url=https://www.nytimes.com/2022/01/31/science/milky-way.html |url-status=live |url-access=subscription |access-date=February 1, 2022 |archive-url=https://web.archive.org/web/20220131224108/https://www.nytimes.com/2022/01/31/science/milky-way.html |archive-date=2022-01-31}}</ref><ref name="ARX-20220128">{{Cite journal |last=Heyood, I. |display-authors=et al. |date=January 28, 2022 |title=The 1.28 GHz MeerKAT Galactic Center Mosaic |journal=The Astrophysical Journal |volume=925 |issue=2 |page=165 |arxiv=2201.10541 |bibcode=2022ApJ...925..165H |doi=10.3847/1538-4357/ac449a |s2cid=246275657 |doi-access=free}}</ref> The oldest stars in the Milky Way are nearly as old as the [[Universe]] itself and thus probably formed shortly after the [[Chronology of the universe#Dark Ages|Dark Ages]] of the [[Big Bang]].<ref name="HD_140283_(arXiv)">{{Cite journal |last1=Bond|first1=H.E. |last2=Nelan|first2=E. P. |last3=VandenBerg|first3=D. A. |last4=Schaefer|first4=G. H. |last5=Harmer|first5=D. |date=February 13, 2013 |title=HD 140283: A Star in the Solar Neighborhood that Formed Shortly After the Big Bang |journal=The Astrophysical Journal |volume=765 |issue=1 |pages=L12 |arxiv=1302.3180 |bibcode=2013ApJ...765L..12B |doi=10.1088/2041-8205/765/1/L12 |s2cid=119247629}}</ref>
[[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 was just one of many galaxies.
== Mythology ==
{{Main|Milky Way (mythology)}}
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 of showing Marduk as superior to the Sumerian deities.<ref name="Lambert1964" />
== Etymology ==
In [[Greek mythology]], [[Zeus]] places his son born to 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.<ref name=waller_hodge2003>{{cite book
|last1=Waller
|first1=William H.
|last2=Hodge
|first2=Paul W.
|author-link2=Paul W. Hodge
|date=2003
|title=Galaxies and the Cosmic Frontier
|publisher=[[Harvard University Press]]
|isbn=978-0-674-01079-6
|url=https://archive.org/details/galaxiescosmicfr0000wall
|url-access=registration
}}</ref> 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>
In Western culture, the name "Milky Way" is derived from its appearance as a dim unresolved "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 /> The English term can be traced back to a story by [[Geoffrey Chaucer]] {{circa|1380}}:
{{Blockquote|See yonder, lo, the Galaxyë<br /> Which men {{linktext|clepe}}th ''the Milky Wey'',<br /> For hit is whyt: and somme, parfey,|''[[The House of Fame]]''<ref name=oed>{{cite web
| title=galaxy
| last=Harper | first=Douglas
| work=[[Online Etymology Dictionary]]
| url=http://www.etymonline.com/index.php?term=galaxy
| access-date=November 11, 2011 | archive-date=November 17, 2011
| archive-url=https://web.archive.org/web/20111117124959/http://www.etymonline.com/index.php?term=galaxy
| url-status=live }}</ref>}}
=== 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 (''Ngân hà''). 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}}). In Vietnamese, "River of Heaven" (''Thiên hà'') means galaxies in general.
*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 ==
[[File:Milky Way Night Sky Black Rock Desert Nevada.jpg|thumb|The Milky Way as seen from a dark site with little [[light pollution]]]]The Milky Way is visible as a hazy band of white light, some 30° wide, arching in the [[night sky]].<ref name=pasachoff1994 /> Although all the individual naked-eye stars in the entire sky are part of the Milky Way Galaxy, the term "Milky Way" is limited to this band of light.<ref>{{Cite book |last=Rey |first=H. A. |url=https://archive.org/details/stars00hare/page/145 |title=The Stars |date=1976 |publisher=Houghton Mifflin Harcourt |isbn=978-0-395-24830-0 |page=[https://archive.org/details/stars00hare/page/145 145]}}</ref><ref>{{Cite book |last1=Pasachoff |first1=Jay M. |url=https://books.google.com/books?id=tZsoAAAAQBAJ&pg=PA384 |title=The Cosmos: Astronomy in the New Millennium |last2=Filippenko |first2=Alex |publisher=Cambridge University Press |year=2013 |isbn=978-1-107-68756-1 |page=384 |access-date=December 18, 2016 |archive-url=https://web.archive.org/web/20230326142950/https://books.google.com/books?id=tZsoAAAAQBAJ&pg=PA384 |archive-date=March 26, 2023 |url-status=live}}</ref> The light originates from the accumulation of [[angular resolution|unresolved]] stars and other material located in the direction of the [[galactic plane]]. Brighter regions around the band appear as soft visual patches known as [[star cloud]]s. The most conspicuous of these is the [[Large Sagittarius Star Cloud]], a portion of the central [[galactic bulge|bulge]] of the galaxy.<ref>{{Cite journal |last=Crossen |first=Craig |date=July 2013 |title=Observing the Milky Way, part I: Sagittarius & Scorpius |journal=Sky & Telescope |language=en |volume=126 |issue=1 |page=24 |bibcode=2013S&T...126a..24C}}</ref> Dark regions within the band, such as the [[Great Rift (astronomy)|Great Rift]] and the [[Coalsack Nebula|Coalsack]], are areas where [[interstellar dust]] blocks light from distant stars. Peoples of the southern hemisphere, including the [[Coricancha#Inca astronomy|Inca]] and [[Australian Aboriginal astronomy#Emu in the sky|Australian Aborigines]], identified these regions as [[dark cloud constellation]]s.<ref>{{Cite book |last=Urton |first=Gary |title=At the Crossroads of the Earth and the Sky: An Andean Cosmology |date=1981 |publisher=Univ. of Texas Pr. |isbn=0-292-70349-X |series=Latin American Monographs |volume=55 |___location=Austin |pages=102–4, 109–11 |author-link=Gary Urton}}</ref> The area of sky that the Milky Way obscures is called the [[Zone of Avoidance]].<ref>{{Cite web |last=Starr |first=Michelle |date=July 14, 2020 |title=A Giant 'Wall' of Galaxies Has Been Found Stretching Across The Universe |url=https://www.sciencealert.com/a-giant-wall-of-galaxies-has-been-found-stretching-across-the-universe |url-status=live |archive-url=https://web.archive.org/web/20210205141100/https://www.sciencealert.com/a-giant-wall-of-galaxies-has-been-found-stretching-across-the-universe |archive-date=February 5, 2021 |access-date=5 May 2022 |website=ScienceAlert |language=en-gb}}</ref>
The Milky Way has a relatively low [[surface brightness]]. Its visibility can be greatly reduced by background light, such as [[light pollution]] or moonlight. The sky needs to be darker than about 20.2 [[magnitude (astronomy)|magnitude]] per square arcsecond in order for the Milky Way to be visible.<ref>{{Cite journal |last=Crumey |first=Andrew |date=2014 |title=Human contrast threshold and astronomical visibility |journal=Monthly Notices of the Royal Astronomical Society |volume=442 |issue=3 |pages=2600–2619 |arxiv=1405.4209 |bibcode=2014MNRAS.442.2600C |doi=10.1093/mnras/stu992 |doi-access=free |s2cid=119210885}}</ref> It should be visible if the [[limiting magnitude]] is approximately +5.1 or better and shows a great deal of detail at +6.1.<ref name="steinicke_sakiel2007" /> This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but very prominent when viewed from [[rural area]]s when the Moon is below the horizon.{{efn|name=bortle}} Maps of artificial night sky brightness show that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution.<ref>{{Cite journal |last1=Falchi |first1=Fabio |last2=Cinzano |first2=Pierantonio |last3=Duriscoe |first3=Dan |last4=Kyba |first4=Christopher C. M. |last5=Elvidge |first5=Christopher D. |last6=Baugh |first6=Kimberly |last7=Portnov |first7=Boris A. |last8=Rybnikova |first8=Nataliya A. |last9=Furgoni |first9=Riccardo |date=June 1, 2016 |title=The new world atlas of artificial night sky brightness |journal=Science Advances |language=en |volume=2 |issue=6 |pages=e1600377 |arxiv=1609.01041 |bibcode=2016SciA....2E0377F |doi=10.1126/sciadv.1600377 |issn=2375-2548 |pmc=4928945 |pmid=27386582}}</ref>
As viewed from Earth, the visible region of the Milky Way's [[galactic plane]] occupies an area of the sky that includes 30 [[constellation]]s.{{efn|name=const}} The [[Galactic Center]] lies in the direction of [[Sagittarius (constellation)|Sagittarius]], where the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass around to the [[galactic anticenter]] in [[Auriga (constellation)|Auriga]]. The band then continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two roughly equal [[celestial sphere|hemispheres]].<ref>{{Cite web |last=Miller |first=James |date=2015-11-14 |title=Which Constellations Can Be Seen Along The Milky Way? |url=https://www.astronomytrek.com/which-constellation-can-be-seen-along-the-milky-way/ |access-date=2024-08-13 |language=en-US}}</ref>
The galactic plane is inclined by about 60° to the [[ecliptic]] (the path of the Sun in the sky). It is tilted at an angle of 63° to the [[celestial equator]].<ref>{{Cite web |last= |first= |title=Galactic Plane {{!}} COSMOS |url=https://astronomy.swin.edu.au/cosmos/G/Galactic+Plane |access-date=2025-07-09 |website=astronomy.swin.edu.au}}</ref>
== Astronomical history ==
{{See also|Galaxy#Observation history}}
=== Ancient, naked eye observations ===
In ''[[Meteorology (Aristotle)|Meteorologica]]'', [[Aristotle]] (384–322 BC) states that the [[Greek philosophy|Greek philosophers]] [[Anaxagoras]] ({{circa|500}}–428 BC) and [[Democritus]] (460–370 BC) proposed that the Milky Way is the glow of stars not directly visible due to Earth's shadow, while other stars receive their light from the Sun, but have their glow obscured by solar rays.<ref>Aristotle with W. D. Ross, ed., ''The Works of Aristotle'' ... (Oxford, England: Clarendon Press, 1931), vol. III, ''Meteorologica'', E. W. Webster, trans., Book 1, Part 8, [https://archive.org/stream/workstranslatedi03arisuoft#page/n37/mode/2up/ pp. 39–40] {{webarchive|url=https://web.archive.org/web/20160411091135/https://archive.org/stream/workstranslatedi03arisuoft|date=April 11, 2016}}: "(2) Anaxagoras, Democritus, and their schools say that the milky way is the light of certain stars ... shaded by the earth from the sun's rays."</ref> Aristotle himself believed that the Milky Way was part of the Earth's upper atmosphere, along with the stars, and that it was a byproduct of stars burning that did not dissipate because of its outermost ___location in the atmosphere, composing its [[great circle]]. He said that the milky appearance of the Milky Way [[Galaxy]] is due to the refraction of the Earth's atmosphere.<ref name="What does your image show">{{Cite web |title=What does your image show |url=http://mo-www.harvard.edu/microobs/guestobserverportal/Galileo/ThenNow/Milky%20Way/mObsMilkyWayweb.htm |url-status=live |archive-url=https://web.archive.org/web/20230315164805/http://mo-www.harvard.edu/microobs/guestobserverportal/Galileo/ThenNow/Milky%20Way/mObsMilkyWayweb.htm |archive-date=March 15, 2023 |access-date=2022-10-20 |website=mo-www.harvard.edu}}</ref><ref name="Montada" /><ref>{{Cite book |last1=Aristotle |url=http://archive.org/details/workstranslatedi03arisuoft |title=Works |translator1-first=William David |translator1-last=Ross |translator2-last=Smith |translator2-first=John Alexander |date=1931<!--Ignore the archive.org metadata. Pay attention to what's actually on the title page--> |___location=Oxford |publisher=Clarendon Press <!--these people just digitized the archive.org copy: |others=Robarts – University of Toronto--> |page=345}}</ref> The [[Neoplatonism|Neoplatonist]] philosopher [[Olympiodorus the Younger]] ({{circa|495|lk=no}}–570 AD) criticized this view, arguing that if the Milky Way were [[sublunary]], it should appear different at different times and places on Earth, and that it should have [[parallax]], which it does not. In his view, the Milky Way is celestial. This idea would be influential later in the [[Muslim world]].<ref name="heidarzadeh2008" />
The [[Persian people|Persian]] astronomer [[Al-Biruni]] (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of [[Nebula|nebulous]] stars".<ref name="mactutor_albirundi" /> The [[Al-Andalus|Andalusian]] astronomer [[Avempace]] (died 1138) proposed that the Milky Way was made up of many stars but appeared to be a continuous image in the Earth's atmosphere, citing his observation of a [[Conjunction (astronomy and astrology)|conjunction]] of Jupiter and Mars in 1106 or 1107 as evidence.<ref name="Montada" /> The Persian astronomer [[Nasir al-Din al-Tusi]] (1201–1274) in his ''Tadhkira'' wrote: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color."<ref name="ragep1993" /> [[Ibn Qayyim al-Jawziyya]] (1292–1350) proposed that the Milky Way is "a myriad of tiny stars packed together in the sphere of the fixed stars".<ref name="Livingston" />
=== Telescopic observations ===
[[File:Herschel-Galaxy.png|thumb|The shape of the Milky Way as deduced from star counts by [[William Herschel]] in 1785. The [[Solar System]] was assumed to be near the center]]Proof of the Milky Way consisting of many stars came in 1610 when [[Galileo Galilei]] used a [[optical telescope|telescope]] to study the Milky Way and discovered that it was composed of a huge number of faint stars. Galileo also concluded that the appearance of the Milky Way was due to [[refraction]] of the Earth's atmosphere.<ref>Galileo Galilei, ''Sidereus Nuncius'' (Venice<!--adding "Italy" is anachronistic-->: Thomas Baglioni, 1610), [https://archive.org/stream/Sidereusnuncius00Gali#page/n37/mode/2up pp. 15–16]. {{webarchive|url=https://web.archive.org/web/20160316173644/https://archive.org/stream/Sidereusnuncius00Gali |date=March 16, 2016 }}<br />
English translation: Galileo Galilei with Edward Stafford Carlos, trans., ''The Sidereal Messenger'' (London: Rivingtons, 1880), [https://archive.org/stream/siderealmessenge80gali#page/42/mode/2up/ pp. 42–43]. {{webarchive|url=https://web.archive.org/web/20121202215542/https://archive.org/stream/siderealmessenge80gali |date=December 2, 2012 }}</ref><ref name=oconnor_robertson2002 /><ref name="What does your image show" /> In a treatise in 1755, [[Immanuel Kant]], drawing on earlier work by [[Thomas Wright (astronomer)|Thomas Wright]],<ref>Thomas Wright, ''An Original Theory or New Hypothesis of the Universe'' (London, England: H. Chapelle, 1750).
* [https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA57 On page 57] {{Webarchive|url=https://web.archive.org/web/20161120195004/https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA57 |date=November 20, 2016 }}, Wright stated that despite their mutual gravitational attraction, the stars in the constellations do not collide because they are in orbit, so centrifugal force keeps them separated: "centrifugal force, which not only preserves them in their orbits, but prevents them from rushing all together, by the common universal law of gravity, ..."
* [https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA48 On page 48] {{Webarchive|url=https://web.archive.org/web/20161120194825/https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA48 |date=November 20, 2016 }}, Wright stated that the form of the Milky Way is a ring: "the stars are not infinitely dispersed and distributed in a promiscuous manner throughout all the mundane space, without order or design, ... this phænomenon [is] no other than a certain effect arising from the observer's situation, ... To a spectator placed in an indefinite space, ... it [i.e. the Milky Way (''Via Lactea'')] [is] a vast ring of stars ..."
* [https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA65 On page 65] {{Webarchive|url=https://web.archive.org/web/20161120174208/https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA65 |date=November 20, 2016 }}, Wright speculated that the central body of the Milky Way, around which the rest of the galaxy revolves, might not be visible to us: "the central body A, being supposed as ''incognitum'' [i.e. an unknown], without [i.e. outside of] the finite view; ..."
* [https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA73 On page 73] {{Webarchive|url=https://web.archive.org/web/20161120194830/https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA73 |date=November 20, 2016 }}, Wright called the Milky Way the ''Vortex Magnus'' (the great whirlpool) and estimated its diameter to be 8.64×10<sup>12</sup> miles (13.9×10<sup>12</sup> km).
* [https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA33 On page 33] {{Webarchive|url=https://web.archive.org/web/20161120172753/https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA33 |date=November 20, 2016 }}, Wright speculated that there are a vast number of inhabited planets in the galaxy: "therefore we may justly suppose, that so many radiant bodies [i.e. stars] were not created barely to enlighten an infinite void, but to ... display an infinite shapeless universe, crowded with myriads of glorious worlds, all variously revolving round them; and ... with an inconceivable variety of beings and states, animate ..."</ref> speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by [[gravitation]]al forces akin to the Solar System but on much larger scales.<ref>Immanuel Kant, [https://books.google.com/books?id=nCcaAQAAMAAJ&pg=PP9 ''Allgemeine Naturgeschichte und Theorie des Himmels''] {{Webarchive|url=https://web.archive.org/web/20161120195036/https://books.google.com/books?id=nCcaAQAAMAAJ&pg=PP9 |date=November 20, 2016 }} [''General Natural History and Theory of Heaven''], (Koenigsberg and Leipzig, (Germany): Johann Friederich Petersen, 1755).
