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Milky Way
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{{About|the galaxy}}{{pp-semi-protected|expiry=July 3, 2019|small=yes}}{{short description|spiral galaxy containing our Solar System}}{{Use mdy dates|date=March 2018}}{{good article}}







factoids
| titlestyle=background: #30D5C8;| headerstyle=background:| belowstyle=background: #30D5C8;| labelstyle=background: inherit;| header1=Observation dataGalaxy morphological classification>TypeFIRST2=CHRISTINE DATE=AUGUST 26, 2005 TITLE=CLASSIFICATION OF THE MILKY WAY GALAXY WORK=SEDS ARCHIVEURL=HTTPS://WEB.ARCHIVE.ORG/WEB/20150531031937/HTTP://MESSIER.SEDS.ORG/MORE/MW_TYPE.HTML, May 31, 2015, (barred spiral galaxy)| label3=Diameter150-200kpclk=on}}| label4=Thickness of thin stellar disk2kpclk=off|sigfig=1}}| label6=Number of stars100–400 billion [(1–4)×1011]}}| label7=| data7=| label8=Mass12}} {{solar mass|link=yes}}| label9=Angular momentum1u=J s}}KARACHENTSEV >FIRST1=IGOR URL=HTTPS://NED.IPAC.CALTECH.EDU/LEVEL5/SEPT02/KEEL/KEEL7.HTML PUBLISHER=IZDATEL'STVO NAUKA DEADURL=NO ARCHIVEDATE=MARCH 4, 2016, | label10=Sun's distance to Galactic Center26.41.0kpclk=off}}Galactic Year>Galactic rotation period| data11=240 MyrDensity wave theory>Spiral pattern rotation period| data12=220–360 MyrBarred spiral galaxy>Bar pattern rotation period| data13=100–120 MyrCosmic microwave background>CMB rest frame631 ± 20 km/s}}HTTP://WWW.NATURE.COM/ARTICLES/S41550-016-0036 >TITLE=THE DIPOLE REPELLER WORK=NATURE ASTRONOMY DATE=AUGUST 22, 2016 ARCHIVEURL=HTTPS://WEB.ARCHIVE.ORG/WEB/20170303111934/HTTP://WWW.NATURE.COM/ARTICLES/S41550-016-0036 ARXIV = 1702.02483, 2017NatAs...1E..36H, | below=See also: Galaxy, List of galaxies| label15=Escape velocity at Sun's position| data15=550 km/s| label16=Dark matter density at Sun's position+0.0024 link=yes}}pc{{sup+0.08-3}}}}The Milky Way is the galaxy that contains our Solar System. The descriptor "milky" is derived from the galaxy's appearance from Earth: a band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin , from the Greek ({{transl|grc|galaxías kýklos}}, "milky circle"). From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. 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.WEB, Milky Way Galaxy: Facts About Our Galactic Home, Space.com,weblink April 8, 2017, no,weblink" title="web.archive.org/web/20170321063305weblink">weblink March 21, 2017, Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.The Milky Way is a barred spiral galaxy with a diameter between 150,000 and 200,000 light-years (ly).WEB, M. López-Corredoira, C. Allende Prieto, F. Garzón, H. Wang, C. Liu and L. Deng,weblink Astronomy and Astrophysics, Disk stars in the Milky Way detected beyond 25 kpc from its center, PRESS RELEASE, David Freeman, The Milky Way galaxy may be much bigger than we thought,weblink May 25, 2018, CNBC, PRESS RELEASE, Mary L. Martialay, The Corrugated Galaxy—Milky Way May Be Much Larger Than Previously Estimated,weblink March 11, 2015, Rensselaer Polytechnic Institute,weblink" title="web.archive.org/web/20150313123405weblink">weblink March 13, 2015, WEB,weblink Size of the Milky Way Upgraded, Solving Galaxy Puzzle, Space.com, Hall, Shannon, May 4, 2015, June 9, 2015, no,weblink" title="web.archive.org/web/20150607104254weblink">weblink June 7, 2015, It is estimated to contain 100–400 billion stars.WEB, Milky Way,weblink BBC,weblink" title="web.archive.org/web/20120302071454weblink">weblink March 2, 2012, WEB,weblink How Many Stars in the Milky Way?, NASA Blueshift, no,weblink" title="web.archive.org/web/20160125140109weblink">weblink January 25, 2016, There are probably at least 100 billion planets in the Milky Way.WEB, Staff, 100 Billion Alien Planets Fill Our Milky Way Galaxy: Study,weblink January 2, 2013, Space.com, January 3, 2013,weblink" title="web.archive.org/web/20130103060601weblink">weblink January 3, 2013, The Solar System is located within the disk, 26,490 (± 100) light-years from the Galactic Center, 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 and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, likely a supermassive black hole of 4.100 (± 0.034) million solar masses.Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed "dark matter". The rotational period is about 240 million years at the position of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.WEB, youtube.com,weblink Laniakea: Our home supercluster, no,weblink" title="web.archive.org/web/20140904162040weblink">weblink September 4, 2014, JOURNAL, 1, R. Brent, Tully, Hélène, Courtois, Yehuda, Hoffman, Daniel, Pomarède, September 4, 2014, The Laniakea supercluster of galaxies, 513, 71–73, 10.1038/nature13674, 2014Natur.513...71T, 7516, 1409.0880, Nature, 25186900,