On pages 2–3, Kant acknowledged his debt to Thomas Wright: {{lang|de|"Dem Herrn Wright von Durham, einen Engeländer, war es vorbehalten, einen glücklichen Schritt zu einer Bemerkung zu thun, welche von ihm selber zu keiner gar zu tüchtigen Absicht gebraucht zu seyn scheinet, und deren nützliche Anwendung er nicht genugsam beobachtet hat. Er betrachtete die Fixsterne nicht als ein ungeordnetes und ohne Absicht zerstreutes Gewimmel, sondern er fand eine systematische Verfassung im Ganzen, und eine allgemeine Beziehung dieser Gestirne gegen einen Hauptplan der Raume, die sie einnehmen."}} ("To Mr. Wright of Durham, an Englishman, it was reserved to take a happy step towards an observation, which seemed, to him and to no one else, to be needed for a clever idea, the exploitation of which he has not studied sufficiently. He regarded the fixed stars not as a disorganized swarm that was scattered without a design; rather, he found a systematic shape in the whole, and a general relation between these stars and the principal plane of the space that they occupy.")</ref> The resulting disk of stars would be seen as a band in the sky from our perspective inside the disk. Wright and Kant also conjectured that some of the [[nebula]]e visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both the Milky Way and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.<ref>Kant (1755), [https://books.google.com/books?id=nCcaAQAAMAAJ&pg=PP49 pages xxxiii–xxxvi of the Preface ({{lang|de|cat=no|Vorrede}})]: {{Webarchive|url=https://web.archive.org/web/20161120231151/https://books.google.com/books?id=nCcaAQAAMAAJ&pg=PP49 |date=November 20, 2016 }}: {{lang|de|"Ich betrachtete die Art neblichter Sterne, deren Herr von Maupertuis in der Abhandlung von der Figur der Gestirne gedenket, und die die Figur von mehr oder weniger offenen Ellipsen vorstellen, und versicherte mich leicht, daß sie nichts anders als eine Häufung vieler Fixsterne seyn können. Die jederzeit abgemessene Rundung dieser Figuren belehrte mich, daß hier ein unbegreiflich zahlreiches Sternenheer, und zwar um einen gemeinschaftlichen Mittelpunkt, müste geordnet seyn, weil sonst ihre freye Stellungen gegen einander, wohl irreguläre Gestalten, aber nicht abgemessene Figuren vorstellen würden. Ich sahe auch ein: daß sie in dem System, darinn sie sich vereinigt befinden, vornemlich auf eine Fläche beschränkt seyn müßten, weil sie nicht zirkelrunde, sondern elliptische Figuren abbilden, und daß sie wegen ihres blassen Lichts unbegreiflich weit von uns abstehen."}} ("I considered the type of nebulous stars, which Mr. de Maupertuis considered in his treatise on the shape of stars, and which present the figures of more or less open ellipses, and I readily assured myself, that they could be nothing else than a cluster of fixed stars. That these figures always measured round informed me that here an inconceivably numerous host of stars, [which were clustered] around a common center, must be orderly, because otherwise their free positions among each other would probably present irregular forms, not measurable figures. I also realized: that in the system in which they find themselves bound, they must be restricted primarily to a plane, because they display not circular, but elliptical figures, and that on account of their faint light, they are located inconceivably far from us.")</ref><ref name="our_galaxy" /><ref>The term {{lang|de|Weltinsel}} (world island) appears nowhere in Kant's book of 1755. The term first appeared in 1850, in the third volume of von Humboldt's ''Kosmos'': Alexander von Humboldt, ''Kosmos'', vol. 3 (Stuttgart & Tübingen, (Germany): J. G. Cotta, 1850), pp. 187, 189. [https://books.google.com/books?id=Su0iAQAAMAAJ&pg=PA187 From p. 187]: {{Webarchive|url=https://web.archive.org/web/20161120174304/https://books.google.com/books?id=Su0iAQAAMAAJ&pg=PA187 |date=November 20, 2016 }} {{lang|de|"Thomas Wright von Durham, Kant, Lambert und zuerst auch William Herschel waren geneigt die Gestalt der Milchstraße und die scheinbare Anhäufung der Sterne in derselben als eine Folge der abgeplatteten Gestalt und ungleichen Dimensionen der ''Weltinsel'' (Sternschict) zu betrachten, in welche unser Sonnensystem eingeschlossen ist."|italic=invert}} ("Thomas Wright of Durham, Kant, Lambert and at first also William Herschel were inclined to regard the shape of the Milky Way and the apparent clustering of stars in it as a consequence of the oblate shape and unequal dimensions of the ''world island'' (star stratum), in which our solar system is included.)<br />
In the English translation{{snd}}Alexander von Humboldt with [[E. C. Otté]], trans., ''Cosmos'' ... (New York City: Harper & Brothers, 1897), vols. 3–5. see [http://babel.hathitrust.org/cgi/pt?id=njp.32101057350447;view=1up;seq=157 p. 147] {{Webarchive|url=https://web.archive.org/web/20181106181344/https://babel.hathitrust.org/cgi/pt?id=njp.32101057350447;view=1up;seq=157 |date=November 6, 2018 }}.</ref>
The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by [[William Herschel]] in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Milky Way with the Solar System close to the center.<ref>William Herschel (1785), "On the Construction of the Heavens", ''Philosophical Transactions of the Royal Society of London'', '''75''': 213–266. Herschel's diagram of the Milky Way appears immediately after the article's last page. See:
* [https://books.google.com/books?id=IU9FAAAAcAAJ&pg=PA213 Google Books] {{Webarchive|url=https://web.archive.org/web/20161120170623/https://books.google.com/books?id=IU9FAAAAcAAJ&pg=PA213 |date=November 20, 2016 }}
* [http://rstl.royalsocietypublishing.org/content/75/213.full.pdf+html The Royal Society of London] {{webarchive|url=https://web.archive.org/web/20160406132029/http://rstl.royalsocietypublishing.org/content/75/213.full.pdf%20html |date=April 6, 2016 }}</ref>
In 1845, [[William Parsons, 3rd Earl of Rosse|Lord Rosse]] constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.<ref name=abbey /><ref>See:
* Rosse revealed the spiral structure of [[Whirlpool Galaxy]] (M51) at the 1845 meeting of the British Association for the Advancement of Science. Rosse's illustration of M51 was reproduced in J. P. Nichol's book of 1846.
** {{cite journal |last1=Rosse |first1=Earl of |title=On the nebula 25 Herschel, or 61 [should read: 51] of Messier's catalogue |journal=Report of the Fifteenth Meeting of the British Association for the Advancement of Science; Held at Cambridge in June 1845 § Notices and Abstracts of Miscellaneous Communications to the Sections |series=Report of the ... Meeting of the British Association for the Advancement of Science (1833) |date=1846 |page=4 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101049356890&view=1up&seq=402 |access-date=February 17, 2020 |archive-date=March 10, 2021 |archive-url=https://web.archive.org/web/20210310235347/https://babel.hathitrust.org/cgi/pt?id=njp.32101049356890&view=1up&seq=402 |url-status=live }}
** {{Cite book |last=Nichol |first=John Pringle |url=https://babel.hathitrust.org/cgi/pt?id=uc2.ark%3A%2F13960%2Ft9k360k8b&view=1up&seq=65 |title=Thoughts on Some Important Points Relating to the System of the World |date=1846 |publisher=William Tait |___location=Edinburgh, Scotland |page=23 |access-date=February 17, 2020 |archive-url=https://web.archive.org/web/20210308094837/https://babel.hathitrust.org/cgi/pt?id=uc2.ark%3A%2F13960%2Ft9k360k8b&view=1up&seq=65 |archive-date=March 8, 2021 |url-status=live}} Rosse's illustration of the Whirlpool Galaxy appears on the plate that immediately precedes p. 23.
* {{cite journal |last1=South |first1=James |title=Auszug aus einem Berichte über Lord ''Rosse's'' grosses Telescop, den Sir ''James South'' in The Times, Nr. 18899, 1845 April 16 bekannt gemacht hat |journal=Astronomische Nachrichten |date=1846 |volume=23 |issue=536 |pages=113–118 |doi=10.1002/asna.18460230802 |url=https://babel.hathitrust.org/cgi/pt?id=pst.000055399548&view=1up&seq=279&size=125 |trans-title=Excerpt from a report about Lord Rosse's great telescope, which Sir James South made known in The Times [of London], no. 18,899, 1845 April 16 |language=de |access-date=February 17, 2020 |archive-date=March 8, 2021 |archive-url=https://web.archive.org/web/20210308145128/https://babel.hathitrust.org/cgi/pt?id=pst.000055399548&view=1up&seq=279&size=125 |url-status=live }} On March 5, 1845, Rosse observed M51, the [[Whirlpool Galaxy]]. From column 115: "The most popularly known nebulæ observed this night were the ring nebulæ in the Canes Venatici, or the 51st of ''Messier's'' catalogue, which was resolved into stars with a magnifying power of 548".
* {{cite journal |last1=Robinson |first1=T. R. |title=On Lord Rosse's telescope |journal=Proceedings of the Royal Irish Academy |date=1845 |volume=3 |issue=50 |pages=114–133 |url=https://www.biodiversitylibrary.org/item/46714#page/132/mode/1up |access-date=February 17, 2020 |archive-date=June 10, 2020 |archive-url=https://web.archive.org/web/20200610193425/https://www.biodiversitylibrary.org/item/46714#page/132/mode/1up |url-status=live }} Rosse's early observations of nebulae and galaxies are discussed on pp. 127–130.
* {{cite journal |last1=Rosse |first1=The Earl of |title=Observations on the nebulae |journal=Philosophical Transactions of the Royal Society of London |date=1850 |volume=140 |pages=499–514 |url=https://books.google.com/books?id=BlFFAAAAcAAJ&pg=PA499 |doi=10.1098/rstl.1850.0026 |doi-access=free |access-date=February 17, 2020 |archive-date=March 26, 2023 |archive-url=https://web.archive.org/web/20230326142957/https://books.google.com/books?id=BlFFAAAAcAAJ&pg=PA499 |url-status=live }} Rosse's illustrations of nebulae and galaxies appear on the plates that immediately precede the article.
* {{cite journal |last1=Bailey |first1=M. E. |last2=Butler |first2=C. J. |last3=McFarland |first3=J. |title=Unwinding the discovery of spiral nebulae |journal=Astronomy & Geophysics |date=April 2005 |volume=46 |issue=2 |pages=2.26–2.28 |doi=10.1111/j.1468-4004.2005.46226.x |doi-access=free}}</ref>
[[File:Pic iroberts1.jpg|thumb|Photograph of the "Great Andromeda Nebula" from 1899, later identified as the [[Andromeda Galaxy]]]]
In 1904, studying the [[proper motion]]s of stars, [[Jacobus Kapteyn]] reported that these were not random, as it was believed in that time; stars could be divided into two streams, moving in nearly opposite directions.<ref>See:
* {{Cite book |last=Kapteyn |first=Jacobus Cornelius |title=Congress of Arts and Science, Universal Exposition, St. Louis, 1904 |date=1906 |publisher=Houghton, Mifflin and Co. |editor-last=Rogers |editor-first=Howard J. |volume=4 |___location=Boston and New York |pages=396–425 |chapter=Statistical methods in stellar astronomy |access-date=February 6, 2020 |chapter-url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044103057121&view=1up&seq=412 |archive-url=https://web.archive.org/web/20210308053515/https://babel.hathitrust.org/cgi/pt?id=hvd.32044103057121&view=1up&seq=412 |archive-date=March 8, 2021 |url-status=live}} From pp. 419–420: "It follows that the one set of the stars must have a systematic motion relative to the other. ... these two main directions of motion must be in reality diametrically opposite."
* {{cite journal |last1=Kapteyn |first1=J. C. |title=Star streaming |journal=Report of the Seventy-fifth Meeting of the British Association for the Advancement of Science, South Africa |series=Report of the ... Meeting of the British Association for the Advancement of Science (1833) |date=1905 |pages=257–265 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101076796737&view=1up&seq=399 |access-date=February 6, 2020 |archive-date=March 8, 2021 |archive-url=https://web.archive.org/web/20210308042552/https://babel.hathitrust.org/cgi/pt?id=njp.32101076796737&view=1up&seq=399 |url-status=live }}</ref> It was later realized that Kapteyn's data had been the first evidence of the rotation of the Milky Way,<ref>See:
* {{Cite journal |last=Schwarzschild |first=K. |date=1907 |title=Ueber die Eigenbewegungen der Fixsterne |trans-title=On the proper motions of the fixed stars |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015048391208&view=1up&seq=654 |url-status=live |journal=Nachrichten von der Königlichen Gesellschaft der Wissenschaften zu Göttingen (Reports of the Royal Society of Science at Göttingen) |language=de |volume=5 |pages=614–632 |bibcode=1907NWGot...5..614S |archive-url=https://web.archive.org/web/20210308142441/https://babel.hathitrust.org/cgi/pt?id=mdp.39015048391208&view=1up&seq=654 |archive-date=March 8, 2021 |access-date=February 6, 2020}}
* {{cite journal |last1=Schwarzschild |first1=K. |title=Ueber die Bestimmung von Vertex und Apex nach der Ellipsoidhypothese aus einer geringeren Anzahl beobachteter Eigenbewegungen |journal=Nachrichten von der Königlichen Gesellschaft der Wissenschaften zu Göttingen |date=1908 |pages=191–200 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015048391554&view=1up&seq=209 |trans-title=On the determination, according to the ellipsoid hypothesis, of the vertex and apex from a small number of observed proper motions |language=de |access-date=February 6, 2020 |archive-date=March 8, 2021 |archive-url=https://web.archive.org/web/20210308024321/https://babel.hathitrust.org/cgi/pt?id=mdp.39015048391554&view=1up&seq=209 |url-status=live }}</ref> which ultimately led to the finding of galactic rotation by [[Bertil Lindblad]] and [[Jan Oort]].
In 1917, [[Heber Doust Curtis]] had observed the nova [[S Andromedae]] within the [[Great Andromeda Nebula]] ([[Messier object]] 31). Searching the photographic record, he found 11 more [[nova]]e. Curtis noticed that these novae were, on average, 10 [[magnitude (astronomy)|magnitudes]] fainter than those that occurred within the Milky Way. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were independent galaxies.<ref>{{Cite journal |last=Curtis |first=Heber D. |date=1917 |title=Novae in spiral nebulae and the island universe theory |journal=Publications of the Astronomical Society of the Pacific |volume=29 |issue=171 |pages=206–207 |bibcode=1917PASP...29..206C |doi=10.1086/122632 |doi-access=free}}</ref><ref name=pasp100_6 /> In 1920 the [[Great Debate (astronomy)|Great Debate]] took place between [[Harlow Shapley]] and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant [[Doppler effect|Doppler shift]].<ref name=weaver />
The controversy was conclusively settled by [[Edwin Hubble]] in the early 1920s using the Mount Wilson observatory [[Hooker telescope|{{convert|100|in|m|abbr=on|sigfig=2|order=flip}} Hooker telescope]]. With the [[light-gathering power]] of this new telescope, he was able to produce [[Astrophotography|astronomical photographs]] that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some [[Cepheid variable]]s that he could use as a [[wikt:benchmark|benchmark]] to estimate the distance to the nebulae. He found that the Andromeda Nebula is 275,000 parsecs from the Sun, far too distant to be part of the Milky Way.<ref name=jrasc83_6 /><ref name=apj69_103 />
=== Satellite observations ===
[[File:Galaxymap.com, map 5000 parsecs (2022).png|thumb|upright=1.3|Map of stars cataloged by the Gaia release in 2021, displayed as density mesh in the diagram]]
The [[European Space Agency|ESA]] spacecraft ''[[Gaia (spacecraft)|Gaia]]'' provides distance estimates by determining the [[parallax]] of a billion stars and is mapping the Milky Way.<ref>{{Cite web |date=September 14, 2016 |title=New Milky Way Map Is a Spectacular Billion-Star Atlas |url=http://news.nationalgeographic.com/2016/09/gaia-milky-way-maps-billion-stars-atlas-space-science |archive-url=https://web.archive.org/web/20160915183426/http://news.nationalgeographic.com/2016/09/gaia-milky-way-maps-billion-stars-atlas-space-science/ |archive-date=September 15, 2016 |access-date=September 15, 2016}}</ref><ref>{{Cite web |title=Gaia > Gaia DR1 |url=http://www.cosmos.esa.int/web/gaia/dr1 |url-status=live |archive-url=https://web.archive.org/web/20160915091251/http://www.cosmos.esa.int/web/gaia/dr1 |archive-date=September 15, 2016 |access-date=September 15, 2016 |website=www.cosmos.esa.int}}</ref>
Data from ''Gaia'' has been described as "transformational". It has been estimated that ''Gaia'' has expanded the number of observations of stars from about 2 million stars, as of the 1990s, to 2 billion. It has expanded the measurable volume of space by a factor of 100 in radius and a factor of 1,000 in precision.<ref name="Skibba">{{Cite journal |last=Skibba |first=Ramin |date=10 June 2021 |title=A galactic archaeologist digs into the Milky Way's history |url=https://knowablemagazine.org/article/physical-world/2021/a-galactic-archaeologist-digs-milky-ways-history |url-status=live |journal=Knowable Magazine |doi=10.1146/knowable-060921-1 |s2cid=236290725 |archive-url=https://web.archive.org/web/20220804193831/https://knowablemagazine.org/article/physical-world/2021/a-galactic-archaeologist-digs-milky-ways-history |archive-date=August 4, 2022 |access-date=4 August 2022 |doi-access=free}}</ref>
A study in 2020 concluded that ''Gaia'' detected a wobbling motion of the galaxy, which might be caused by "[[torques]] from a misalignment of the disc's rotation axis with respect to the principal axis of a non-spherical halo, or from [[Accretion (astrophysics)|accreted]] matter in the halo acquired during late infall, or from nearby, interacting satellite galaxies and their consequent tides".<ref>{{Cite journal |last1=Poggio |first1=E. |last2=Drimmel |first2=R. |last3=Andrae |first3=R. |last4=Bailer-Jones |first4=C. A. L. |last5=Fouesneau |first5=M. |last6=Lattanzi |first6=M. G. |last7=Smart |first7=R. L. |last8=Spagna |first8=A. |date=2020 |title=Evidence of a dynamically evolving Galactic warp |journal=Nature Astronomy |volume=4 |issue=6 |pages=590–596 |arxiv=1912.10471 |bibcode=2020NatAs...4..590P |doi=10.1038/s41550-020-1017-3 |s2cid=209444772}}</ref> In April 2024, initial studies and related maps, involving the [[magnetic field]]s of the Milky Way were reported.<ref name="NYT-20240419">{{cite news |last=Overbye |first=Dennis |authorlink=Dennis Overbye |title=The Dusty Magnets of the Milky Way |url=https://www.nytimes.com/2024/04/19/science/space/astronomy-milky-way.html |date=19 April 2024 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20240419104158/https://www.nytimes.com/2024/04/19/science/space/astronomy-milky-way.html |archivedate=19 April 2024 |accessdate=19 April 2024 }}</ref>
== Astrography ==
=== Sun's ___location and neighborhood ===
{{See also|Location of Earth}}
[[File:Galaxymap.com, map 12000 parsecs (2022).png|thumb|300x300px|Map of stars cataloged by the ''Gaia'' release in 2021, overlay on top of artist's conception of the Milky Way overall shape]]
The [[Sun]] is near the inner rim of the [[Orion Arm]], within the [[Local Fluff]] of the [[Local Bubble]], between the [[Radcliffe wave]] and ''Split'' linear structures (formerly [[Gould Belt]]).<ref name="Alves Zucker Goodman Speagle 2020">{{Cite journal |last1=Alves |first1=João |last2=Zucker |first2=Catherine |last3=Goodman |first3=Alyssa A. |last4=Speagle |first4=Joshua S. |last5=Meingast |first5=Stefan |last6=Robitaille |first6=Thomas |last7=Finkbeiner |first7=Douglas P. |last8=Schlafly |first8=Edward F. |last9=Green |first9=Gregory M. |date=7 January 2020 |title=A Galactic-scale gas wave in the Solar Neighborhood |journal=Nature |volume=578 |pages=237–239 |arxiv=2001.08748 |bibcode=2020Natur.578..237A |doi=10.1038/s41586-019-1874-z |pmid=31910431 |s2cid=210086520 |number=7794}}</ref> Based upon studies of stellar orbits around Sgr A* by Gillessen ''et al.'' (2016), the Sun lies at an estimated distance of {{convert|8.32|±|0.14|kpc|kly|order=flip|abbr=on|lk=off}}<ref name="Gillessen2016" /> from the Galactic Center. Boehle ''et al.'' (2016) found a smaller value of {{convert|7.86|±|0.14|kpc|kly|order=flip|abbr=on|lk=off}}, also using a star orbit analysis.<ref name="boehle2016" /> The Sun is currently {{convert|5|-|30|pc|ly}} above, or north of, the central plane of the Galactic disk.<ref name=majaess2009 /> The distance between the local arm and the next arm out, the [[Perseus Arm]], is about {{convert|2000|pc|ly}}.<ref name="fn9" /> The Sun, and thus the Solar System, is located in the Milky Way's [[galactic habitable zone]].<ref name="mullen-2001">{{Cite news |last=Mullen, Leslie |date=May 18, 2001 |title=Galactic Habitable Zones |work=NAI Features Archive |agency=Nasa Astrobiology Institute |url=http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=157 |access-date=May 9, 2013 |archive-url=https://web.archive.org/web/20130409002532/http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=157 |archive-date=April 9, 2013}}</ref><ref name="sundin-2006">{{Cite journal |last=Sundin |first=M. |year=2006 |title=The galactic habitable zone in barred galaxies |journal=International Journal of Astrobiology |volume=5 |issue=4 |pages=325–326 |bibcode=2006IJAsB...5..325S |doi=10.1017/S1473550406003065 |s2cid=122018103}}</ref>
There are about 208 stars brighter than [[absolute magnitude]] 8.5 within a sphere with a radius of {{convert|15|pc|ly}} from the Sun,<!--not counting the Sun because that would give a biased estimate of the density--> giving a density of one star per 69 cubic parsecs, or one star per 2,360 cubic light-years (from [[List of nearest bright stars]]). On the other hand, there are 64 known stars (of any magnitude, not counting 4 [[brown dwarf]]s) within {{convert|5|pc|ly}} of the Sun<!--again, not counting the Sun-->, giving a density of about one star per 8.2 cubic parsecs, or one per 284 cubic light-years (from [[List of nearest stars]]). This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about 500 stars brighter than [[apparent magnitude]] 4 but 15.5 million stars brighter than apparent magnitude 14.<ref>{{Cite web |title=Magnitude |url=http://www.nso.edu/PR/answerbook/magnitude.html |archive-url=https://web.archive.org/web/20080206074842/http://www.nso.edu/PR/answerbook/magnitude.html |archive-date=February 6, 2008 |access-date=August 9, 2013 |publisher=National Solar Observatory{{snd}}Sacramento Peak}}</ref>
The apex of the Sun's way, or the [[solar apex]], is the direction that the Sun travels through the [[Local standard of rest]] in the Milky Way. The general direction of the Sun's Galactic motion is towards the star [[Deneb]] near the constellation of [[Cygnus (constellation)|Cygnus]], at an angle of roughly 90 sky degrees to the direction of the Galactic Center. The Sun's orbit about the Milky Way is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun passes through the Galactic plane approximately 2.7 times per orbit.<ref>{{Cite book |last1=Moore |first1=Patrick |url=https://books.google.com/books?id=2FNfjWKBZx8C&pg=PA4 |title=Patrick Moore's Data Book of Astronomy |last2=Rees |first2=Robin |date=2014 |publisher=Cambridge University Press |isbn=978-1-139-49522-6 |edition=2nd |page=4 |archive-url=https://web.archive.org/web/20170215095853/https://books.google.com/books?id=2FNfjWKBZx8C&pg=PA4 |archive-date=February 15, 2017 |url-status=live}}</ref> This is very similar to how a [[simple harmonic oscillator]] works with no drag force (damping) term. These oscillations were until recently thought to coincide with [[mass extinction|mass lifeform extinction]] periods on Earth.<ref name="extinction" /> A reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation.<ref name="overholt_etal" />
It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a [[galactic year]]),<ref name="sparke_gallagher" /> so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the [[origin of humans]]. The [[orbital speed]] of the Solar System about the center of the Milky Way is approximately {{convert|220|km/s|mph|sigfig=2|abbr=on}} or 0.073% of the [[speed of light]]. The Sun moves through the heliosphere at {{convert|52000|mi/h|km/h|order=flip|abbr=on}}. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU ([[astronomical unit]]).<ref name=garlick2002 /> The Solar System is headed in the direction of the zodiacal constellation [[Scorpius]], which follows the ecliptic.<ref>{{Cite web |date=April 8, 2011 |title=Solar System's 'Nose' Found; Aimed at Constellation Scorpius |url=http://news.nationalgeographic.com/news/2011/04/110407-sun-nose-scorpius-solar-system-nasa-ibex-ribbon-space-science/ |archive-url=https://web.archive.org/web/20150907130456/http://news.nationalgeographic.com/news/2011/04/110407-sun-nose-scorpius-solar-system-nasa-ibex-ribbon-space-science/ |archive-date=September 7, 2015}}</ref>
=== Galactic quadrants ===
{{Main|Galactic quadrant}}
[[File:Galactic longitude.JPG|thumb|A diagram of the Sun's ___location in the Milky Way; the angles represent longitudes in the [[galactic coordinate system]]]]
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>
Quadrants are described using [[Ordinal number (linguistics)|ordinals]]{{snd}}for example, "1st galactic quadrant",<ref name="toolsofradio">{{Citation |last1=Wilson |first1=Thomas L. |title=Tools of Radio Astronomy |date=2009 |url=https://books.google.com/books?id=9KHw6R8rQEMC&pg=PA347 |archive-url=https://web.archive.org/web/20160426084004/https://books.google.com/books?id=9KHw6R8rQEMC&pg=PA347 |url-status=live |publisher=Springer Science & Business Media |isbn=978-3-540-85121-9 |display-authors=1 |archive-date=April 26, 2016 |last2=Rohlfs |first2=Kristen |last3=Hüttemeister |first3=Susanne|bibcode=2009tra..book.....W }}</ref> "second galactic quadrant",<ref name=NASAAD131K>{{cite journal |bibcode=2004A&A...418..131K |doi=10.1051/0004-6361:20034530 |author=Kiss, Cs |author2=Moór, A. |author3=Tóth, L. V. |title=Far-infrared loops in the 2nd Galactic Quadrant |journal=Astronomy and Astrophysics |volume=418 |date=April 2004 |arxiv=astro-ph/0401303 |pages=131–141|s2cid=7825138|url=https://www.aanda.org/articles/aa/pdf/2004/16/aa0530.pdf|access-date=August 17, 2010}}</ref> or "third quadrant of the Milky Way".<ref name="Lampton1997">{{Cite journal |author=Lampton, M. |author2=Lieu, R. |author3=Schmitt, J. H. M. M. |author4=Bowyer, S. |author5=Voges, W. |author6=Lewis, J. |author7=Wu, X. |display-authors=1 |date=February 1997 |title=An All-Sky Catalog of Faint Extreme Ultraviolet Sources |journal=The Astrophysical Journal Supplement Series |volume=108 |issue=2 |pages=545–557 |bibcode=1997ApJS..108..545L |doi=10.1086/312965 |doi-access=free}}</ref> Viewing from the [[north galactic pole]] with 0° [[degree (angle)|(zero degrees)]] as the [[ray (geometry)|ray]] that runs starting from the Sun and through the Galactic Center, the quadrants are:
::{|
! Galactic<br />quadrant<br />
! Galactic<br />longitude<br />[[galactic longitude|(ℓ)]]
! <br />Reference<br />
|-
!align="center"|1st
|align="right" | 0° ≤ ℓ ≤ 90°
|align="center"|<ref name="vanWoerden-Strom-2006">{{Cite journal |last1=van Woerden |first1=Hugo |last2=Strom |first2=Richard G. |date=June 2006 |title=The beginnings of radio astronomy in the Netherlands |url=http://www.astron.nl/~leeuwen/video/dloo/JAHH9p3.pdf |journal=Journal of Astronomical History and Heritage |volume=9 |issue=1 |pages=3–20 |bibcode=2006JAHH....9....3V |doi=10.3724/SP.J.1440-2807.2006.01.01 |s2cid=16816839 |archive-url=https://web.archive.org/web/20100919025431/http://astron.nl/~leeuwen/video/dloo/JAHH9p3.pdf |archive-date=September 19, 2010}}</ref>
|-
!align="center"|2nd
|align="right" | 90° ≤ ℓ ≤ 180°
|align="center"|<ref name=NASAAD131K />
|-
!align="center"|3rd
|align="right" |180° ≤ ℓ ≤ 270°
|align="center"|<ref name="Lampton1997" />
|-
!align="center"|4th<br />
|align="right" |270° ≤ ℓ ≤ 360°<br />(360° ≅ 0°)
|align="center"|<ref name="toolsofradio" /><br />
|}
with the galactic longitude [[galactic longitude|(ℓ)]] increasing in the counter-clockwise direction ([[Angle#Positive and negative angles|positive rotation]]) as viewed from [[galactic north|north]] of the [[Galactic Center]] (a view-point several hundred thousand [[light-year]]s distant from Earth in the direction of the constellation [[Coma Berenices]]); if viewed from south of the Galactic Center (a view-point similarly distant in the constellation [[Sculptor (constellation)|Sculptor]]), [[galactic longitude|ℓ]] would increase in the clockwise direction ([[Angle#Positive and negative angles|negative rotation]]).