Appearance

File:Milky Way Night Sky Black Rock Desert Nevada.jpg|thumb|A view of the Milky Way toward the constellation Sagittarius (including the Galactic Center), as seen from a dark site with little light pollution (the Black Rock Desert, Nevada). The bright object on the lower right is Jupiter, just above AntaresAntaresFile:Following the Milky Way over ALMA.webm|thumb|This time-lapse video captures the Milky Way arching over ALMA.]]The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching across the night sky. In night sky observing, although all the individual naked-eye stars in the entire sky are part of the Milky Way, the term “Milky Way” is limited to this band of light.BOOK, H. A., Rey, The Stars, 145, 1976, 978-0395248300, Houghton Mifflin Harcourt, BOOK, The Cosmos: Astronomy in the New Millennium, Jay M., Pasachoff, Alex, Filippenko, 2013, Cambridge University Press, 384,weblink no,weblink December 2, 2017, mdy-all, 9781107687561, The light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars. The area of sky that the Milky Way obscures is called the Zone of Avoidance.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 per square arcsecond in order for the Milky Way to be visible.JOURNAL, 10.1093/mnras/stu992, Crumey, Andrew, Human contrast threshold and astronomical visibility, Monthly Notices of the Royal Astronomical Society, 2014, 442, 3, 2600–19, 2014MNRAS.442.2600C, 1405.4209, It should be visible if the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1. This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but very prominent when viewed from rural areas when the Moon is below the horizon. "The new world atlas of artificial night sky brightness" shows that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution.JOURNAL, Falchi, Fabio, Cinzano, Pierantonio, Duriscoe, Dan, Kyba, Christopher C. M., Elvidge, Christopher D., Baugh, Kimberly, Portnov, Boris A., Rybnikova, Nataliya A., Furgoni, Riccardo, June 1, 2016, The new world atlas of artificial night sky brightness,weblink Science Advances, en, 2, 6, e1600377, 10.1126/sciadv.1600377, 2375-2548, 4928945, 27386582, no,weblink" title="web.archive.org/web/20160711121713weblink">weblink July 11, 2016, 1609.01041, 2016SciA....2E0377F, As viewed from Earth, the visible region of the Milky Way's galactic plane occupies an area of the sky that includes 30 constellations.The bright center of the galaxy is located in the constellation Sagittarius. From Sagittarius, the hazy band of white light appears to pass westward through the constellations of Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion and Gemini, Taurus, to the galactic anticenter in Auriga. From there, it passes through Perseus, Andromeda, Cassiopeia, Cepheus and Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius. The Galactic Center lies in the direction of 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. The band then continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two roughly equal hemispheres.The galactic plane is inclined by about 60° to the ecliptic (the plane of Earth's orbit). Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth’s equatorial plane and the plane of the ecliptic, relative to the galactic plane. The north galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near β Comae Berenices, and the south galactic pole is near α Sculptoris. Because of this high inclination, depending on the time of night and year, the arch of the Milky Way may appear relatively low or relatively high in the sky. For observers from latitudes approximately 65° north to 65° south, the Milky Way passes directly overhead twice a day.File:Milky Way Arch.jpg|thumb|center|600px|The Milky Way arching at a high inclination across the night sky. (This composited panorama was taken at Paranal Observatory in northern Chile). The bright object is Jupiter in the constellation Sagittarius, and the Magellanic CloudsMagellanic Clouds

Size and mass

File:UGC 12158.jpg|thumb|right|250px|An HST image of galaxy UGC 12158UGC 12158The Milky Way is the second-largest galaxy in the Local Group, with its stellar disk approximately {{convert|100000|ly|kpc|abbr=on|sigfig=1}} in diameter and, on average, approximately {{convert|1000|ly|kpc|1|abbr=on|sigfig=1}} thick.JOURNAL, Rix, Hans-Walter, Bovy, Jo, The Milky Way's Stellar Disk, The Astronomy and Astrophysics Review, 2013, 21, 1301.3168, 10.1007/s00159-013-0061-8, 2013A&ARv..21...61R, To compare the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter ({{convert|0.955|in|mm|abbr=on|order=flip}}), the Milky Way would be approximately the size of the contiguous United States.WEB,weblink NASA-Smithsonian Education Forum on the Structure and Evolution of the Universe, at the Harvard Smithsonian Center for Astrophysics, March 13, 2013, How Big is Our Universe: How far is it across the Milky Way?, no,weblink" title="web.archive.org/web/20130305005817weblink">weblink March 5, 2013, A ring-like filament of stars wrapping around the Milky Way may belong to the Milky Way itself, rippling above and below the relatively flat galactic plane. If so, that would mean a diameter of {{convert|150,000–180,000|ly|kpc}}.JOURNAL, 1, Newberg, Heidi Jo, Xu, Yan, Carlin, Jeffrey L., Liu, Chao, Deng, Licai, Li, Jing, Schoenrich, Ralph, Yanny, Brian, March 1, 2015, Rings and Radial Waves in the Disk of the Milky Way, The Astrophysical Journal, 801, 2, 10.1088/0004-637X/801/2/105, 1503.00257, 2015ApJ...801..105X, 105, (File:Milky way profile.svg|thumb|left|250px|A schematic profile of the Milky Way)Estimates of the mass of the Milky Way vary, depending upon the method and data used. At the low end of the estimate range, the mass of the Milky Way is 5.8{{e|11}} solar masses ({{solar mass}}), somewhat less than that of the Andromeda Galaxy. 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. 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. In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kiloparsecs is 7{{e|11}} {{solar mass}}. According to a study published in 2014, the mass of the entire Milky Way is estimated to be 8.5{{e|11}} {{solar mass}},JOURNAL
, 1
, Jorge, Peñarrubia
, Yin-Zhe, Ma
, Matthew G., Walker
, Alan, McConnachie
, A dynamical model of the local cosmic expansion
, Monthly Notices of the Royal Astronomical Society
, 433
, 3
, 2204–2222
, 2014MNRAS.443.2204P
, 10.1093/mnras/stu879, 1405.0306, 2014, which is about half the mass of the Andromeda Galaxy.
Much of the mass of the Milky Way appears to be dark matter, an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A dark matter halo is 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}}.JOURNAL, Slobodan Ninković, Mass Distribution and Gravitational Potential of the Milky Way, Open Astronomy, 26, April 2017, 1–6, 10.1515/astro-2017-0002,weblink no,weblink April 22, 2017, 2017OAst...26....1N, Recent studies indicate a range in mass, as large as 4.5{{e|12}} {{solar mass}} JOURNAL, 1, Phelps, Steven, Nusser, Adi, Desjacques, Vincent, The Mass of the Milky Way and M31 Using the Method of Least Action, The Astrophysical Journal, 775, 2, 102, 12, October 2013, 10.1088/0004-637X/775/2/102, 2013ApJ...775..102P, 1306.4013, and as small as 8{{e|11}} {{solar mass}}.JOURNAL, 1, Kafle, Prajwal Raj, Sharma, Sanjib, Lewis, Geraint F., Bland-Hawthorn, Joss, On the Shoulders of Giants: Properties of the Stellar Halo and the Milky Way Mass Distribution, The Astrophysical Journal, 794, 1, 59, 17, October 2014, 10.1088/0004-637X/794/1/59, 2014ApJ...794...59K, 1408.1787, The total mass of all the stars in the Milky Way is estimated to be between 4.6{{e|10}} {{solar mass}}JOURNAL
, Licquia, Timothy, Newman, J.
, Improved Constraints on the Total Stellar Mass, Color, and Luminosity of the Milky Way
, American Astronomical Society, AAS Meeting #221, #254.11
, 2013
, 2013AAS...22125411L
, and 6.43{{e|10}} {{solar mass}}. In addition to the stars, there is also interstellar gas, comprising 90% hydrogen and 10% helium by mass, with two thirds of the hydrogen found in the atomic form and the remaining one-third as molecular hydrogen. The mass of this gas is equal to between 10%WEB, Lecture Seven: The Milky Way: Gas,weblink May 2, 2015, no,weblink" title="web.archive.org/web/20150708011238weblink">weblink July 8, 2015, and 15%WEB, The Interstellar Medium,weblink May 2, 2015, no,weblink" title="web.archive.org/web/20150419065745weblink">weblink April 19, 2015, of the total mass of the galaxy's stars. Interstellar dust accounts for an additional 1% of the total mass of the gas.