== Size and mass ==
=== Size ===
[[File:Local Group Galaxies Comparison.png|thumb|right|300px|A size comparison of the six largest galaxies of the [[Local Group]], including the Milky Way]]
The Milky Way is one of the two largest galaxies in the [[Local Group]] (the other being the [[Andromeda Galaxy]]), although the size for its [[galactic disc]] and how much it defines the isophotal diameter is not well understood.<ref name="Lopez2018" /> It is estimated that the significant bulk of stars in the galaxy lies within the {{convert|26|kpc|ly|sigfig=1|abbr=off}} diameter, and that the number of stars beyond the outermost disc dramatically reduces to a very low number, with respect to an extrapolation of the exponential disk with the scale length of the inner disc.<ref name=arxiv0909.3857 /><ref name="Lopez2018" />
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, nor were among the [[List of largest galaxies|largest known]] (if the former not being the largest) as previously widely believed, but rather average ordinary spiral galaxies.<ref name="Goodwin_Hubble">{{Cite arXiv |eprint=astro-ph/9704289 |first1=S. P. |last1=Goodwin |first2=J. |last2=Gribbin |title=New Determination of the Hubble Parameter Using the Principle of Terrestrial Mediocrity |date=30 April 1997 |last3=Hendry |first3=M. A.}}</ref> To compare the relative physical scale of the Milky Way, if the [[Solar System]] out to [[Neptune]] were the size of a [[quarter (United States coin)|US quarter]] ({{convert|0.955|in|mm|abbr=on|order=flip}}), the Milky Way would be approximately at least the greatest north–south line of the [[contiguous United States]].<ref>{{Cite web |title=How Big is Our Universe: How far is it across the Milky Way? |url=http://www.cfa.harvard.edu/seuforum/howfar/across.html |url-status=live |archive-url=https://web.archive.org/web/20130305005817/http://www.cfa.harvard.edu/seuforum/howfar/across.html |archive-date=March 5, 2013 |access-date=March 13, 2013 |website=NASA-Smithsonian Education Forum on the Structure and Evolution of the Universe, at the Harvard Smithsonian Center for Astrophysics}}</ref> An even older study from 1978 gave a lower diameter for Milky Way of about {{convert|23|kpc|ly|sigfig=2|abbr=on}}.<ref name="Goodwin" />
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>
Another 2018 study revealed the very probable presence of disk stars at {{convert|26|-|31.5|kpc|ly|sigfig=3|abbr=on}} from the Galactic Center or perhaps even farther, significantly beyond approximately {{convert|13|-|20|kpc|ly|sigfig=1|abbr=on}}, in which it was once believed to be the abrupt drop-off of the stellar density of the disk, meaning that few or no stars were expected to be above this limit, save for stars that belong to the old population of the galactic halo.<ref name="Lopez2018">{{Cite journal |last1=López-Corredoira |first1=M. |last2=Allende Prieto |first2=C. |last3=Garzón |first3=F. |last4=Wang |first4=H. |last5=Liu |first5=C. |last6=Deng |first6=L. |date=9 April 2018 |title=Disk stars in the Milky Way detected beyond 25 kpc from its center |url=https://www.aanda.org/articles/aa/abs/2018/04/aa32880-18/aa32880-18.html |journal=Astronomy & Astrophysics |volume=612 |pages=L8 |arxiv=1804.03064 |bibcode=2018A&A...612L...8L |doi=10.1051/0004-6361/201832880 |s2cid=59933365}}</ref><ref name="nbcnews1" /><ref>{{Cite web |author=Elizabeth Howell |date=July 2, 2018 |title=It Would Take 200,000 Years at Light Speed to Cross the Milky Way |url=https://www.space.com/41047-milky-way-galaxy-size-bigger-than-thought.html |url-status=live |archive-url=https://web.archive.org/web/20200416141212/https://www.space.com/41047-milky-way-galaxy-size-bigger-than-thought.html |archive-date=April 16, 2020 |access-date=May 31, 2020 |website=Space.com}}</ref>
A 2020 study predicted the edge of the Milky Way's [[dark matter halo]] being around {{convert|292|±|61|kpc|ly|sigfig=3|abbr=on|lk=on}}, which translates to a diameter of {{convert|584|±|122|kpc|Mly|sigfig=4|abbr=on|lk=on}}.<ref name="croswell2020" /><ref name="dearson2020" /> The Milky Way's stellar disk is also estimated to be approximately up to {{convert|1.35|kpc|ly|abbr=on|sigfig=1}} thick.<ref name="ask-astro" /><ref name="Rix_Bovy">{{Cite journal |last1=Rix |first1=Hans-Walter |last2=Bovy |first2=Jo |date=2013 |title=The Milky Way's Stellar Disk |journal=The Astronomy and Astrophysics Review |volume=21 |page=61 |arxiv=1301.3168 |bibcode=2013A&ARv..21...61R |doi=10.1007/s00159-013-0061-8 |s2cid=117112561}}</ref>
=== Mass ===
[[File:Milky way profile.svg|thumb|upright=1.2|A schematic profile of the Milky Way.<br />Abbreviations: GNP/GSP: Galactic North and South Poles]]
The Milky Way is approximately 0.88 trillion times the mass of the [[Sun]] in total (0.88{{e|12}} solar masses), using a cutoff of 200kpc to define the galaxy.<ref>{{Cite journal |last1=Bobylev |first1=V. V. |last2=Baykova |first2=A. T. |date=August 2023 |title=Modern Estimates of the Mass of the Milky Way |url=https://www.researchgate.net/publication/375619138 |journal=Astronomy Reports |language=en |volume=67 |issue=8 |pages=812–823 |doi=10.1134/S1063772923080024 |bibcode=2023ARep...67..812B |issn=1063-7729}}</ref> Estimates of the mass of the Milky Way vary, depending upon the method and data used. The low end of the estimate range is 5.8{{e|11}} [[solar mass]]es ({{solar mass}}), somewhat less than that of the [[Andromeda Galaxy]].<ref name="Karachentsevetal2006" /><ref name="mass" /><ref name="galmass" />
Measurements using the [[Very Long Baseline Array]] in 2009 found velocities as large as {{convert|254|km/s|mph|abbr=on}} for stars at the outer edge of the Milky Way.<ref name=nrao20090105 />
Because the orbital velocity depends on the total mass inside the orbital radius, this suggests that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7{{e|11}} {{solar mass}} within {{convert|160000|ly|kpc|abbr=on}} of its center.<ref name="reid2009" /> In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kilo[[parsec]]s is 7{{e|11}} {{solar mass}}.<ref name="gnedin2010" /> In a 2014 study, the mass of the entire Milky Way is estimated to be 8.5{{e|11}} {{solar mass}},<ref name="Jorge Peñarrubia2014">{{Cite journal |last1=Peñarrubia |first1=Jorge |last2=Ma |first2=Yin-Zhe |last3=Walker |first3=Matthew G. |last4=McConnachie |first4=Alan |display-authors=1 |year=2014 |title=A dynamical model of the local cosmic expansion |journal=Monthly Notices of the Royal Astronomical Society |volume=433 |issue=3 |pages=2204–2222 |arxiv=1405.0306 |bibcode=2014MNRAS.443.2204P |doi=10.1093/mnras/stu879 |doi-access=free |s2cid=119295582}}</ref> but this is only half the mass of the Andromeda Galaxy.<ref name="Jorge Peñarrubia2014" /> A recent 2019 mass estimate for the Milky Way is 1.29{{e|12}} {{solar mass}}.<ref>{{Cite journal |last1=Grand |first1=Robert J J. |last2=Deason |first2=Alis J. |last3=White |first3=Simon D M. |last4=Simpson |first4=Christine M. |last5=Gómez |first5=Facundo A. |last6=Marinacci |first6=Federico |last7=Pakmor |first7=Rüdiger |year=2019 |title=The effects of dynamical substructure on Milky Way mass estimates from the high-velocity tail of the local stellar halo |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=487 |issue=1 |pages=L72–L76 |arxiv=1905.09834 |bibcode=2019MNRAS.487L..72G |doi=10.1093/mnrasl/slz092 |doi-access=free |s2cid=165163524}}</ref>
Much of the mass of the Milky Way seems to be [[dark matter]], an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A [[dark matter halo]] is conjectured to spread out relatively uniformly to a distance beyond one hundred kiloparsecs (kpc) from the Galactic Center. Mathematical models of the Milky Way suggest that the mass of dark matter is 1–1.5{{e|12}} {{solar mass}}.<ref name=McMillan2011 /><ref name=McMillan2016 /><ref>{{Cite journal |last=Slobodan Ninković |date=April 2017 |title=Mass Distribution and Gravitational Potential of the Milky Way |journal=Open Astronomy |volume=26 |issue=1 |pages=1–6 |bibcode=2017OAst...26....1N |doi=10.1515/astro-2017-0002 |doi-access=free}}</ref> 2013 and 2014 studies indicate a range in mass, as large as 4.5{{e|12}} {{solar mass}}<ref>{{Cite journal |last1=Phelps |first1=Steven |last2=Nusser |first2=Adi |last3=Desjacques |first3=Vincent |display-authors=1 |date=October 2013 |title=The Mass of the Milky Way and M31 Using the Method of Least Action |journal=The Astrophysical Journal |volume=775 |issue=2 |pages=102–113 |arxiv=1306.4013 |bibcode=2013ApJ...775..102P |doi=10.1088/0004-637X/775/2/102 |s2cid=21656852 |id=102}}</ref> and as small as 8{{e|11}} {{solar mass}}.<ref>{{Cite journal |last1=Kafle |first1=Prajwal Raj |last2=Sharma |first2=Sanjib |last3=Lewis |first3=Geraint F. |last4=Bland-Hawthorn |first4=Joss |display-authors=1 |date=October 2014 |title=On the Shoulders of Giants: Properties of the Stellar Halo and the Milky Way Mass Distribution |journal=The Astrophysical Journal |volume=794 |issue=1 |page=17 |arxiv=1408.1787 |bibcode=2014ApJ...794...59K |doi=10.1088/0004-637X/794/1/59 |s2cid=119040135 |id=59}}</ref> By comparison, the total mass of all the stars in the Milky Way is estimated to be between 4.6{{e|10}} {{solar mass}}<ref name="Licquia2013">{{Cite journal |last1=Licquia |first1=Timothy |last2=Newman |first2=J. |date=2013 |title=Improved Constraints on the Total Stellar Mass, Color, and Luminosity of the Milky Way |journal=American Astronomical Society, AAS Meeting #221, #254.11 |volume=221 |pages=254.11 |bibcode=2013AAS...22125411L}}</ref> and 6.43{{e|10}} {{solar mass}}.<ref name="McMillan2011" />
In addition to the stars, there is also interstellar gas, comprising 90% [[hydrogen]] and 10% [[helium]] by mass,<ref name="ism1" /> with two thirds of the hydrogen found in the [[neutral hydrogen|atomic form]] and the remaining one-third as [[molecular hydrogen]].<ref name="ism2" /> The mass of the Milky Way's interstellar gas is equal to between 10%<ref name="ism2">{{Cite web |title=Lecture Seven: The Milky Way: Gas |url=http://www.astro.rug.nl/~etolstoy/pog14/resources/lectures/PoG-lecture7pr.pdf |archive-url=https://web.archive.org/web/20150708011238/http://www.astro.rug.nl/~etolstoy/pog14/resources/lectures/PoG-lecture7pr.pdf |archive-date=July 8, 2015 |access-date=May 2, 2015}}</ref> and 15%<ref name="ism1">{{Cite web |title=The Interstellar Medium |url=http://csep10.phys.utk.edu/astr162/lect/milkyway/ism.html |archive-url=https://web.archive.org/web/20150419065745/http://csep10.phys.utk.edu/astr162/lect/milkyway/ism.html |archive-date=April 19, 2015 |access-date=May 2, 2015}}</ref> of the total mass of its stars. [[Interstellar dust]] accounts for an additional 1% of the total mass of the gas.<ref name="ism1" />
In March 2019, astronomers reported that the [[virial mass]] of the Milky Way Galaxy is {{Val|1.54e12|ul=solar masses}} within a [[radius]] of about {{convert|39.5|kpc|ly|sigfig=2|abbr=on}}, over twice as much as was determined in earlier studies, suggesting that about 90% of the mass of the galaxy is [[dark matter]].<ref name="SA-20190308">{{Cite news |last=Starr |first=Michelle |date=March 8, 2019 |title=The Latest Calculation of Milky Way's Mass Just Changed What We Know About Our Galaxy |work=ScienceAlert.com |url=https://www.sciencealert.com/the-most-accurate-measurement-yet-of-the-milky-way-s-mass-puts-us-ahead-of-andromeda |url-status=live |access-date=March 8, 2019 |archive-url=https://web.archive.org/web/20190308125656/https://www.sciencealert.com/the-most-accurate-measurement-yet-of-the-milky-way-s-mass-puts-us-ahead-of-andromeda |archive-date=March 8, 2019}}</ref><ref name="ARX-20190208">{{Cite journal |last=Watkins, Laura L. |display-authors=et al. |date=February 2, 2019 |title=Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions |journal=The Astrophysical Journal |volume=873 |issue=2 |page=118 |arxiv=1804.11348 |bibcode=2019ApJ...873..118W |doi=10.3847/1538-4357/ab089f |s2cid=85463973 |doi-access=free}}</ref>
In September 2023, astronomers reported that the [[virial mass]] of the Milky Way Galaxy is only {{Val|2.06e11|ul=solar masses}}, only a tenth of the mass of previous studies. The mass was determined from data of the [[Gaia (spacecraft)|''Gaia'' spacecraft]].<ref name="aA-20230927">{{cite journal | last1=Jiao | first1=Y.-J. | last2=Hammer | first2=F. | last3=Wang | first3=H.-F. | last4=Wang | first4=J.-L. | last5=Amram | first5=P. | last6=Chemin | first6=L. | last7=Yang | first7=Y.-B. | title=Detection of the Keplerian decline in the Milky Way rotation curve | journal=Astronomy & Astrophysics | publisher=EDP Sciences | date=2023-09-27 | volume=678 | pages=A208 | issn=0004-6361 | doi=10.1051/0004-6361/202347513 | doi-access=free | arxiv=2309.00048 | bibcode=2023A&A...678A.208J }}</ref>
{{clear}}
== Contents ==
The Milky Way contains between 100 and 400 billion stars<ref name=frommert_kronberg2005 /><ref name=wethington2010 /> and at least that many planets.<ref name=villard20120111 /> An exact figure would depend on counting the number of very-low-mass stars, which are difficult to detect, especially at distances of more than {{convert|300|ly|pc|-1|abbr=on}} from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (10<sup>12</sup>) stars.<ref name=young2006 /> The Milky Way may contain ten billion [[white dwarf]]s, a billion [[neutron star]]s, and a hundred million stellar [[black hole]]s.<!--Start of the "number" note-->{{efn|name=number|1=These estimates are very uncertain, as most non-star objects are difficult to detect; for example, black hole estimates range from ten million to one billion.<ref>{{Cite web |title=Black Holes {{!}} Science Mission Directorate |url=https://science.nasa.gov/astrophysics/focus-areas/black-holes |url-status=live |archive-url=https://web.archive.org/web/20171117232123/https://science.nasa.gov/astrophysics/focus-areas/black-holes |archive-date=November 17, 2017 |access-date=April 5, 2018 |website=NASA |language=en}}</ref><ref>{{Cite journal |last1=Oka |first1=Tomoharu |last2=Tsujimoto |first2=Shiho |last3=Iwata |first3=Yuhei |last4=Nomura |first4=Mariko |last5=Takekawa |first5=Shunya |date=October 2017 |title=Millimetre-wave emission from an intermediate-mass black hole candidate in the Milky Way |url=http://www.nature.com/articles/s41550-017-0224-z |url-status=live |journal=Nature Astronomy |language=en |volume=1 |issue=10 |pages=709–712 |arxiv=1707.07603 |bibcode=2017NatAs...1..709O |doi=10.1038/s41550-017-0224-z |issn=2397-3366 |s2cid=119400213 |archive-url=https://web.archive.org/web/20220424231649/https://www.nature.com/articles/s41550-017-0224-z |archive-date=April 24, 2022 |access-date=April 24, 2022}}</ref>}}<!--End of the "number" note--><ref>Napiwotzki, R. (2009). The galactic population of white dwarfs. In Journal of Physics: Conference Series (Vol. 172, No. 1, p. 012004). IOP Publishing.</ref><ref>{{Cite web |title=NASA – Neutron Stars |url=https://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html |url-status=live |archive-url=https://web.archive.org/web/20180908042354/https://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html |archive-date=September 8, 2018 |access-date=April 5, 2018 |website=NASA |language=en}}</ref> Filling the space between the stars is a disk of gas and dust called the [[interstellar medium]]. This disk has at least a comparable extent in radius to the stars,<ref name=science312_1773 /> whereas the thickness of the gas layer ranges from hundreds of light-years for the colder gas to thousands of light-years for the warmer gas.<ref name=araa28_215 /><ref name=apj702_1472 />
The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars decreases with distance from the center of the Milky Way. Beyond a radius of roughly 40,000 light years (13 kpc) from the center, the number of stars per cubic [[parsec]] drops much faster with radius.<ref name=arxiv0909.3857 /> Surrounding the galactic disk is a spherical [[galactic halo]] of stars and [[globular cluster]]s that extends farther outward, but is limited in size by the orbits of two Milky Way satellites, the Large and Small [[Magellanic Clouds]], whose [[Apsis|closest approach]] to the Galactic Center is about {{convert|180000|ly|kpc|abbr=on}}.<ref name=mnras371_1_108 /> At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way. The integrated [[absolute visual magnitude]] of the Milky Way is estimated to be around −20.9.<ref name=coffey /><ref name="Karachentsev2004">{{Cite journal |last1=Karachentsev |first1=Igor D. |last2=Karachentseva |first2=Valentina E. |last3=Huchtmeier |first3=Walter K. |last4=Makarov |first4=Dmitry I. |date=2003 |title=A Catalog of Neighboring Galaxies |journal=The Astronomical Journal |volume=127 |issue=4 |pages=2031–2068 |bibcode=2004AJ....127.2031K |doi=10.1086/382905 |doi-access=free}}</ref>{{efn|name=milky way mag}}
Both [[gravitational microlensing]] and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way,<ref name="Nature-20120111" /><ref name=borenstein2011 /> and microlensing measurements indicate that there are more [[rogue planet]]s not bound to host stars than there are stars.<ref>{{Cite journal |last1=Sumi |first1=T. |last2=Kamiya |first2=K. |last3=Bennett |first3=D. P. |last4=Bond |first4=I. A. |last5=Abe |first5=F. |last6=Botzler |first6=C. S. |last7=Fukui |first7=A. |last8=Furusawa |first8=K. |last9=Hearnshaw |first9=J. B. |last10=Itow |first10=Y. |last11=Kilmartin |first11=P. M. |last12=Korpela |first12=A. |last13=Lin |first13=W. |last14=Ling |first14=C. H. |last15=Masuda |first15=K. |display-authors=1 |year=2011 |title=Unbound or distant planetary mass population detected by gravitational microlensing |journal=Nature |volume=473 |issue=7347 |pages=349–352 |arxiv=1105.3544 |bibcode=2011Natur.473..349S |doi=10.1038/nature10092 |pmid=21593867 |s2cid=4422627 |last16=Matsubara |first16=Y. |last17=Miyake |first17=N. |last18=Motomura |first18=M. |last19=Muraki |first19=Y. |last20=Nagaya |first20=M. |last21=Nakamura |first21=S. |last22=Ohnishi |first22=K. |last23=Okumura |first23=T. |last24=Perrott |first24=Y. C. |last25=Rattenbury |first25=N. |last26=Saito |first26=To. |last27=Sako |first27=T. |last28=Sullivan |first28=D. J. |last29=Sweatman |first29=W. L. |last30=Tristram |first30=P. J. |last31=Udalski |first31=A. |last32=Szymański |first32=M. K. |last33=Kubiak |first33=M. |last34=Pietrzyński |first34=G. |last35=Poleski |first35=R. |last36=Soszyński |first36=I. |last37=Wyrzykowski |first37=Ł. |last38=Ulaczyk |first38=K.}}</ref><ref name=nasa20110526 /> The Milky Way contains an average of at least one planet per star, resulting in 100–400 billion planets, according to a January 2013 study of the five-planet star system [[Kepler-32]] by the [[Kepler (spacecraft)|Kepler]] space observatory.<ref name="Space-20130102" /> A different January 2013 analysis of Kepler data estimated that at least 17 billion [[Terrestrial planet|Earth-sized]] [[exoplanet]]s reside in the Milky Way.<ref name="Space-Billions">{{cite news |author=<!--Not stated--> |title=17 Billion Earth-Size Alien Planets Inhabit Milky Way |url=http://www.space.com/19157-billions-earth-size-alien-planets-aas221.html |date=January 7, 2013 |work=[[Space.com]] |access-date=January 8, 2013 |archive-url=https://web.archive.org/web/20141006095334/http://www.space.com/19157-billions-earth-size-alien-planets-aas221.html |archive-date=October 6, 2014}}</ref>
In November 2013, astronomers reported, based on [[Kepler (spacecraft)|''Kepler'' space mission]] data, that there could be as many as 40 billion Earth-sized [[extrasolar planet|planets]] orbiting in the [[habitable zone]]s of [[Solar analog|Sun-like stars]] and [[red dwarf]]s within the Milky Way.<ref name="NYT-20131104">{{Cite news |last=Overbye |first=Dennis |date=November 4, 2013 |title=Far-Off Planets Like the Earth Dot the Galaxy |work=[[The New York Times]] |url=https://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |url-status=live |access-date=November 5, 2013 |archive-url=https://web.archive.org/web/20131105023653/http://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |archive-date=November 5, 2013}}</ref><ref name="PNAS-20131031">{{Cite journal |last1=Petigura |first1=Eric A. |last2=Howard |first2=Andrew W. |last3=Marcy |first3=Geoffrey W. |date=October 31, 2013 |title=Prevalence of Earth-size planets orbiting Sun-like stars |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=110 |issue=48 |pages=19273–19278 |arxiv=1311.6806 |bibcode=2013PNAS..11019273P |doi=10.1073/pnas.1319909110 |pmc=3845182 |pmid=24191033 |doi-access=free}}</ref><ref name="nov2013_planet_estimate" /> 11 billion of these estimated planets may be orbiting Sun-like stars.<ref name="LATimes-20131104">{{Cite news |last=Khan |first=Amina |date=November 4, 2013 |title=Milky Way may host billions of Earth-size planets |work=[[Los Angeles Times]] |url=https://www.latimes.com/science/la-sci-earth-like-planets-20131105,0,2673237.story |url-status=live |access-date=November 5, 2013 |archive-url=https://web.archive.org/web/20131106030558/http://www.latimes.com/science/la-sci-earth-like-planets-20131105%2C0%2C2673237.story |archive-date=November 6, 2013}}</ref> The nearest exoplanet may be 4.2 light-years away, orbiting the [[red dwarf]] [[Proxima Centauri]], according to a 2016 study.<ref name="Planet-Alpha-Centauri">{{Cite journal |last1=Anglada-Escudé |first1=Guillem |last2=Amado |first2=Pedro J. |last3=Barnes |first3=John |last4=Berdiñas |first4=Zaira M. |last5=Butler |first5=R. Paul |last6=Coleman |first6=Gavin A. L. |last7=de la Cueva |first7=Ignacio |last8=Dreizler |first8=Stefan |last9=Endl |first9=Michael |last10=Giesers |first10=Benjamin |last11=Jeffers |first11=Sandra V. |last12=Jenkins |first12=James S. |last13=Jones |first13=Hugh R. A. |last14=Kiraga |first14=Marcin |last15=Kürster |first15=Martin |display-authors=3 |year=2016 |title=A terrestrial planet candidate in a temperate orbit around Proxima Centauri |url=https://www.nature.com/articles/nature19106 |url-status=live |journal=Nature |volume=536 |issue=7617 |pages=437–440 |arxiv=1609.03449 |bibcode=2016Natur.536..437A |doi=10.1038/nature19106 |pmid=27558064 |s2cid=4451513 |archive-url=https://web.archive.org/web/20211003204853/https://www.nature.com/articles/nature19106 |archive-date=October 3, 2021 |access-date=September 11, 2021 |last16=López-González |first16=María J. |last17=Marvin |first17=Christopher J. |last18=Morales |first18=Nicolás |last19=Morin |first19=Julien |last20=Nelson |first20=Richard P. |last21=Ortiz |first21=José L. |last22=Ofir |first22=Aviv |last23=Paardekooper |first23=Sijme-Jan |last24=Reiners |first24=Ansgar |last25=Rodríguez |first25=Eloy |last26=Rodríguez-López |first26=Cristina |last27=Sarmiento |first27=Luis F. |last28=Strachan |first28=John P. |last29=Tsapras |first29=Yiannis |last30=Tuomi |first30=Mikko |first31=Mathias |last31=Zechmeister}}</ref> Such Earth-sized planets may be more numerous than gas giants,<ref name="Nature-20120111" /> though harder to detect at great distances given their small size. Besides exoplanets, "[[exocomet]]s", [[comet]]s beyond the Solar System, have also been detected and may be common in the Milky Way.<ref name="Space-Exocomets">{{cite news |author=<!--Not stated--> |title='Exocomets' Common Across Milky Way Galaxy |url=http://www.space.com/19156-exocomets-alien-solar-systems.html |date=January 7, 2013 |work=[[Space.com]] |access-date=January 8, 2013 |archive-url=https://web.archive.org/web/20140916085824/http://www.space.com/19156-exocomets-alien-solar-systems.html |archive-date=September 16, 2014}}</ref> More recently, in November 2020, over 300 million habitable exoplanets are estimated to exist in the Milky Way Galaxy.<ref name="NYT-20201105">{{Cite news |last=Overbye |first=Dennis |date=November 5, 2020 |title=Looking for Another Earth? Here Are 300 Million, Maybe – A new analysis of data from NASA's Kepler spacecraft increases the number of habitable exoplanets thought to exist in this galaxy. |work=[[The New York Times]] |url=https://www.nytimes.com/2020/11/05/science/astronomy-exoplanets-kepler.html |url-status=live |url-access=subscription |access-date=November 5, 2020 |archive-url=https://web.archive.org/web/20201105164943/https://www.nytimes.com/2020/11/05/science/astronomy-exoplanets-kepler.html |archive-date=2020-11-05}}</ref>
When compared to other more distant galaxies in the universe, the Milky Way galaxy has a below average amount of [[neutrino]] luminosity making our galaxy a "neutrino desert".<ref>{{Cite journal |last1=Fang |first1=Ke |last2=Gallagher |first2=John S. |last3=Halzen |first3=Francis |date=February 2024 |title=The Milky Way revealed to be a neutrino desert by the IceCube Galactic plane observation |url=https://www.nature.com/articles/s41550-023-02128-0 |journal=Nature Astronomy |language=en |volume=8 |issue=2 |pages=241–246 |doi=10.1038/s41550-023-02128-0 |issn=2397-3366|arxiv=2306.17275 |bibcode=2024NatAs...8..241F }}</ref>
== Structure ==
[[File:Milky Way side view.png|thumb|upright=1.35|Overview of different elements of the overall structure of the Milky Way]]
The Milky Way consists of a bar-shaped core region surrounded by a warped disk of [[interstellar medium|gas, dust]] and stars.<ref>{{Cite web |title=The Milky Way is warped |url=https://phys.org/news/2019-02-milky-warped.html |url-status=live |archive-url=https://web.archive.org/web/20190207015954/https://phys.org/news/2019-02-milky-warped.html |archive-date=February 7, 2019 |access-date=February 22, 2019 |website=phys.org}}</ref><ref>{{Cite journal |last1=Chen |first1=Xiaodian |last2=Wang |first2=Shu |last3=Deng |first3=Licai |last4=de Grijs |first4=Richard |last5=Liu |first5=Chao |last6=Tian |first6=Hao |date=February 4, 2019 |title=An intuitive 3D map of the Galactic warp's precession traced by classical Cepheids |journal=Nature Astronomy |language=en |volume=3 |issue=4 |pages=320–325 |arxiv=1902.00998 |bibcode=2019NatAs...3..320C |doi=10.1038/s41550-018-0686-7 |issn=2397-3366 |s2cid=119290364}}</ref> The mass distribution within the Milky Way closely resembles the type Sbc in the [[Hubble sequence|Hubble classification]], which represents spiral galaxies with relatively loosely wound arms.<ref name=ssr100_1_129 /> Astronomers first began to conjecture that the Milky Way is a [[barred spiral galaxy]], rather than an ordinary [[spiral galaxy]], in the 1960s.<ref>Gerard de Vaucouleurs (1964), [http://adsabs.harvard.edu/abs/1964IAUS...20..195D Interpretation of velocity distribution of the inner regions of the Galaxy] {{Webarchive|url=https://web.archive.org/web/20190203053710/http://adsabs.harvard.edu/abs/1964IAUS...20..195D |date=February 3, 2019 }}</ref><ref>Peters, W.L. III. (1975), [http://adsabs.harvard.edu/abs/1975ApJ...195..617P Models for the inner regions of the Galaxy. I] {{Webarchive|url=https://web.archive.org/web/20190203185645/http://adsabs.harvard.edu/abs/1975ApJ...195..617P |date=February 3, 2019 }}</ref><ref>Hammersley, P. L.; Garzon, F.; Mahoney, T.; Calbet, X. (1994), [http://adsabs.harvard.edu/abs/1994MNRAS.269..753H Infrared Signatures of the Inner Spiral Arms and Bar] {{Webarchive|url=https://web.archive.org/web/20190203062819/http://adsabs.harvard.edu/abs/1994MNRAS.269..753H |date=February 3, 2019 }}</ref> These conjectures were confirmed by the [[Spitzer Space Telescope]] observations in 2005 that showed the Milky Way's central bar to be larger than previously thought.<ref name="fn3" />
=== Galactic Center ===
{{Main|Galactic Center|Sagittarius A*}}
{{multiple image
| align = right
| direction = horizontal
| total_width = 400
| image1 = EHT Saggitarius A black hole.tif
| caption1 = [[Supermassive black hole]] {{nowrap|[[Sagittarius A*]]}} imaged by the [[Event Horizon Telescope]] in radio waves. The central dark spot is the black hole's shadow, which is larger than the [[event horizon]].
| alt1 = A dark spot surrounded by doughnut shaped orange-yellow ring
| image2 = X-RayFlare-BlackHole-MilkyWay-20140105.jpg
| caption2 = Bright [[X-ray]] flares from {{nowrap|Sagittarius A*}} (inset) in the center of the Milky Way, as detected by the [[Chandra X-ray Observatory]].<ref name="NASA-20150105" />
}}
The Sun is {{convert|25000|-|28000|ly|kpc|abbr=on}} from the Galactic Center. This value is estimated using [[geometric]]-based methods or by measuring selected astronomical objects that serve as [[standard candles]], with different techniques yielding various values within this approximate range.<ref name="apj692_2_1075" /><ref name=boehle2016 /><ref name=Gillessen2016 /><ref name=ReideaSgrb2009 /><ref name="vanhollebeke09" /><ref name="majaess10" /> In the inner few kiloparsecs (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called [[Bulge (astronomy)|the bulge]].<ref name="Galactic bulge" /> It has been proposed that the Milky Way lacks a [[Galactic bulge#Classical bulges|bulge]] due to a [[galactic merger|collision and merger between previous galaxies]], and that instead it only has a [[Galactic bulge#Disk-like Bulges|''pseudobulge'']] formed by its central bar.<ref name=apjl_1_l72 /> However, confusion in the literature between the (peanut shell)-shaped structure created by instabilities in the bar, versus a possible bulge with an expected half-light radius of 0.5 kpc, abounds.<ref>{{Cite journal |last1=Ciambur |first1=Bogdan C. |last2=Graham |first2=Alister W. |last3=Bland-Hawthorn |first3=Joss |year=2017 |title=Quantifying the (X/peanut)-shaped structure of the Milky Way – new constraints on the bar geometry |journal=Monthly Notices of the Royal Astronomical Society |volume=471 |issue=4 |page=3988 |arxiv=1706.09902 |bibcode=2017MNRAS.471.3988C |doi=10.1093/mnras/stx1823 |doi-access=free |s2cid=119376558}}</ref>
The Galactic Center is marked by an intense [[radio astronomy|radio source]] named {{nowrap|[[Sagittarius A*]]}} (pronounced ''Sagittarius A-star''). The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object.<ref name="Jones2004" /> This concentration of mass is best explained as a [[supermassive black hole]]{{efn|name=chandra2003}}<ref name="apj692_2_1075" /><ref name="apj689_2_1044" /> (SMBH) with an estimated mass of 4.1–4.5 million times the [[solar mass|mass of the Sun]].<ref name="apj689_2_1044" /> The rate of accretion of the SMBH is consistent with an [[Active galactic nucleus|inactive galactic nucleus]], being estimated at {{Val|1|e=−5}} {{Solar mass}} per year.<ref name="wang13" /> Observations indicate that there are SMBHs located near the center of most normal galaxies.<ref name="blandford1999" /><ref name="frolov_zelnikov2011" />
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 ====
[[File:SRG-eROSITA all-sky image.jpg|thumb|upright=1.6|All-sky x-ray image]]
Since 1970, various gamma-ray detection missions have discovered 511-[[keV]] [[gamma rays]] coming from the general direction of the Galactic Center. These gamma rays are produced by [[positrons]] (antielectrons) annihilating with [[electron]]s. In 2008 it was found that the distribution of the sources of the gamma rays resembles the distribution of low-mass [[X-ray binaries]], seeming to indicate that these X-ray binaries are sending positrons (and electrons) into interstellar space where they slow down and annihilate.<ref>{{Cite journal |last=Georg Weidenspointner |display-authors=etal |date=January 10, 2008 |title=An asymmetric distribution of positrons in the Galactic disk revealed by γ-rays |journal=Nature |volume=451 |issue=7175 |pages=159–162 |bibcode=2008Natur.451..159W |doi=10.1038/nature06490 |pmid=18185581 |s2cid=4333175}}</ref><ref name="NASA-2008">{{Cite web |date=January 9, 2008 |title= Satellite Explains Giant Cloud of Antimatter |url=https://www.nasa.gov/topics/universe/features/antimatter_binary.html |url-status=live |archive-url=https://web.archive.org/web/20210506215302/http://www.nasa.gov/topics/universe/features/antimatter_binary.html |archive-date=May 6, 2021 |access-date=July 2, 2021 |website=NASA |first1=Bob |last1=Naeye |language=en}}</ref><ref>{{Cite web |title=Antimatter Clouds and Fountains – NASA Press Release 97-83 |url=https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/anti01.html |url-status=live |archive-url=https://web.archive.org/web/20210709190307/https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/anti01.html |archive-date=July 9, 2021 |access-date=July 2, 2021 |website=HEASARC |date=April 28, 1997 }}</ref> The observations were made by both [[NASA]] and [[European Space Agency|ESA]]'s satellites. In 1970 gamma ray detectors found that the emitting region was about 10,000 light-years across with a luminosity of about 10,000 Suns.<ref name="NASA-2008" />
[[File:800 nasa structure renderin2.jpg|thumb|upright=1.35|Illustration of the two gigantic [[X-ray]]/[[gamma-ray]] bubbles (blue-violet) of the Milky Way (center)]]
In 2010, two gigantic spherical bubbles of high energy gamma-emission were detected to the north and the south of the Milky Way core, using data from the [[Fermi Gamma-ray Space Telescope]]. The diameter of each of the bubbles is about {{convert|25000|ly|kpc}} (or about 1/4 of the galaxy's estimated diameter); they stretch up to [[Grus (constellation)|Grus]] and to [[Virgo (constellation)|Virgo]] on the night-sky of the southern hemisphere.<ref name=overbye2010 /><ref name=sz2010 /> Subsequently, observations with the [[Parkes Telescope]] at radio frequencies identified polarized emission that is associated with the Fermi bubbles. These observations are best interpreted as a magnetized outflow driven by star formation in the central {{convert|640|ly|pc|abbr=on}} of the Milky Way.<ref>{{Cite journal |last1=Carretti |first1=E. |last2=Crocker |first2=R. M. |last3=Staveley-Smith |first3=L. |last4=Haverkorn |first4=M. |last5=Purcell |first5=C. |last6=Gaensler |first6=B. M. |last7=Bernardi |first7=G. |last8=Kesteven |first8=M. J. |last9=Poppi |first9=S. |year=2013 |title=Giant magnetized outflows from the centre of the Milky Way |journal=Nature |volume=493 |issue=7430 |pages=66–69 |arxiv=1301.0512 |bibcode=2013Natur.493...66C |doi=10.1038/nature11734 |pmid=23282363 |s2cid=4426371}}</ref>
Later, on January 5, 2015, [[NASA]] reported observing an [[X-ray]] flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of an [[asteroid]] falling into the black hole or by the entanglement of [[magnetic field lines]] within gas flowing into Sagittarius A*.<ref name="NASA-20150105">{{Cite web |last1=Chou |first1=Felicia |last2=Anderson |first2=Janet |last3=Watzke |first3=Megan |date=January 5, 2015 |title=Release 15-001 – NASA's Chandra Detects Record-Breaking Outburst from Milky Way's Black Hole |url=http://www.nasa.gov/press/2015/january/nasa-s-chandra-detects-record-breaking-outburst-from-milky-way-s-black-hole/ |url-status=live |archive-url=https://web.archive.org/web/20150106100532/http://www.nasa.gov/press/2015/january/nasa-s-chandra-detects-record-breaking-outburst-from-milky-way-s-black-hole/ |archive-date=January 6, 2015 |access-date=January 6, 2015 |website=[[NASA]]}}</ref>
===Spiral arms=== <!-- Note: this section is linked to above. Do not rename without checking links to [[#Spiral arms]] -->
{{Further|Spiral galaxy}}
[[File:Milky Way Arms.svg|thumb|upright=1.45|Observed (normal lines) and extrapolated (dotted lines) structure of the spiral arms of the Milky Way, viewed from north of the galaxy – the galaxy rotates clockwise in this view. The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations]]Outside the gravitational influence of the Galactic bar, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms.<ref name="churchwell09" /> Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation, as traced by [[H II region]]s<ref name="TaylorCordes" /><ref name="russeil03" /> and [[molecular cloud]]s.<ref>{{Cite journal |last1=Dame |first1=T. M. |last2=Hartmann |first2=D. |last3=Thaddeus |first3=P. |year=2001 |title=The Milky Way in molecular clouds: A new complete CO survey |journal=The Astrophysical Journal |volume=547 |issue=2 |pages=792–813 |arxiv=astro-ph/0009217 |bibcode=2001ApJ...547..792D |doi=10.1086/318388 |s2cid=118888462}}</ref>