Contents

{{Further|Exoplanet}}The Milky Way contains between 200 and 400 billion stars and at least 100 billion planets. The exact figure depends on the number of very-low-mass stars, which are hard 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 (1012) stars. The Milky Way may also contain perhaps ten billion white dwarfs, a billion neutron stars, and a hundred million black holes.These estimates are very uncertain, as most non-star objects are hard to detect; for example, black hole estimates range from ten million to one billion.Napiwotzki, R. (2009). The galactic population of white dwarfs. In Journal of Physics: Conference Series (Vol. 172, No. 1, p. 012004). IOP Publishing.WEB, NASA – Neutron Stars,weblink NASA, 5 April 2018, en, WEB, Black Holes {{!, Science Mission Directorate|url=https://science.nasa.gov/astrophysics/focus-areas/black-holes|website=NASA|accessdate=5 April 2018|language=en}} 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, whereas the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas.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. For reasons that are not understood, beyond a radius of roughly 40,000 ly (13 kpc) from the center, the number of stars per cubic parsec drops much faster with radius. Surrounding the galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward but is limited in size by the orbits of two Milky Way satellites, the Large and Small Magellanic Clouds, whose closest approach to the Galactic Center is about {{convert|180000|ly|kpc|abbr=on}}. 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.JOURNAL
, Karachentsev, Igor D.
, Karachentseva, Valentina E.
, Huchtmeier, Walter K.
, Makarov, Dmitry I.
, A Catalog of Neighboring Galaxies
, The Astronomical Journal
, 2003
, 127
, 4
, 2031–2068
, 2004AJ....127.2031K
, 10.1086/382905, {{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, and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.JOURNAL, 1, Sumi, T., Kamiya, K., Bennett, D. P., Bond, I. A., Abe, F., Botzler, C. S., Fukui, A., Furusawa, K., Hearnshaw, J. B., Itow, Y., Kilmartin, P. M., Korpela, A., Lin, W., Ling, C. H., Masuda, K., Matsubara, Y., Miyake, N., Motomura, M., Muraki, Y., Nagaya, M., Nakamura, S., Ohnishi, K., Okumura, T., Perrott, Y. C., Rattenbury, N., Saito, To., Sako, T., Sullivan, D. J., Sweatman, W. L., Tristram, P. J., Udalski, A., Szymański, M. K., Kubiak, M., Pietrzyński, G., Poleski, R., Soszyński, I., Wyrzykowski, Ł., Ulaczyk, K., 10.1038/nature10092, Unbound or distant planetary mass population detected by gravitational microlensing, Nature, 473, 7347, 349–352, 2011, 2011Natur.473..349S, 1105.3544, 21593867, The Milky Way contains 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 with the Kepler space observatory. A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in the Milky Way.WEB, Staff, 17 Billion Earth-Size Alien Planets Inhabit Milky Way,weblink January 7, 2013, Space.com, January 8, 2013,weblink" title="web.archive.org/web/20141006095334weblink">weblink October 6, 2014, On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way.NEWS, Overbye, Dennis, Far-Off Planets Like the Earth Dot the Galaxy,weblink November 4, 2013, New York Times, November 5, 2013, no,weblink" title="web.archive.org/web/20131105023653weblink">weblink November 5, 2013, JOURNAL, Petigura, Eric A., Howard, Andrew W., Marcy, Geoffrey W., Prevalence of Earth-size planets orbiting Sun-like stars,weblink October 31, 2013, Proceedings of the National Academy of Sciences of the United States of America, 10.1073/pnas.1319909110, November 5, 2013, 1311.6806, 2013PNAS..11019273P, 110, 48, 19273–19278, 24191033, 3845182, no,weblink" title="web.archive.org/web/20131109010831weblink">weblink November 9, 2013, 11 billion of these estimated planets may be orbiting Sun-like stars.NEWS, Khan, Amina, Milky Way may host billions of Earth-size planets,weblink November 4, 2013, Los Angeles Times, November 5, 2013, no,weblink" title="web.archive.org/web/20131106030558weblink">weblink November 6, 2013, The nearest such planet may be 4.2 light-years away, according to a 2016 study.JOURNAL, Anglada-Escudé, Guillem, et. al., A terrestrial planet candidate in a temperate orbit around Proxima Centauri, 2016, Nature, 536, 7617, 437–440, 10.1038/nature19106, 27558064, 1609.03449, 2016Natur.536..437A, Such Earth-sized planets may be more numerous than gas giants. Besides exoplanets, "exocomets", comets beyond the Solar System, have also been detected and may be common in the Milky Way.WEB, Staff, 'Exocomets' Common Across Milky Way Galaxy,weblink January 7, 2013, Space.com, January 8, 2013,weblink" title="web.archive.org/web/20140916085824weblink">weblink September 16, 2014, File:ESO - The Milky Way panorama (by).jpg|360-degree panorama view of the Milky Way (an assembled mosaic of photographs) by ESO. The galactic centre is in the middle of the view, with galactic north up.

Structure

(File:Artist's impression of the Milky Way.ogv|thumb|Artist's impression of how the Milky Way would look from different vantage points. From edge-on lines-of-sight, the (peanut shell)-shaped structure, not to be confused with the galaxy's central bulge, is evident. Viewed from above, the central narrow bar that is responsible for this structure appears clearly, as would many spiral arms and their associated dust clouds.){{multiple image| align = right| direction = vertical| width = 220| image1 = Artist's impression of the Milky Way (updated - annotated).jpg| alt1 =| caption1 = Artist's conception of the spiral structure of the Milky Way with two major stellar arms and a bar| image2 = Milky Way IR Spitzer.jpg| alt2 =Spitzer Space Telescope>Spitzer reveals what cannot be seen in visible light: cooler stars (blue), heated dust (reddish hue), and Sgr A* as bright white spot in the middle.| image3=X-RayFlare-BlackHole-MilkyWay-20140105.jpg| caption3=Bright X-ray flares from Sagittarius A*, location of the supermassive black hole at the center of the Milky Way.}}The Milky Way consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. The mass distribution within the Milky Way closely resembles the type Sbc in the Hubble classification, which represents spiral galaxies with relatively loosely wound arms. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1960s.Gerard de Vaucouleurs (1964), Interpretation of velocity distribution of the inner regions of the GalaxyPeters, W.L. III. (1975), Models for the inner regions of the Galaxy. IHammersley, P. L.; Garzon, F.; Mahoney, T.; Calbet, X. (1994), Infrared Signatures of the Inner Spiral Arms and Bar Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005 that showed the Milky Way's central bar to be larger than previously thought.

Galactic quadrants

A galactic quadrant, or quadrant of the Milky Way, refers to one of four circular sectors in the division of the Milky Way. In actual astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the origin of the mapping system.{{citation | display-authors=1 | last1=Blaauw | first1=A. | last2=Gum | first2=C. S. | last3=Pawsey | first3=J. L. | last4=Westerhout | first4=G. | title=The new I. A. U. system of galactic coordinates (1958 revision) | journal=Monthly Notices of the Royal Astronomical Society | volume=121 | issue=2 | pages=123–131 | date=1960 | bibcode=1960MNRAS.121..123B | doi=10.1093/mnras/121.2.123}}Quadrants are described using ordinals—for example, "1st galactic quadrant",{{citation|display-authors=1 |first1=Thomas L. |last1=Wilson |first2=Kristen |last2=Rohlfs |first3=Susanne |last3=Hüttemeister |title=Tools of Radio Astronomy |date=2009 |publisher=Springer Science & Business Media |isbn=978-3540851219 |url=https://books.google.com/books?id=9KHw6R8rQEMC&pg=PA347 |deadurl=no |archiveurl=https://web.archive.org/web/20160426084004weblink |archivedate=April 26, 2016 }} "second galactic quadrant",JOURNAL, 2004A&A...418..131K, 10.1051/0004-6361:20034530, Kiss, Cs, Moór, A., Tóth, L. V., Far-infrared loops in the 2nd Galactic Quadrant, Astronomy and Astrophysics, 418, April 2004, astro-ph/0401303, 131–141, or "third quadrant of the Milky Way".JOURNAL, 1, Lampton, M., Lieu, R., Schmitt, J. H. M. M., Bowyer, S., Voges, W., Lewis, J., Wu, X., An All-Sky Catalog of Faint Extreme Ultraviolet Sources, The Astrophysical Journal Supplement Series, 108, 2, 545–557, February 1997, 10.1086/312965, 1997ApJS..108..545L, Viewing from the north galactic pole with 0 degrees (°) as the ray that runs starting from the Sun and through the Galactic Center, the quadrants are as follows:
  • 1st galactic quadrant â€“ 0° ≤ longitude (â„“) ≤ 90°JOURNAL, van Woerden, Hugo, Strom, Richard G., The beginnings of radio astronomy in the Netherlands, Journal of Astronomical History and Heritage, 9, 1, 3–20, June 2006,weblink 2006JAHH....9....3V, no,weblink" title="web.archive.org/web/20100919025431weblink">weblink September 19, 2010,
  • 2nd galactic quadrant â€“ 90° ≤ â„“ ≤ 180°
  • 3rd galactic quadrant â€“ 180° ≤ â„“ ≤ 270°
  • 4th galactic quadrant â€“ 270° ≤ â„“ ≤ 360° (0°)

Galactic Center

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. In the inner few kpc (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge. It has been proposed that the Milky Way lacks a bulge formed due to a collision and merger between previous galaxies, and that instead it only has a pseudobulge formed by its central bar. 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,Ciambur, Bogdan C.; Graham, Alister W.; Bland-Hawthorn, Joss (2017), Quantifying the (X/peanut)-shaped structure of the Milky Way – new constraints on the bar geometry abound.The Galactic Center is marked by an intense radio source named Sagittarius A* (pronounced Sagittarius A-star). The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object. This concentration of mass is best explained as a supermassive black hole (SMBH) with an estimated mass of 4.1–4.5 million times the mass of the Sun. The rate of accretion of the SMBH is consistent with an inactive galactic nucleus, being estimated at around {{Val|1|e=−5}} {{Solar mass}} y−1. Observations indicate that there are SMBH located near the center of most normal galaxies.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. Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other. However, RR Lyrae variables do not trace a prominent Galactic bar. 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. X-ray emission from the core is aligned with the massive stars surrounding the central barJOURNAL, Wang, Q. D., Nowak, M. A., Markoff, S. B., Baganoff, F. K., Nayakshin, S., Yuan, F., Cuadra, J., Davis, J., Dexter, J., Fabian, A. C., Grosso, N., Haggard, D., Houck, J., Ji, L., Li, Z., Neilsen, J., Porquet, D., Ripple, F., Shcherbakov, R. V., Dissecting X-ray-Emitting Gas Around the Center of Our Galaxy, 10.1126/science.1240755, Science, 341, 6149, 981–983, 2013, 23990554, 1307.5845, 2013Sci...341..981W, and the Galactic ridge.JOURNAL, Bhat, C. L., Kifune, T., Wolfendale, A. W., A cosmic-ray explanation of the galactic ridge of cosmic X-rays, Nature, 318, 267–269, November 21, 1985, 10.1038/318267a0, 1985Natur.318..267B, 6043, File:800 nasa structure renderin2.jpg|thumb|left|Illustration of the two gigantic X-ray/gamma-raygamma-rayIn 2010, two gigantic spherical bubbles of high energy 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}}; they stretch up to Grus and to Virgo on the night-sky of the southern hemisphere. 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.JOURNAL, Carretti, E., Crocker, R. M., Staveley-Smith, L., Haverkorn, M., Purcell, C., Gaensler, B. M., Bernardi, G., Kesteven, M. J., Poppi, S., 10.1038/nature11734, Giant magnetized outflows from the centre of the Milky Way, Nature, 493, 7430, 66–69, 2013, 23282363, bibcode = 2013Natur.493...66C, 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*.WEB, Chou, Felicia, Anderson, Janet, Watzke, Megan, RELEASE 15-001 – NASA’s Chandra Detects Record-Breaking Outburst from Milky Way’s Black Hole,weblink January 5, 2015, NASA, January 6, 2015, no,weblink" title="web.archive.org/web/20150106100532weblink">weblink January 6, 2015,