The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's arms.<ref name=two_arms /> Perfect logarithmic spiral patterns only crudely describe features near the Sun,<ref name="russeil03" /><ref name="majaess09b" /> because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.<ref name="majaess10" /><ref name="majaess09b" /><ref name="lepine10" /> The possible scenario of the Sun within a spur / Local arm<ref name="russeil03" /> emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way.<ref name="majaess09b" /> Estimates of the pitch angle of the arms range from about 7° to 25°.<ref name="science312_1773" /><ref name="Drimmel2000" /> There are thought to be four spiral arms that all start near the Milky Way Galaxy's center.<ref>{{Cite journal |last1=Sanna |first1=A. |last2=Reid |first2=M. J. |last3=Dame |first3=T. M. |last4=Menten |first4=K. M. |last5=Brunthaler |first5=A. |year=2017 |title=Mapping spiral structure on the far side of the Milky Way |journal=Science |volume=358 |pages=227–230 |arxiv=1710.06489 |bibcode=2017Sci...358..227S |doi=10.1126/science.aan5452 |pmid=29026043 |s2cid=206660521 |number=6360}}</ref> These are named as follows, with the positions of the arms shown in the image:
{|class="wikitable"
|-
!Color
!Arm(s)
|-
| style="background:#28c7b4;"|turquoise
|[[Near 3 kpc Arm|Near 3 kpc]] and [[Perseus Arm]]
|-
| style="background:#928bff;"|blue
|[[Norma Arm|Norma]] and [[Outer arm]] (Along with extension discovered in 2004<ref name="mcclure-griffiths" />)
|-
| style="background:#07c459;"|green
|[[Far 3 kpc Arm|Far 3 kpc]] and [[Scutum–Centaurus Arm]]
|-
| style="background:#ff6c6c;"|red
|[[Carina–Sagittarius Arm]]
|-
|colspan="2" style="text-align:center;"|''There are at least two smaller arms or spurs, including:''
|-
| style="background:#ff9b00;"|orange
|[[Orion–Cygnus Arm]] (which contains the Sun and Solar System)
|}
Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum–Centaurus Arm contains approximately 30% more [[red giant]]s than would be expected in the absence of a spiral arm.<ref name="Drimmel2000" /><ref name="Benjamin2005" />
This observation suggests that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess old stars.<ref name=two_arms /> In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way.<ref>{{Cite press release |title=Massive stars mark out Milky Way's 'missing' arms |date=December 17, 2013 |publisher=University of Leeds |url=http://www.leeds.ac.uk/news/article/3470/massive_stars_mark_out_milky_ways_missing_arms |access-date=December 18, 2013 |archive-url=https://web.archive.org/web/20131218150156/http://www.leeds.ac.uk/news/article/3470/massive_stars_mark_out_milky_ways_missing_arms |archive-date=December 18, 2013 |place=Leeds, UK}}</ref><ref>{{Cite news |last=Westerholm |first=Russell |date=December 18, 2013 |title=Milky Way Galaxy has four arms, reaffirming old data and contradicting recent research |work=University Herald |url=http://www.universityherald.com/articles/6299/20131218/milky-way-galaxy-has-four-arms-reaffirming-old-data-and-contradicting-recent-research.htm |url-status=live |access-date=December 18, 2013 |archive-url=https://web.archive.org/web/20131219010711/http://www.universityherald.com/articles/6299/20131218/milky-way-galaxy-has-four-arms-reaffirming-old-data-and-contradicting-recent-research.htm |archive-date=December 19, 2013}}</ref><ref name="urquart2013">{{Cite journal |last1=Urquhart |first1=J. S. |last2=Figura |first2=C. C. |last3=Moore |first3=T. J. T. |last4=Hoare |first4=M. G. |last5=Lumsde |first5=S. L. |last6=Mottram |first6=J. C. |last7=Thompson |first7=M. A. |last8=Oudmaijer |first8=R. D. |display-authors=4 |date=January 2014 |title=The RMS Survey: Galactic distribution of massive star formation |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=437 |issue=2 |pages=1791–1807 |arxiv=1310.4758 |bibcode=2014MNRAS.437.1791U |doi=10.1093/mnras/stt2006 |doi-access=free |s2cid=14266458}}</ref> Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear.<ref name="urquart2013" />
The ''[[Near 3 kpc Arm]]'' (also called the ''Expanding 3 kpc Arm'' or simply the ''3 kpc Arm'') was discovered in the 1950s by astronomer van Woerden and collaborators through [[Hydrogen line|21 centimeter]] radio measurements of H{{sup|I}} ([[atomic hydrogen]]).<ref>{{Cite journal |last1=van Woerden |first1=H. |last2=Rougoor |first2=G. W. |last3=Oort |first3=J. H. |display-authors=1 |date=1957 |title=Expansion d'une structure spirale dans le noyau du Système Galactique, et position de la radiosource Sagittarius A |journal=Comptes Rendus de l'Académie des Sciences |language=fr |volume=244 |pages=1691–1695 |bibcode=1957CRAS..244.1691V}}</ref><ref name="Dame">{{Cite journal |last1=Dame |first1=T. M. |last2=Thaddeus |first2=P. |year=2008 |title=A New Spiral Arm of the Galaxy: The Far 3-Kpc Arm |journal=The Astrophysical Journal |volume=683 |issue=2 |pages=L143–L146 |arxiv=0807.1752 |bibcode=2008ApJ...683L.143D |doi=10.1086/591669 |s2cid=7450090}}</ref> It was found to be expanding away from the central bulge at more than 50 [[kilometer per second|km/s]]. It is located in the fourth galactic quadrant at a distance of about 5.2 [[parsec|kpc]] from the [[Sun]] and 3.3 kpc from the [[Galactic Center]]. The Far 3 kpc Arm was discovered in 2008 by astronomer Tom Dame ([[Harvard–Smithsonian Center for Astrophysics|Center for Astrophysics {{!}} Harvard & Smithsonian]]). It is located in the first galactic quadrant at a distance of 3 [[parsec|kpc]] (about 10,000 [[light year|ly]]) from the Galactic Center.<ref name="Dame" /><ref>{{Cite news |date=June 3, 2008 |title=Milky Way's Inner Beauty Revealed |publisher=Center for Astrophysics {{!}} Harvard & Smithsonian |url=http://www.cfa.harvard.edu/news/2008/pr200813.html |url-status=live |access-date=July 7, 2015 |archive-url=https://web.archive.org/web/20130705045749/http://www.cfa.harvard.edu/news/2008/pr200813.html |archive-date=July 5, 2013}}</ref>
A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the [[Sagittarius Dwarf Elliptical Galaxy]].<ref>{{Cite web |last=Matson |first=John |date=September 14, 2011 |title=Star-Crossed: Milky Way's Spiral Shape May Result from a Smaller Galaxy's Impact |url=http://www.scientificamerican.com/article.cfm?id=sagittarius-satellite-spiral |url-status=live |archive-url=https://web.archive.org/web/20131203000625/http://www.scientificamerican.com/article.cfm?id=sagittarius-satellite-spiral |archive-date=December 3, 2013 |access-date=July 7, 2015 |website=Scientific American}}</ref>
It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer [[Galaxy morphological classification#De Vaucouleurs system|pseudoring]],<ref name=pseudoring2 /> and the two patterns would be connected by the Cygnus arm.<ref name=pseudoring />
Outside of the major spiral arms is the [[Monoceros Ring]] (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped [[thick disk]] of the Milky Way.<ref>{{Cite arXiv |eprint=1207.2749 |class=astro-ph.GA |first1=M. |last1=Lopez-Corredoira |first2=A. |last2=Moitinho |title=Comments on the "Monoceros" affair |date=July 2012 |last3=Zaggia |first3=S. |last4=Momany |first4=Y. |last5=Carraro |first5=G. |last6=Hammersley |first6=P. L. |last7=Cabrera-Lavers |first7=A. |last8=Vazquez |first8=R. A. |display-authors=1}}</ref> The structure of the Milky Way's disk is warped along an [[Sigmoid function|"S" curve]].<ref>{{Cite web |last=Byrd |first=Deborah |author-link=Deborah Byrd |date=February 5, 2019 |title=The Milky Way is warped |url=https://earthsky.org/space/milky-way-warped-twisted-study-cepheids |url-status=live |archive-url=https://web.archive.org/web/20190206010840/https://earthsky.org/space/milky-way-warped-twisted-study-cepheids |archive-date=February 6, 2019 |access-date=February 6, 2019 |website=[[EarthSky]]}}</ref>
=== Halo ===
{{Main|Galactic halo}}
The Galactic disk is surrounded by a [[Galactic spheroid|spheroidal halo]] of old stars and [[globular cluster]]s, of which 90% lie within {{convert|100000|ly|kpc|-1}} of the Galactic Center.<ref name=harris2003 /> However, a few globular clusters have been found farther, such as PAL 4 and AM 1 at more than 200,000 light-years from the Galactic Center. About 40% of the Milky Way's clusters are on [[retrograde orbit]]s, which means they move in the opposite direction from the Milky Way rotation.<ref name=aaa313_119 /> The globular clusters can follow [[rosette orbit]]s about the Milky Way, in contrast to the [[elliptical orbit]] of a planet around a star.<ref name=apj522 />
Although the disk contains dust that obscures the view at some wavelengths, the halo component does not. Active [[star formation]] takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little cool gas to collapse into stars.<ref name="sparke_gallagher" /> [[Open cluster]]s are also located primarily on the disk.<ref name="aj108">{{Cite journal |last1=Janes, K.A. |last2=Phelps, R.L. |date=1980 |title=The galactic system of old star clusters: The development of the galactic disk |journal=The Astronomical Journal |volume=108 |pages=1773–1785 |bibcode=1994AJ....108.1773J |doi=10.1086/117192 |doi-access=free}}</ref>
Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much farther than previously thought,<ref name="fn5" /> the possibility of the disk of the Milky Way extending farther is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the [[Cygnus Arm]]<ref name="mcclure-griffiths" /><ref name="fn6" /> and of a similar extension of the [[Scutum–Centaurus Arm]].<ref>{{Cite journal |last1=T.M. Dame |last2=P. Thaddeus |date=2011 |title=A Molecular Spiral Arm in the Far Outer Galaxy |journal=The Astrophysical Journal |volume=734 |issue=1 |page=L24 |arxiv=1105.2523 |bibcode=2011ApJ...734L..24D |doi=10.1088/2041-8205/734/1/l24 |s2cid=118301649}}</ref> With the discovery of the [[Sagittarius Dwarf Elliptical Galaxy]] came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Upon the 2004 discovery of a ring of galactic debris in an in-plane orbit around the Milky Way, it was initially believed that the debris was the remnant of a system dubbed the [[Canis Major Dwarf Galaxy]].<ref>{{Cite journal |last1=Martin |first1=N. F. |last2=Ibata |first2=R. A. |last3=Bellazzini |first3=M. |last4=Irwin |first4=M. J. |last5=Lewis |first5=G. F. |last6=Dehnen |first6=W. |date=2004 |title=A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion on to the Milky Way |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=348 |issue=1 |pages=12–23 |doi=10.1111/j.1365-2966.2004.07331.x|doi-access=free |arxiv=astro-ph/0311010 |bibcode=2004MNRAS.348...12M }}</ref> Other scholars believed it to be due to the Galactic warp,<ref>{{Cite journal |last1=Momany |first1=Y. |last2=Zaggia |first2=S. R. |last3=Bonifacio |first3=P. |last4=Piotto |first4=G. |last5=Angeli |first5=F. De |last6=Bedin |first6=L. R. |last7=Carraro |first7=G. |date=2004-07-01 |title=Probing the Canis Major stellar over-density as due to the Galactic warp |url=https://www.aanda.org/articles/aa/abs/2004/26/aagd202/aagd202.html |journal=Astronomy & Astrophysics |language=en |volume=421 |issue=2 |pages=L29–L32 |doi=10.1051/0004-6361:20040183 |arxiv=astro-ph/0405526 |bibcode=2004A&A...421L..29M |issn=0004-6361|hdl=11577/2467288 |hdl-access=free }}</ref> a view which has been supported by more recent evidence as of 2021.<ref>{{Cite journal |last1=Carballo-Bello |first1=Julio A |last2=Martínez-Delgado |first2=David |last3=Corral-Santana |first3=Jesús M |last4=Alfaro |first4=Emilio J |last5=Navarrete |first5=Camila |last6=Vivas |first6=A Katherina |last7=Catelan |first7=Márcio |date=2020-12-31 |title=A revised view of the Canis Major stellar overdensity with DECam and Gaia : new evidence of a stellar warp of blue stars |url=https://academic.oup.com/mnras/article/501/2/1690/5923573 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=501 |issue=2 |pages=1690–1700 |doi=10.1093/mnras/staa2655 |issn=0035-8711 |doi-access=free|arxiv=2009.01855 }}</ref>
The [[Sloan Digital Sky Survey]] of the northern sky shows a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Milky Way. The proposed likely interpretation is that a [[dwarf galaxy]] is merging with the Milky Way. This galaxy is tentatively named the [[Virgo Stellar Stream]] and is found in the direction of Virgo about {{convert|30000|ly|kpc|0}} away.<ref name=apj673_2_864 />
==== Gaseous halo ====
In addition to the stellar halo, the [[Chandra X-ray Observatory]], [[XMM-Newton]], and [[Suzaku (satellite)|''Suzaku'']] have provided evidence that there is also a gaseous halo containing a large amount of hot gas. This halo extends for hundreds of thousands of light-years, much farther than the stellar halo and close to the distance of the Large and Small [[Magellanic Clouds]]. The mass of this hot halo is nearly equivalent to the mass of the Milky Way itself.<ref name="chandra_hotgas">{{Cite web |last=Boen |first=Brooke |title=NASA's Chandra Shows Milky Way is Surrounded by Halo of Hot Gas |url=http://www.nasa.gov/mission_pages/chandra/news/H-12-331.html |url-status=live |archive-url=https://web.archive.org/web/20121023023929/http://www.nasa.gov/mission_pages/chandra/news/H-12-331.html |archive-date=October 23, 2012 |access-date=October 28, 2012 |publisher=Brooke Boen}}</ref><ref>{{Cite journal |last1=Gupta |first1=A. |last2=Mathur |first2=S. |last3=Krongold |first3=Y. |last4=Nicastro |first4=F. |last5=Galeazzi |first5=M. |year=2012 |title=A Huge Reservoir of Ionized Gas Around the Milky Way: Accounting for the Missing Mass? |journal=The Astrophysical Journal |volume=756 |issue=1 |page=L8 |arxiv=1205.5037 |bibcode=2012ApJ...756L...8G |doi=10.1088/2041-8205/756/1/L8 |s2cid=118567708}}</ref><ref>{{Cite web |date=September 24, 2012 |title=Galactic Halo: Milky Way is Surrounded by Huge Halo of Hot Gas |url=http://chandra.si.edu/photo/2012/halo/ |url-status=live |archive-url=https://web.archive.org/web/20121029155611/http://chandra.si.edu/photo/2012/halo/ |archive-date=October 29, 2012 |website=Smithsonian Astrophysical Observatory}}</ref> The temperature of this halo gas is between 1 and 2.5 million K (1.8 and 4.5 million °F).<ref>{{Cite web |last=Communications |first=Discovery |title=Our Galaxy Swims Inside a Giant Pool of Hot Gas |work=Discovery News |url=http://news.discovery.com/space/massive-pocket-of-hot-gas-envelopes-milky-way-120924.html |url-status=live |archive-url=https://web.archive.org/web/20121029004855/http://news.discovery.com/space/massive-pocket-of-hot-gas-envelopes-milky-way-120924.html |archive-date=October 29, 2012 |access-date=October 28, 2012 |publisher=Discovery Communications}}</ref>
Observations of distant galaxies indicate that the Universe had about one-sixth as much [[baryon]]ic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way.<ref name="Harrington_feature" /> If the finding that the mass of the halo is comparable to the mass of the Milky Way is confirmed, it could be the identity of the missing baryons around the Milky Way.<ref name="Harrington_feature">{{Cite news |last1=J.D. Harrington |last2=Janet Anderson |last3=Peter Edmonds |date=September 24, 2012 |title=NASA's Chandra Shows Milky Way is Surrounded by Halo of Hot Gas |work=NASA |url=http://www.nasa.gov/mission_pages/chandra/news/H-12-331.html |url-status=live |archive-url=https://web.archive.org/web/20121023023929/http://www.nasa.gov/mission_pages/chandra/news/H-12-331.html |archive-date=October 23, 2012}}</ref>
=== Galactic rotation ===
[[File:Rotation curve (Milky Way).svg|thumb|400px|[[Galaxy rotation curve]] for the Milky Way – vertical axis is speed of rotation about the galactic center; horizontal axis is distance from the galactic center in kpcs; the sun is marked with a yellow ball; the observed curve of speed of rotation is blue; the predicted curve based upon stellar mass and gas in the Milky Way is red; scatter in observations roughly indicated by gray bars, the difference is due to dark matter.<ref name=Koupelis_Kuhn2007 /><ref name="Jones1">{{Cite book |last1=Jones |first1=Mark H. |url=https://books.google.com/books?id=36K1PfetZegC&q=Milky+Way+%22rotation+curve%22&pg=PA20 |title=An Introduction to Galaxies and Cosmology |last2=Lambourne |first2=Robert J. |last3=Adams |first3=David John |date=2004 |publisher=Cambridge University Press |isbn=978-0-521-54623-2 |page=21; Fig. 1.13 |access-date=October 27, 2020 |archive-url=https://web.archive.org/web/20230326143523/https://books.google.com/books?id=36K1PfetZegC&q=Milky+Way+%22rotation+curve%22&pg=PA20 |archive-date=March 26, 2023 |url-status=live}}</ref> Note that close to the center the speed is actually proportional to the distance, going to zero at the center, as shown in a reference.<ref name="Schneider">{{Cite book |last=Peter Schneider |url=https://books.google.com/books?id=uP1Hz-6sHaMC&pg=PA5 |title=Extragalactic Astronomy and Cosmology |date=2006 |publisher=Springer |isbn=978-3-540-33174-2 |at=page 4, Fig. 1.4 |access-date=October 27, 2020 |archive-url=https://web.archive.org/web/20230326143010/https://www.google.com/books/edition/Extragalactic_Astronomy_and_Cosmology/uP1Hz-6sHaMC?hl=en&gbpv=1&bsq=rotation+Milky+way&pg=PA100&printsec=frontcover |archive-date=March 26, 2023 |url-status=live}}</ref>]]
The stars and gas in the Milky Way rotate about its center [[differential rotation|differentially]], meaning that the rotation period varies with ___location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 200 and 220 km/s.<ref>{{Cite journal |last1=Camarillo |first1=Tia |last2=Dredger |first2=Pauline |last3=Ratra |first3=Bharat |date=May 4, 2018 |title=Median Statistics Estimate of the Galactic Rotational Velocity |journal=Astrophysics and Space Science |volume=363 |issue=12 |page=268 |arxiv=1805.01917 |bibcode=2018Ap&SS.363..268C |doi=10.1007/s10509-018-3486-8 |s2cid=55697732}}</ref> Hence the [[orbital period]] of the typical star is approximately proportional to the length of the path traveled. This is unlike the situation in the Solar System, where two-body gravitational dynamics dominate, and different orbits have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation.