Spiral arms

{{Details|Spiral galaxy}}Outside the gravitational influence of the Galactic bars, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms. 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 regions and molecular clouds.JOURNAL, Dame, T. M., Hartmann, D., Thaddeus, P., 10.1086/318388, The Milky Way in Molecular Clouds: A New Complete CO Survey, The Astrophysical Journal, 547, 2, 792–813, 2001, astro-ph/0009217, 2001ApJ...547..792D, The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's spiral arms. Perfect logarithmic spiral patterns only crudely describe features near the Sun, because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity. The possible scenario of the Sun within a spur / Local arm emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way. Estimates of the pitch angle of the arms range from about 7° to 25°. There are thought to be four spiral arms that all start near the Milky Way's center.JOURNAL, Sanna, A., Reid, M. J., Dame, T. M., Menten, K. M., Brunthaler, A., 2017, Mapping spiral structure on the far side of the Milky Way, Science, 358, 6360, 227–230, 10.1126/science.aan5452, 29026043, 1710.06489, 2017Sci...358..227S, These are named as follows, with the positions of the arms shown in the image at right:(File:Milky Way Arms.svg|thumb|500px|Observed (normal lines) and extrapolated (dotted lines) structure of the spiral arms of the Milky Way, viewed from "north" of the galaxy. Stars generally move clockwise in this view. The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations.){|class="wikitable"!Color!Arm(s)cyan|Near 3 kpc Arm and Perseus ArmpurpleNorma Arm>Norma and Outer arm (Along with extension discovered in 2004)green|Scutum–Centaurus Armpink|Sagittarius-Carina ArmThere are at least two smaller arms or spurs, including: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 giants than would be expected in the absence of a spiral arm. 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. 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."Massive stars mark out Milky Way's 'missing' arms" {{webarchive|url=https://web.archive.org/web/20131218150156weblink |date=December 18, 2013 }}, University of Leeds. December 17, 2013. Retrieved December 18, 2013.NEWS, Russell, Westerholm,weblink Milky Way Galaxy Has Four Arms, Reaffirming Old Data and Contradicting Recent Research, University Herald, December 18, 2013, December 18, 2013, no,weblink" title="web.archive.org/web/20131219010711weblink">weblink December 19, 2013, JOURNAL, Monthly Notices of the Royal Astronomical Society, 437, 2, 1791–1807, January 2014, 4, J. S., Urquhart, C. C., Figura, T. J. T., Moore, M. G., Hoare, S. L., Lumsde, J. C., Mottram, M. A., Thompson, R. D., Oudmaijer, The RMS Survey: Galactic distribution of massive star formation, 2014MNRAS.437.1791U, 10.1093/mnras/stt2006, 1310.4758, 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.File:PIA19341-MilkyWayGalaxy-SpiralArmsData-WISE-20150603.jpg|thumb|left|200px|Clusters detected by WISE used to trace the Milky Way's spiral arms]]The Near 3 kpc Arm (also called Expanding 3 kpc Arm or simply 3 kpc Arm) was discovered in the 1950s by astronomer van Woerden and collaborators through 21-centimeter radio measurements of HI (atomic hydrogen).Expansion d'une structure spirale dans le noyau du Système Galactique, et position de la radiosource Sagittarius A, Comptes Rendus l'Académie des Sciences, Vol. 244, p. 1691-1695, 1957JOURNAL, 0807.1752, A New Spiral Arm of the Galaxy: The Far 3-Kpc Arm, T. M., Dame, P., Thaddeus, The Astrophysical Journal, 683, 2, L143–L146, 2008ApJ...683L.143D, 10.1086/591669, 2008, It was found to be expanding away from the central bulge at more than 50 km/s. It is located in the fourth galactic quadrant at a distance of about 5.2 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 CfA). It is located in the first galactic quadrant at a distance of 3 kpc (about 10,000 ly) from the Galactic Center.NEWS,weblink Milky Way's Inner Beauty Revealed, Harvard-Smithsonian Center for Astrophysics, June 3, 2008, July 7, 2015, no,weblink" title="web.archive.org/web/20130705045749weblink">weblink July 5, 2013, 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.WEB, John, Matson, Star-Crossed: Milky Way's Spiral Shape May Result from a Smaller Galaxy's Impact, Scientific American, September 14, 2011,weblink July 7, 2015, no,weblink" title="web.archive.org/web/20131203000625weblink">weblink December 3, 2013, 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 pseudoring, and the two patterns would be connected by the Cygnus arm.(File:Nessie molecular cloud.png|thumb|The long filamentary molecular cloud dubbed "Nessie" probably forms a dense "spine" of the Scutum–Centarus Arm)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.ARXIV, Comments on the "Monoceros" affair, Lopez-Corredoira, M., Moitinho, A., Zaggia, S., Momany, Y., Carraro, G., Hammersley, P. L., Cabrera-Lavers, A., Vazquez, R. A., 1, July 2012, 1207.2749, astro-ph.GA,

Halo

The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within {{convert|100000|ly|kpc|-1}} of the Galactic Center. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years from the Galactic Center. About 40% of the Milky Way's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation. The globular clusters can follow rosette orbits about the Milky Way, in contrast to the elliptical orbit of a planet around a star.Although the disk contains dust that obscures the view in 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 gas cool enough to collapse into stars. Open clusters are also located primarily in the disk.JOURNAL, Janes, K.A., Phelps, R.L., 1980, The galactic system of old star clusters: The development of the galactic disk, The Astronomical Journal, 108, 1773–1785, 10.1086/117192, 1994AJ....108.1773J, 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 further than previously thought, the possibility of the disk of the Milky Way extending further is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm and of a similar extension of the Scutum-Centaurus Arm.JOURNAL, T.M. Dame, P. Thaddeus, The Astrophysical Journal, A Molecular Spiral Arm in the Far Outer Galaxy, 2011, 1105.2523, 10.1088/2041-8205/734/1/l24, 734, L24, 2011ApJ...734L..24D, 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. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.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.