If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotational speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter.<ref name=Koupelis_Kuhn2007 /> The rotation curve of the Milky Way agrees with the [[universal rotation curve]] of spiral galaxies, the best evidence for the existence of [[dark matter]] in galaxies. Alternatively, a minority of astronomers propose that a [[MOND|modification of the law of gravity]] may explain the observed rotation curve.<ref name="Schneider_p413">{{Cite book |last=Peter Schneider |url=https://books.google.com/books?id=uP1Hz-6sHaMC&q=rotation+Milky+way&pg=PA100 |title=Extragalactic Astronomy and Cosmology |date=2006 |publisher=Springer |isbn=978-3-540-33174-2 |page=413 |access-date=October 27, 2020 |archive-url=https://web.archive.org/web/20230326143010/https://www.google.com/books/edition/Extragalactic_Astronomy_and_Cosmology/uP1Hz-6sHaMC?hl=en&gbpv=1&bsq=rotation+Milky+way&pg=PA100&printsec=frontcover |archive-date=March 26, 2023 |url-status=live}}</ref>
== Formation ==
{{Main|Galaxy formation and evolution}}
<!-- Need to say where the dark matter halo came from. -->
=== History ===
[[File:Galaxy color-magnitude diagram-en.svg|thumb|224x224px|A [[galaxy color–magnitude diagram]] showing the red sequence (old galaxies, typically elliptical galaxies), the green valley (where the Milky Way is believed to be in), and the blue cloud (young galaxies, typically spiral galaxies).]]
The Milky Way began as one or several small overdensities in the mass distribution in the [[Universe]] shortly after the [[Big Bang]] 13.61 billion years ago.<ref name="PHYS-20170727">{{cite web |author=<!--Not stated--> |title=Milky Way's origins are not what they seem |url=https://phys.org/news/2017-07-milky.html |date=July 27, 2017 |work=[[Phys.org]] |access-date=July 27, 2017 |url-status=live |archive-url=https://web.archive.org/web/20170727094803/https://phys.org/news/2017-07-milky.html |archive-date=July 27, 2017}}</ref><ref name="borah">{{Cite journal |last1=Borah |first1=Debasish |last2=Dutta |first2=Manoranjan |last3=Mahapatra |first3=Satyabrata |last4=Sahu |first4=Narendra |year=2022 |title=Boosted self-interacting dark matter and XENON1T excess |journal=Nuclear Physics B |volume=979 |page=115787 |arxiv=2107.13176 |bibcode=2022NuPhB.97915787B |doi=10.1016/j.nuclphysb.2022.115787 |s2cid=236469147}}</ref><ref name="legassick">{{Cite arXiv |eprint=1509.02832 |class=astro-ph.GA |first=Daniel |last=Legassick |title=The Age Distribution of Potential Intelligent Life in the Milky Way |year=2015}}</ref> Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. Nearly half the matter in the Milky Way may have come from other distant galaxies.<ref name="PHYS-20170727" /> These stars and clusters now comprise the stellar halo of the Milky Way. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to [[conservation of angular momentum]], this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk.<ref name=ut20090527 /><ref name="Buser" />
Since the first stars began to form, the Milky Way has grown through both [[galaxy merger]]s (particularly early in the Milky Way's growth) and accretion of gas directly from the Galactic halo.<ref name="Buser" /> The Milky Way is currently accreting material from several small galaxies, including two of its largest satellite galaxies, the [[Large Magellanic Cloud|Large]] and [[Small Magellanic Cloud|Small]] Magellanic Clouds, through the [[Magellanic Stream]]. Direct accretion of gas is observed in [[high-velocity cloud]]s like the [[Smith Cloud]].<ref name=araa35_217 /><ref name=apj679_L21 />
Cosmological simulations indicate that, 11 billion years ago, it merged with a particularly large galaxy that has been labeled the [[Kraken galaxy|Kraken]].<ref>{{Cite journal |last1=Kruijssen |first1=J M Diederik |last2=Pfeffer |first2=Joel L |last3=Chevance |first3=Mélanie |last4=Bonaca |first4=Ana |last5=Trujillo-Gomez |first5=Sebastian |last6=Bastian |first6=Nate |last7=Reina-Campos |first7=Marta |last8=Crain |first8=Robert A |last9=Hughes |first9=Meghan E |date=October 2020 |title=Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations |url=https://academic.oup.com/mnras/article-abstract/498/2/2472/5893320 |url-status=live |journal=Monthly Notices of the Royal Astronomical Society |volume=498 |issue=2 |pages=2472–2491 |arxiv=2003.01119 |doi=10.1093/mnras/staa2452 |doi-access=free |archive-url=https://web.archive.org/web/20201116101953/https://academic.oup.com/mnras/article-abstract/498/2/2472/5893320 |archive-date=November 16, 2020 |access-date=November 15, 2020}}</ref><ref>{{Cite web |last=Young |first=Monica |date=November 13, 2020 |title=Star Clusters reveal the "Kraken" in the Milky Way's Past |url=https://skyandtelescope.org/astronomy-news/stellar-fossils-reveal-the-kraken-in-the-milky-ways-past/ |url-status=live |archive-url=https://web.archive.org/web/20201115190620/https://skyandtelescope.org/astronomy-news/stellar-fossils-reveal-the-kraken-in-the-milky-ways-past/ |archive-date=November 15, 2020 |access-date=November 15, 2020 |website=Sky and Telescope}}</ref> Properties of the Milky Way such as stellar mass, [[angular momentum]], and [[metallicity]] in its outermost regions suggest it has undergone no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies. Its neighbour the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies.<ref>{{Cite journal |last1=Yin, J. |last2=Hou |first2=J.L |last3=Prantzos |first3=N. |last4=Boissier |first4=S. |last5=Chang |first5=R. X. |last6=Shen |first6=S. Y. |last7=Zhang |first7=B. |display-authors=4 |date=2009 |title=Milky Way versus Andromeda: a tale of two disks |journal=Astronomy and Astrophysics |volume=505 |issue=2 |pages=497–508 |arxiv=0906.4821 |bibcode=2009A&A...505..497Y |doi=10.1051/0004-6361/200912316 |s2cid=14344453}}</ref><ref>{{Cite journal |last1=Hammer, F. |last2=Puech |first2=M. |last3=Chemin |first3=L. |last4=Flores |first4=H. |last5=Lehnert |first5=M. D. |display-authors=4 |date=2007 |title=The Milky Way, an Exceptionally Quiet Galaxy: Implications for the Formation of Spiral Galaxies |journal=The Astrophysical Journal |volume=662 |issue=1 |pages=322–334 |arxiv=astro-ph/0702585 |bibcode=2007ApJ...662..322H |doi=10.1086/516727 |s2cid=18002823}}</ref>
According to recent studies, the Milky Way as well as the Andromeda Galaxy lie in what in the [[galaxy color–magnitude diagram]] is known as the "green valley", a region populated by galaxies in transition from the "blue cloud" (galaxies actively forming new stars) to the "red sequence" (galaxies that lack star formation). Star-formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both the Milky Way and the Andromeda Galaxy.<ref>{{Cite journal |last1=Mutch, S.J. |last2=Croton |first2=D.J. |last3=Poole |first3=G.B. |date=2011 |title=The Mid-life Crisis of the Milky Way and M31 |journal=The Astrophysical Journal |volume=736 |issue=2 |page=84 |arxiv=1105.2564 |bibcode=2011ApJ...736...84M |doi=10.1088/0004-637X/736/2/84 |s2cid=119280671}}</ref> Measurements of other galaxies similar to the Milky Way suggest it is among the reddest and brightest spiral galaxies that are still forming new stars and it is just slightly bluer than the bluest red sequence galaxies.<ref>{{Cite journal |last1=Licquia, T. |last2=Newman |first2=J.A. |last3=Poole |first3=G.B. |date=2012 |title=What Is The Color Of The Milky Way? |journal=American Astronomical Society |volume=219 |pages=252.08 |bibcode=2012AAS...21925208L}}</ref>
=== Age and cosmological history ===
[[File:Early Milky Way.jpg|thumb|upright=1.3|Comparison of the night sky with the night sky of a hypothetical planet within the Milky Way 10 billion years ago, at an age of about 3.6 billion years and 5 billion years before the Sun formed.<ref>{{Cite web |title=A firestorm of star birth (artist's illustration) |url=http://www.spacetelescope.org/images/opo1511a/ |url-status=live |archive-url=https://web.archive.org/web/20150413031322/http://www.spacetelescope.org/images/opo1511a/ |archive-date=April 13, 2015 |access-date=April 14, 2015 |website=www.spacetelescope.org |publisher=ESA/Hubble}}</ref>]]
Globular clusters are among the oldest objects in the Milky Way, which thus set a lower limit on the age of the Milky Way. The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived [[radioactive element]]s such as [[thorium-232]] and [[uranium-238]], then comparing the results to estimates of their original abundance, a technique called [[nucleocosmochronology]]. These yield values of about {{nowrap|12.5 ± 3 billion years}} for [[Cayrel's Star|CS 31082-001]]<ref>{{Cite journal |last=Cayrel |display-authors=etal |date=2001 |title=Measurement of stellar age from uranium decay |journal=Nature |volume=409 |issue=6821 |pages=691–692 |arxiv=astro-ph/0104357 |bibcode=2001Natur.409..691C |doi=10.1038/35055507 |pmid=11217852 |s2cid=17251766}}</ref> and {{nowrap|13.8 ± 4 billion years}} for [[BD +17° 3248]].<ref>{{Cite journal |last1=Cowan |first1=J. J. |last2=Sneden |first2=C. |last3=Burles |first3=S. |last4=Ivans |first4=I. I. |last5=Beers |first5=T. C. |last6=Truran |first6=J. W. |last7=Lawler |first7=J. E. |last8=Primas |first8=F. |author8-link=Francesca Primas|last9=Fuller |first9=G. M. |last10=Pfeiffer |first10=B. |last11=Kratz |first11=K. L. |display-authors=9 |year=2002 |title=The Chemical Composition and Age of the Metal-poor Halo Star BD +17o3248 |journal=The Astrophysical Journal |volume=572 |issue=2 |pages=861–879 |arxiv=astro-ph/0202429 |bibcode=2002ApJ...572..861C |doi=10.1086/340347 |s2cid=119503888}}</ref>
Once a [[white dwarf]] is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperatures, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as {{nowrap|12.7 ± 0.7 billion years}}. Age estimates of the oldest of these clusters give a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.<ref name=science299_5603_65 />
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]], has been estimated at either 13.7 ± 0.7 billion years, 12.2 ± 0.6 billion years,<ref name=creevey>{{cite journal |bibcode=2015A&A...575A..26C |title=Benchmark stars for Gaia Fundamental properties of the Population II star HD 140283 from interferometric, spectroscopic, and photometric data |journal=Astronomy and Astrophysics |volume=575 |pages=A26 |last1=Creevey|first1=O. L. |last2=Thévenin |first2=F. |last3=Berio |first3=P. |last4=Heiter |first4=U. |last5=von Braun |first5=K. |last6=Mourard |first6=D. |last7=Bigot |first7=L. |last8=Boyajian |first8=T.S. |last9=Kervella |first9=P. |last10=Morel |first10=P. |last11=Pichon |first11=B. |last12=Chiavassa |first12=A. |last13=Nardetto |first13=N. |last14=Perraut |first14=K. |last15=Meilland |first15=A. |last16=Mc Alister |first16=H. A. |last17=Ten Brummelaar |first17=T.A. |last18=Farrington |first18=C. |last19=Sturmann |first19=J. |last20=Sturmann |first20=L. |last21=Turner |first21=N. |year=2015 |arxiv=1410.4780 |doi=10.1051/0004-6361/201424310|s2cid=18003446 }}</ref> or 12.0 ± 0.5 billion years.<ref name=Tang>{{cite journal|journal= [[Research Notes of the AAS]]|volume= 5|issue=5|id=117|year= 2021|title= Revised Best Estimates for the Age and Mass of the Methuselah Star HD 140283 Using MESA and Interferometry and Implications for 1D Convection|author1= Jiangling Tang|author2= Meredith Joyce|page= 117|doi=10.3847/2515-5172/ac01ca|bibcode=2021RNAAS...5..117T|arxiv=2105.11311|s2cid= 235166094|doi-access= free}}</ref>
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://livescience.com/65059-milky-way-bulge-hides-old-stars.html |url-status=live |archive-url=https://web.archive.org/web/20190323223957/https://livescience.com/65059-milky-way-bulge-hides-old-stars.html |archive-date=March 23, 2019 |access-date=March 24, 2019 |website=Live Science}}</ref>
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>
The satellite galaxies surrounding the Milky Way are not randomly distributed but seem to be the result of a breakup of some larger system producing a ring structure 500,000 light-years in diameter and 50,000 light-years wide.<ref>{{Cite journal |last=Lynden-Bell |first=D. |date=March 1, 1976 |title=Dwarf Galaxies and Globular Clusters in High Velocity Hydrogen Streams |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=174 |issue=3 |pages=695–710 |bibcode=1976MNRAS.174..695L |doi=10.1093/mnras/174.3.695 |issn=0035-8711 |doi-access=free}}</ref> Close encounters between galaxies, like that expected in 4 billion years with the Andromeda Galaxy, can rip off huge tails of gas, which, over time can coalesce to form dwarf galaxies in a ring at an arbitrary angle to the main disc.<ref>{{Cite journal |last1=Kroupa |first1=P. |last2=Theis |first2=C. |last3=Boily |first3=C. M. |year=2005 |title=The great disk of Milky-Way satellites and cosmological sub-structures |journal=Astronomy and Astrophysics |volume=431 |issue=2 |pages=517–521 |arxiv=astro-ph/0410421 |bibcode=2005A&A...431..517K |doi=10.1051/0004-6361:20041122 |doi-access=free}}</ref>
== Intergalactic neighbourhood ==
{{multiple image
| align = right
| direction = vertical
| width = 220
| image1 = 06-Local Group (LofE06240).png
| alt1 =
| caption1 = A diagram of the galaxies in the [[Local Group]] relative to the Milky Way
| image2 = 07-Laniakea (LofE07240).png
| alt2 =
| caption2 = The position of the Local Group within the [[Laniakea Supercluster]]
}}
{{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/>
The smallest dwarf galaxies of the Milky Way are only 500 light-years in diameter. These include [[Carina Dwarf]], [[Draco Dwarf]], and [[Leo II (dwarf galaxy)|Leo II Dwarf]]. There may still be undetected dwarf galaxies that are dynamically bound to the Milky Way, which is supported by the detection of nine new satellites of the Milky Way in a relatively small patch of the night sky in 2015.<ref name=Koposov2015>{{cite journal |title=Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds |author1=Sergey E. Koposov |author2=Vasily Belokurov |author3=Gabriel Torrealba |author4=N. Wyn Evans |date=March 10, 2015 |arxiv=1503.02079 |journal=The Astrophysical Journal |bibcode=2015ApJ...805..130K |doi=10.1088/0004-637X/805/2/130 |volume=805 |issue=2 |page=130|s2cid=118267222 }}</ref> There are some dwarf galaxies that have already been absorbed by the Milky Way, such as the progenitor of [[Omega Centauri]].<ref>{{cite journal |last1=Noyola |first1=E. |last2=Gebhardt |first2=K. |last3=Bergmann |first3=M. |title=Gemini and Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in ω Centauri |journal=The Astrophysical Journal |arxiv=0801.2782 |date=April 2008 |volume=676 |issue=2 |pages=1008–1015 |doi=10.1086/529002 |bibcode=2008ApJ...676.1008N|s2cid=208867075 }}</ref>
In 2005<ref>{{cite journal |last1=Kroupa |first1=P. |last2=Theis |first2=C. |last3=Boily |first3=C.M. |title=The great disk of Milky-Way satellites and cosmological sub-structures
|journal=Astronomy and Astrophysics |arxiv=astro-ph/0410421 |date=February 2005 |volume=431 |issue=2 |pages=517–521 |doi=10.1051/0004-6361:20041122 |bibcode=2005A&A...431..517K |s2cid=55827105 }}</ref>
with further confirmation in 2012<ref>{{cite journal |last1=Pawlowski |first1=M. |last2=Pflamm-Altenburg |first2=J. |last3=Kroupa |first3=P. |title=The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way |journal=Monthly Notices of the Royal Astronomical Society |arxiv=1204.5176 |date=June 2012 |volume=423 |issue=2 |pages=1109–1126 |doi=10.1111/j.1365-2966.2012.20937.x |doi-access=free |bibcode=2012MNRAS.423.1109P|s2cid=55501752 }}</ref> researchers reported that most satellite galaxies of the Milky Way lie in a very large disk and orbit in the same direction. This came as a surprise: according to standard cosmology, satellite galaxies should form in dark matter halos, and they should be widely distributed and moving in random directions. This discrepancy is still not explained.<ref>{{cite journal |last1=Pawlowski |first1=M. |last2=Famaey |first2=B. |last3=Jerjen |first3=H.|last4=Merritt |first4=D. |last5=Kroupa |first5=P. |last6=Dabringhausen |first6=J. |last7=Lueghausen |first7=F. |last8=Forbes |first8=D. |last9=Hensler |first9=G.|last10=Hammer |first10=F. |last11=Puech |first11=M.|last12=Fouquet |first12=S.|last13=Flores |first13=H.|last14=Yang |first14=Y. |title=Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies |journal=Monthly Notices of the Royal Astronomical Society |arxiv=1406.1799 |date=August 2014 |volume=423 |issue=3 |pages=2362–2380 |doi=10.1093/mnras/stu1005 |doi-access=free |bibcode=2014MNRAS.442.2362P }}</ref>
In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they orbit the Milky Way, causing vibrations when they pass through its edges. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, in a computer model, the movement of these two galaxies creates a dark matter wake that amplifies their influence on the larger Milky Way.<ref name=berkeley20060109 />
Current measurements suggest the Andromeda Galaxy is approaching the Milky Way at {{convert|100|to|140|km/s|mph|abbr=on|sigfig=2}}. In 4.3 billion years, there may be an [[Andromeda–Milky Way collision]], depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, the chance of individual [[Stellar collision|stars colliding]] with each other is extremely low,<ref name=wong20000414 /> but instead the two galaxies will merge to form a single [[elliptical galaxy]] or perhaps a large [[disk galaxy]]<ref name="Ueda2014">{{cite journal |author=Junko Ueda |display-authors=etal |title=Cold molecular gas in merger remnants. I. Formation of molecular gas disks |journal=The Astrophysical Journal Supplement Series |volume=214 |issue=1 |page=1 |bibcode=2014ApJS..214....1U |doi=10.1088/0067-0049/214/1/1 |arxiv=1407.6873 |year=2014|s2cid=716993 }}</ref> over the course of about six billion years.<ref name="Schiavi">{{cite journal|last1=Schiavi|first1=Riccardo|last2=Capuzzo-Dolcetta|first2=Roberto|last3=Arca-Sedda|first3=Manuel|last4=Spera|first4=Mario|title=Future merger of the Milky Way with the Andromeda galaxy and the fate of their supermassive black holes|journal=Astronomy & Astrophysics|date=October 2020|volume=642|pages=A30|doi=10.1051/0004-6361/202038674|bibcode=2020A&A...642A..30S|arxiv=2102.10938|s2cid=224991193}}</ref>
== Velocity ==
Although [[special relativity]] states that there is no "preferred" [[inertial frame of reference]] in space with which to compare the Milky Way, the Milky Way does have a velocity with respect to cosmological [[frames of reference]].<ref>{{Cite web |date=July 21, 2017 |title=Ned Wright's Cosmology Tutorial pt. 1 |url=https://www.astro.ucla.edu/~wright/cosmo_01.htm |access-date=May 19, 2025 |website=www.astro.ucla.edu}}</ref>
One such frame of reference is the [[Hubble flow]], the apparent motions of galaxy clusters due to the [[expansion of space]]. Individual galaxies, including the Milky Way, have [[peculiar velocities]] relative to the average flow. Thus, to compare the Milky Way to the Hubble flow, one must consider a volume large enough so that the expansion of the Universe dominates over local, random motions. A large enough volume means that the mean motion of galaxies within this volume is equal to the Hubble flow. Astronomers believe the Milky Way is moving at approximately {{convert|630|km/s|mph|abbr=on|sigfig=2}} with respect to this local co-moving frame of reference.<ref>{{cite web |title=The Velocity of Our Galaxy: the End of a 40-Year Mystery |url=https://www.cea.fr/drf/english/Pages/News/Scientific-results/2017/velocity-of-our-galaxy-the-end-of-a-40-year-mystery.aspx |website=CEA/The Knowledge Factory |access-date=5 May 2022 |language=en |date=31 January 2017 |archive-date=June 2, 2022 |archive-url=https://web.archive.org/web/20220602152114/https://www.cea.fr/drf/english/Pages/News/Scientific-results/2017/velocity-of-our-galaxy-the-end-of-a-40-year-mystery.aspx |url-status=live }}</ref><ref>{{cite magazine |title=The Milky Way is being pushed through space by a void called the Dipole Repeller |url=https://www.wired.co.uk/article/dipole-repeller-milky-way |magazine=Wired UK |access-date=5 May 2022 |archive-date=January 6, 2019 |archive-url=https://web.archive.org/web/20190106104313/https://www.wired.co.uk/article/dipole-repeller-milky-way |url-status=live }}</ref>
The Milky Way is moving in the general direction of the [[Great Attractor]] and other [[galaxy cluster]]s, including the [[Shapley Supercluster]], behind it.<ref>{{Cite journal |last1=Kocevski |first1=D. D. |last2=Ebeling |first2=H. |doi=10.1086/503666 |title=On the origin of the Local Group's peculiar velocity |journal=The Astrophysical Journal |volume=645 |issue=2 |pages=1043–1053 |year=2006 |arxiv=astro-ph/0510106 |bibcode=2006ApJ...645.1043K |s2cid=2760455 }}</ref> The Local Group, a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda Galaxy, is part of a [[supercluster]] called the [[Local Supercluster]], centered near the [[Virgo Cluster]]: although they are moving away from each other at {{convert|967|km/s|mph|abbr=on}} as part of the Hubble flow, this velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.<ref>{{cite journal |doi=10.1016/j.newast.2005.08.008 |title=Mass determination of groups of galaxies: Effects of the cosmological constant |date=2006 |author=Peirani, S |journal=New Astronomy |volume=11 |issue=4 |pages=325–330 |last2=Defreitaspacheco |first2=J |arxiv=astro-ph/0508614 |bibcode=2006NewA...11..325P|s2cid=685068 }}</ref>
{{anchor|cmb}}Another reference frame is provided by the [[cosmic microwave background]] (CMB), in which the CMB temperature is least distorted by Doppler shift (zero dipole moment). The Milky Way is moving at {{nowrap|{{convert|552|±|6| km/s|mph|abbr=on}}}}<ref name="COBE1993" /> with respect to this frame, toward 10.5 right ascension, −24° declination ([[J2000]] epoch, near the center of [[Hydra (constellation)|Hydra]]). This motion is observed by satellites such as the [[Cosmic Background Explorer]] (COBE) and the [[Wilkinson Microwave Anisotropy Probe]] (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get [[Doppler effect|blue-shifted]] in the direction of the motion and [[redshift|red-shifted]] in the opposite direction.<ref name="COBE1993" />
== See also ==
{{portal|solar system|outer space|astronomy}}
* [[Baade's Window]]
* [[Galactic astronomy]]
* [[Galactic Center GeV excess]]
* [[Oort constants]]
== Notes ==
{{notelist|refs=
{{efn|name=distance|1=The distance towards its [[Galactic Center|center]] ([[Sagittarius A*]]).}}
{{efn|name=bortle|1=See also [[Bortle Dark-Sky Scale]].}}
{{efn|name=const|1=The bright center of the galaxy is located in the constellation [[Sagittarius (constellation)|Sagittarius]]. From Sagittarius, the hazy band of white light appears to pass westward through the constellations of [[Scorpius]], [[Ara (constellation)|Ara]], [[Norma (constellation)|Norma]], [[Triangulum Australe]], [[Circinus]], [[Centaurus]], [[Musca]], [[Crux]], [[Carina (constellation)|Carina]], [[Vela (constellation)|Vela]], [[Puppis]], [[Canis Major]], [[Monoceros]], [[Orion (constellation)|Orion]] and [[Gemini (constellation)|Gemini]], [[Taurus (constellation)|Taurus]], to the [[galactic anticenter]] in [[Auriga (constellation)|Auriga]]. From there, it passes through [[Perseus (constellation)|Perseus]], [[Andromeda (constellation)|Andromeda]], [[Cassiopeia (constellation)|Cassiopeia]], [[Cepheus (constellation)|Cepheus]] and [[Lacerta]], [[Cygnus (constellation)|Cygnus]], [[Vulpecula]], [[Sagitta]], [[Aquila (constellation)|Aquila]], [[Ophiuchus]], [[Scutum (constellation)|Scutum]], and back to [[Sagittarius (constellation)|Sagittarius]].}}
{{efn|name=chandra2003|1=For a photo see: {{cite web |title=Sagittarius A*: Milky Way monster stars in cosmic reality show |work=Chandra X-ray Observatory |publisher=Center for Astrophysics {{!}} Harvard & Smithsonian |date=January 6, 2003 |url=http://chandra.harvard.edu/photo/2003/0203long/ |access-date=May 20, 2012 |url-status=live |archive-url=https://web.archive.org/web/20080317061809/http://chandra.harvard.edu/photo/2003/0203long/ |archive-date=March 17, 2008}} }}
{{efn|name=milky way mag|1=Karachentsev et al. give a ''blue'' absolute magnitude of −20.8. Combined with a [[color index]] of 0.55 estimated [http://adsabs.harvard.edu/abs/2014MNRAS.440..405M here], an absolute visual magnitude of −21.35 (−20.8 − 0.55 = −21.35) is obtained. Determining the absolute magnitude of the Milky Way is very difficult, because Earth is inside it.}}
}}
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<ref name=berkeley20060109>{{cite press release |title=Milky Way Galaxy is warped and vibrating like a drum |publisher=[[University of California, Berkeley]] |date=January 9, 2006 |url=http://www.berkeley.edu/news/media/releases/2006/01/09_warp.shtml |access-date=October 18, 2007 |archive-url=https://web.archive.org/web/20140716013619/http://berkeley.edu/news/media/releases/2006/01/09_warp.shtml |archive-date=July 16, 2014}}</ref>
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<ref name="our_galaxy">{{cite web |last1=Evans |first1=J. C. |date=November 24, 1998 |url=http://physics.gmu.edu/~jevans/astr103/CourseNotes/ECText/ch20_txt.htm |title=Our Galaxy |publisher=George Mason University |access-date=January 4, 2007 |url-status=live |archive-url=https://archive.today/20120630/http://physics.gmu.edu/~jevans/astr103/CourseNotes/ECText/ch20_txt.htm |archive-date=June 30, 2012}}</ref>
<ref name=mactutor_albirundi>{{MacTutor Biography | id=Al-Biruni | title=Abu Rayhan Muhammad ibn Ahmad al-Biruni }}<!--source supports this comment, but the source is not published--></ref>
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<ref name=rotation_pattern_speeds>{{cite arXiv |last1=Gerhard |first1=O. |title=Pattern speeds in the Milky Way |eprint=1003.2489v1 |year=2010|class=astro-ph.GA }}</ref>
<ref name=Kafle2012>{{Cite journal |first1=P.R. |last1=Kafle |first2=S. |last2=Sharma |first3=G.F. |last3=Lewis |first4=J. |last4=Bland-Hawthorn |title=Kinematics of the Stellar Halo and the Mass Distribution of the Milky Way Using Blue Horizontal Branch Stars |journal=The Astrophysical Journal |volume=761 |issue=2 |date=2012 |page=17 |doi=10.1088/0004-637X/761/2/98 |bibcode=2012ApJ...761...98K |arxiv=1210.7527|s2cid=119303111 }}</ref>
<ref name=Kafle2014>{{Cite journal |first1=P.R. |last1=Kafle |first2=S. |last2=Sharma |first3=G.F. |last3=Lewis |first4=J. |last4=Bland-Hawthorn |title=On the Shoulders of Giants: Properties of the Stellar Halo and the Milky Way Mass Distribution |journal=The Astrophysical Journal |volume=794 |issue=1 |date=2014 |page=17 |doi=10.1088/0004-637X/794/1/59 |bibcode=2014ApJ...794...59K |arxiv=1408.1787|s2cid=119040135 }}</ref>
}}
== Further reading ==
* {{cite journal |last=Dambeck |first=Thorsten |date=March 2008 |title=Gaia's Mission to the Milky Way |journal=[[Sky & Telescope]] |volume=115 |issue=3 |pages=36–39|bibcode=2008S&T...115c..36D }}
* {{cite journal |last=Chiappini |first=Cristina |date=November–December 2001 |title=The Formation and Evolution of the Milky Way |url=http://www.astro.caltech.edu/~george/ay20/Chiappini-MilkyWay.pdf |journal=[[American Scientist]] |volume=89 |issue=6 |pages=506–515 |doi=10.1511/2001.40.745}}
* {{cite book | last=McTier | first=Moiya | title=The Milky Way | publisher = Grand Central Publishing | date=August 16, 2022 | isbn=978-1-5387-5415-3}}
* [[Phil Plait|Plait, Phil]], "The Milky Way's Secrets: Our galaxy's night-sky spectacle sparked [[scientific revolution]]s", ''[[Scientific American]]'', vol. 329, no. 4 (November 2023), pp. 86–87.
== External links ==
{{Commons category|Milky Way Galaxy}}
{{Wikiquote}}
* [http://www.sky-map.org/?ra=12.0593794293245&de=-20.27239516216098&zoom=0&show_grid=1&show_constellation_lines=1&show_constellation_boundaries=1&show_const_names=0&show_galaxies=1&img_source=IRAS Milky Way – IRAS (infrared) survey] – wikisky.org
* [http://www.sky-map.org/?ra=12.0593794293245&de=-20.27239516216098&zoom=0&show_grid=1&show_constellation_lines=1&show_constellation_boundaries=1&show_const_names=0&show_galaxies=1&img_source=HALPHA Milky Way – H-Alpha survey] – wikisky.org
* [https://asd.gsfc.nasa.gov/archive/mwmw/mmw_sci.html Multiwavelength Milky Way] – Images and [[VRML]] models (NASA)
* [http://www.360pano.eu/show/?id=736 Milky Way – Panorama (9 billion pixels)] {{Webarchive|url=https://web.archive.org/web/20170806131326/http://www.360pano.eu/show/?id=736 |date=August 6, 2017 }}.
* [http://messier.seds.org/more/mw.html Milky Way] – SEDS Messier website
* [http://galaxymap.org/drupal/node/127 Milky Way] – Infrared Images
* [https://www.sciencealert.com/feast-your-eyes-on-this-1-250-hour-exposure-of-the-milky-way Milky Way] – Mosaic of galactic plane (March 19, 2021)
* [http://astrojan.nhely.hu/milk.htm The clickable Milky Way]
{{Milky Way}}
{{Earth's ___location}}
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[[Category:Milky Way| ]]
[[Category:Articles containing video clips]]
[[Category:Astronomical objects known since antiquity]]
[[Category:Barred spiral galaxies]]
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