Gaseous halo

In addition to the stellar halo, the Chandra X-ray Observatory, XMM-Newton, and Suzaku have provided evidence that there is a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousand of light years, much further 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.WEB, Boen, Brooke, NASA's Chandra Shows Milky Way is Surrounded by Halo of Hot Gas09.24.12,weblink Brooke Boen, October 28, 2012, no,weblink" title="web.archive.org/web/20121023023929weblink">weblink October 23, 2012, JOURNAL, Gupta, A., Mathur, S., Krongold, Y., Nicastro, F., Galeazzi, M., A Huge Reservoir of Ionized Gas Around the Milky Way: Accounting for the Missing Mass?, 10.1088/2041-8205/756/1/L8, The Astrophysical Journal, 756, L8, 2012, 1205.5037, 2012ApJ...756L...8G, WEB,weblink Smithsonian Astrophysical Observatory, Galactic Halo: Milky Way is Surrounded by Huge Halo of Hot Gas, September 24, 2012, no,weblink" title="web.archive.org/web/20121029155611weblink">weblink October 29, 2012, The temperature of this halo gas is between 1 and 2.5 million K (1.8 and 4.5 million oF).WEB, Communications, Discovery, OUR GALAXY SWIMS INSIDE A GIANT POOL OF HOT GAS,weblink Discovery Communications, October 28, 2012, no,weblink" title="web.archive.org/web/20121029004855weblink">weblink October 29, 2012, Observations of distant galaxies indicate that the Universe had about one-sixth as much baryonic (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. 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.NEWS,weblink J.D. Harrington, Janet Anderson, Peter Edmonds, NASA's Chandra Shows Milky Way is Surrounded by Halo of Hot Gas, September 24, 2012, NASA, no,weblink" title="web.archive.org/web/20121023023929weblink">weblink October 23, 2012,

Sun’s location and neighborhood

{{multiple image| align = right| direction = vertical| width = 220| image1 = Galactic longitude.JPG| alt1 =| caption1 = Diagram of the Sun’s location in the Milky Way. The angles represent longitudes in the galactic coordinate system.| image2 = 04-Closest Stars (LofE04240).png| alt2 =| caption2 = Diagram of the stars in the Solar neighborhood}}The Sun is near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, and in the Gould Belt, at a distance of {{convert|26.4|±|1.0|kly|kpc|abbr=on|lk=off}} from the Galactic Center. The Sun is currently {{convert|5|-|30|pc|ly}} from the central plane of the Galactic disk. The distance between the local arm and the next arm out, the Perseus Arm, is about {{convert|2000|pc|ly}}. The Sun, and thus the Solar System, is located in the Milky Way's galactic habitable zone.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, 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 dwarfs) within {{convert|5|pc|ly}} of 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.WEB,weblinkweblink" title="web.archive.org/web/20080206074842weblink">weblink February 6, 2008, Magnitude, National Solar Observatory—Sacramento Peak, August 9, 2013, The apex of the Sun's way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 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.BOOK, Moore, Patrick, Rees, Robin, Patrick Moore's Data Book of Astronomy, 2014, Cambridge University Press, 978-1-139-49522-6, 4, 2nd,weblink no,weblink February 15, 2017, 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 lifeform extinction periods on Earth. However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation.It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a galactic year), 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). The Solar System is headed in the direction of the zodiacal constellation Scorpius, which follows the ecliptic.WEB,weblink Solar System's "Nose" Found; Aimed at Constellation Scorpius, no,weblink" title="web.archive.org/web/20150907130456weblink">weblink September 7, 2015,

Galactic rotation

File:Rotation curve (Milky Way).JPG|thumb|right|200px|Galaxy rotation curveGalaxy rotation curveThe stars and gas in the Milky Way rotate about its center 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 {{convert|210|+/-|10|km/s|mph|abbr=on|sigfig=2}}.weblink Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within 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. Toward the center of the Milky Way the orbit speeds are too low, whereas beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation.If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation 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. 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 modification of the law of gravity may explain the observed rotation curve.BOOK, Extragalactic Astronomy and Cosmology,weblink Peter Schneider, Springer, 978-3-540-33174-2, 2006, 413,

Formation

The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang.WEB, Staff, Milky Way's origins are not what they seem,weblink July 27, 2017, Phys.org, July 27, 2017, no,weblink July 27, 2017, mdy-all, 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. Nonetheless, 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.Since the first stars began to form, the Milky Way has grown through both galaxy mergers (particularly early in the Milky Way's growth) and accretion of gas directly from the Galactic halo. The Milky Way is currently accreting material from several small galaxies, including two of its largest satellite galaxies, the Large and Small Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in high-velocity clouds like the Smith Cloud. However, 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.JOURNAL, 10.1051/0004-6361/200912316, Milky Way versus Andromeda: a tale of two disks, 2009, 4, Yin, J., Astronomy and Astrophysics, 505, 2, 497–508, Hou, J.L, Prantzos, N., Boissier, S., Chang, R. X., Shen, S. Y., Zhang, B., 2009A&A...505..497Y, 0906.4821, JOURNAL, 10.1086/516727, The Milky Way, an Exceptionally Quiet Galaxy: Implications for the Formation of Spiral Galaxies, 2007, 4, Hammer, F., The Astrophysical Journal, 662, 1, 322–334, Puech, M., Chemin, L., Flores, H., Lehnert, M. D., 2007ApJ...662..322H, astro-ph/0702585, 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.JOURNAL, 10.1088/0004-637X/736/2/84, The Mid-life Crisis of the Milky Way and M31, 2011, Mutch, S.J., The Astrophysical Journal, 736, 2, Croton, D.J., Poole, G.B., 2011ApJ...736...84M, 1105.2564, 84, In fact, 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.JOURNAL, What Is The Color Of The Milky Way?, 2012, Licquia, T., American Astronomical Society, Newman, J.A., Poole, G.B., 2012AAS...21925208L,

Age and cosmological history

(File:A firestorm of star birth (artist’s illustration).jpg|thumb|left|150px|Night sky from a hypothetical planet within the Milky Way 10 billion years ago.WEB, A firestorm of star birth (artist’s illustration),weblink www.spacetelescope.org, ESA/Hubble, April 14, 2015, no,weblink" title="web.archive.org/web/20150413031322weblink">weblink April 13, 2015, )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 elements 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 CS 31082-001JOURNAL, Nature, 2001, 409, 6821, 691–692, astro-ph/0104357, 2001Natur.409..691C, Cayrel, etal, Measurement of stellar age from uranium decay, 10.1038/35055507, 11217852, and {{nowrap|13.8 ± 4 billion years}} for BD +17° 3248.JOURNAL, Cowan, J. J., Sneden, C., Burles, S., Ivans, I. I., Beers, T. C., Truran, J. W., Lawler, J. E., Primas, F., Fuller, G. M., Pfeiffer, 10.1086/340347, B., Kratz, K. L., The Chemical Composition and Age of the Metal‐poor Halo Star BD +17o3248, The Astrophysical Journal, 572, 2, 861–879, 2002, 9, astro-ph/0202429, 2002ApJ...572..861C, 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 temperature, 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 gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.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. This estimate was made using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of thorium and other elements created by the R-process. The line strengths yield abundances of different elemental isotopes, from which an estimate of the age of the star can be derived using nucleocosmochronology. Another star, HD 140283, is 14.5 ± 0.7 billion years old.JOURNAL, HD 140283: A Star in the Solar Neighborhood that Formed Shortly After the Big Bang, February 13, 2013, H.E. Bond, 4, E. P. Nelan, D. A. VandenBerg, G. H. Schaefer, D. Harmer, 10.1088/2041-8205/765/1/L12, The Astrophysical Journal, 765, L12, 1, 1302.3180, 2013ApJ...765L..12B, WEB,weblink NASA, Hubble Finds Birth Certificate of Oldest Known Star in the Milky Way, March 7, 2013,weblink" title="web.archive.org/web/20140811210821weblink">weblink August 11, 2014, 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. 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.Skibba, Ramon (2016), "Milky Way retired early from star making" (New Scientist, March 5, 2016), p.9The satellite galaxies surrounding the Milky way are not randomly distributed, but seemed to be the result of a break-up of some larger system producing a ring structure 500,000 light years in diameter and 50,000 light years wide.JOURNAL, Lynden-Bell, D., March 1, 1976, Dwarf Galaxies and Globular Clusters in High Velocity Hydrogen Streams,weblink Monthly Notices of the Royal Astronomical Society, en, 174, 3, 695–710, 10.1093/mnras/174.3.695, 0035-8711, 1976MNRAS.174..695L, Close encounters between galaxies, like that expected in 4 billion years with the Andromeda Galaxy rips 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.Kroupa, P., C. Theis and C. M. Boily (2005) "The great disk of Milky-Way satellites and cosmological sub-structures" (Astronomy and Astrophysics, Volume 431, Number 2, February IV 2005) pp. 517 – 521

Environment

{{multiple image| align = right| direction = vertical| width = 220| image1 = 06-Local Group (LofE06240).png| alt1 =| caption1 = 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}}The Milky Way and the Andromeda Galaxy are a 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 Virgo Supercluster. Surrounding the Virgo Supercluster are a number of voids, devoid of many galaxies, the Microscopium Void to the "north", the Sculptor Void to the "left", the Bootes 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,Hadhazy, Adam (2016), "Nothing Really Matters: Gaping Cosmic Voids" (Discover, Dec 2016) which in turn forms part of a greater structure, called Laniakea.JOURNAL, The Laniakea supercluster of galaxies, R. Brent Tully, Helene Courtois, Yehuda Hoffman, Daniel Pomarède, September 2, 2014, September 4, 2014, Nature, 513, 7516, 71–73, 2014Natur.513...71T, 1409.0880, 10.1038/nature13674, 25186900, Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. The largest of these is the Large Magellanic Cloud with a diameter of 14,000 light-years. 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.JOURNAL, Putman, M. E., Staveley‐Smith, L., Freeman, K. C., Gibson, B. K., Barnes, D. G., The Magellanic Stream, High‐Velocity Clouds, and the Sculptor Group, 10.1086/344477, The Astrophysical Journal, 586, 170–194, 2003, astro-ph/0209127, 2003ApJ...586..170P, Some of the 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. 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. 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.JOURNAL, Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds, Sergey E. Koposov, Vasily Belokurov, Gabriel Torrealba, N. Wyn Evans, March 10, 2015, 1503.02079, The Astrophysical Journal, 2015ApJ...805..130K, 10.1088/0004-637X/805/2/130, 805, 2, 130, There are also some dwarf galaxies that have already been absorbed by the Milky Way, such as the progenitor of Omega Centauri.JOURNAL
, Noyola, E.
, Gebhardt, K.
, Bergmann, M.
, Gemini and Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in ω Centauri
, The Astrophysical Journal
, 0801.2782
, April 2008
, 676
, 2
, 1008–1015
, 10.1086/529002
, 2008ApJ...676.1008N
, In 2014 researchers reported that most satellite galaxies of the Milky Way actually lie in a very large disk and orbit in the same direction.WEB, Lea Kivivali, Nearby satellite galaxies challenge standard model of galaxy formation, Swinburne University of Technology, June 11, 2014,weblink no,weblink" title="web.archive.org/web/20150316134533weblink">weblink March 16, 2015, This came as a surprise: according to standard cosmology, the satellite galaxies should form in dark matter halos, and they should be widely distributed and moving in random directions. This discrepancy is still not fully explained.JOURNAL, Pawlowski, etal, Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies, Monthly Notices of the Royal Astronomical Society, June 10, 2014, 1406.1799, 2014MNRAS.442.2362P, 10.1093/mnras/stu1005, 442, 3, 2362–2380, 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.Current measurements suggest the Andromeda Galaxy is approaching us at {{convert|100|to|140|km/s|mph|abbr=on|sigfig=2}}. In 3 to 4 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 stars colliding with each other is extremely low, but instead the two galaxies will merge to form a single elliptical galaxy or perhaps a large disk galaxyJOURNAL
, Junko Ueda, etal
, Cold molecular gas in merger remnants. I. Formation of molecular gas disks
, The Astrophysical Journal Supplement Series
, 214
, 1
, 1
, 2014ApJS..214....1U
, 10.1088/0067-0049/214/1/1, 1407.6873, 2014
, over the course of about a billion years.

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.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.BOOK, An Introduction to Galaxies and Cosmology, 298, 978-0-521-54623-2, Mark H. Jones, Robert J. Lambourne, David John Adams, 2004,weblink Cambridge University Press, The Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters, including the Shapley supercluster, behind it.JOURNAL, Kocevski, D. D., Ebeling, H., 10.1086/503666, On the origin of the Local Group's peculiar velocity, The Astrophysical Journal, 645, 2, 1043–1053, 2006, astro-ph/0510106, 2006ApJ...645.1043K, 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.JOURNAL, 10.1016/j.newast.2005.08.008, Mass determination of groups of galaxies: Effects of the cosmological constant, 2006, Peirani, S, New Astronomy, 11, 4, 325–330, Defreitaspacheco, J, astro-ph/0508614, 2006NewA...11..325P, {{anchor|cmb}}Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at {{nowrap|{{convert|552|±|6| km/s|mph|abbr=on}}}} with respect to the photons of the CMB, toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of 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 blue-shifted in the direction of the motion and red-shifted in the opposite direction.

Etymology and mythology

File:Jacopo Tintoretto - The Origin of the Milky Way - Google Art Project.jpg|thumb|upright=1.3|The Origin of the Milky Way ({{circa}} 1575–1580) by TintorettoTintorettoIn the Babylonian epic poem Enûma Eliš, the Milky Way is created from the severed tail of the primeval salt water dragoness Tiamat, set in the sky by Marduk, the Babylonian national god, after slaying her.BOOK, Brown, William P., The Seven Pillars of Creation: The Bible, Science, and the Ecology of Wonder, 2010, Oxford University Press, Oxford, England, 978-0-19-973079-7, 25,weblink harv, BOOK, MacBeath, Alastair, Tiamat's Brood: An Investigation Into the Dragons of Ancient Mesopotamia, 1999, Dragon's Head, 9780952438755, 41,weblink harv, This story was once thought to have been based on an older Sumerian version in which Tiamat is instead slain by Enlil of Nippur,BOOK, James, E. O., The Worship of the Skygod: A Comparative Study in Semitic and Indo-European Religion, London, England, University of London, Jordan Lectures in Comparative religion, 1963, 24, 27f, harv, JOURNAL, Lambert, W. G., Bulletin of the School of Oriental and African Studies, London, England, University of London, 27, 1, 1964, 157–158, harv, but is now thought to be purely an invention of Babylonian propagandists with the intention to show Marduk as superior to the Sumerian deities.Llys Dôn (literally "The Court of Dôn") is the traditional Welsh name for the constellation Cassiopeia. At least three of Dôn's children also have astronomical associations: Caer Gwydion ("The fortress of Gwydion") is the traditional Welsh name for the Milky Way, and Caer Arianrhod ("The Fortress of Arianrhod") being the constellation of Corona Borealis.In western culture, the name "Milky Way" is derived from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term is a translation of the Classical Latin via lactea, in turn derived from the Hellenistic Greek , short for ({{transl|grc|galaxías kýklos}}, "milky circle"). The Ancient Greek ({{transl|grc|galaxias}}) – from root -, ("milk") + (forming adjectives) – is also the root of "galaxy", the name for our, and later all such, collections of stars.In Greek mythology, the Milky Way was formed after the trickster god Hermes suckled the infant Heracles at the breast of Hera, the queen of the gods, while she was asleep.BOOK, Leeming, David Adams, Mythology: The Voyage of the Hero, 1998, Oxford University Press, Oxford, England, 978-0-19-511957-2, 44, Third,weblink harv, BOOK, Pache, Corinne Ondine, Hercules, Ancient Greece and Rome, 2010, Oxford University Press, Oxford, England, 978-0-19-538839-8, Gargarin, Michael, Fantham, Elaine, 1: Academy-Bible, 400,weblink harv, When Hera awoke, she tore Heracles away from her breast and splattered her breast milk across the heavens. In another version of the story, Athena, the patron goddess of heroes, tricked Hera into suckling Heracles voluntarily, but he bit her nipple so hard that she flung him away, spraying milk everywhere.The Milky Way, or "milk circle", was just one of 11 "circles" the Greeks identified in the sky, others being the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, Arctic and Antarctic circles, and two colure circles passing through both poles.

Astronomical history

{{See also|Galaxy#Observation history}}File:Herschel-Galaxy.png|thumb|The shape of the Milky Way as deduced from star counts by William HerschelWilliam HerschelIn Meteorologica (DK 59 A80), Aristotle (384–322 BC) wrote that the Greek philosophers Anaxagoras ({{circa|lk=no|500}}–428 BC) and Democritus (460–370 BC) proposed that the Milky Way might consist of distant stars.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, pp. 39–40 {{webarchive|url=https://web.archive.org/web/20160411091135weblink |date=April 11, 2016 }} : "(2) Anaxagoras, Democritus, and their schools say that the milky way is the light of certain stars." However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together"(Aristotle with Ross, 1931), p. 41: {{webarchive|url=https://web.archive.org/web/20160411091135weblink |date=April 11, 2016 }} "For it is natural to suppose that, if the motion of a single star excites a flame, that of all the stars should have a similar result, and especially in that region in which the stars are biggest and most numerous and nearest to one another." and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions."(Aristotle with Ross, 1931), p. 43 : {{webarchive|url=https://web.archive.org/web/20160411091135weblink |date=April 11, 2016 }} "We have now explained the phenomena that occur in that part of the terrestrial world which is continuous with the motions of the heavens, namely, shooting-stars and the burning flame, comets and the milky way, these being the chief affections that appear in that region." The Neoplatonist philosopher Olympiodorus the Younger ({{circa|495}}–570 A.D.) 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 Islamic world.The Persian astronomer Abū Rayhān al-Bīrūnī (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars". The Andalusian astronomer Avempace ({{abbr|d|died}} 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in Earth's atmosphere, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence. 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" and that these stars are larger than planets.According to Jamil Ragep, the Persian astronomer Naṣīr al-Dīn al-Ṭūsī (1201–1274) in his Tadhkira writes:"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."Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars.Galileo Galilei, Sidereus Nuncius (Venice, (Italy): Thomas Baglioni, 1610), pages 15 and 16. {{webarchive|url=https://web.archive.org/web/20160316173644weblink |date=March 16, 2016 }}English translation: Galileo Galilei with Edward Stafford Carlos, trans., The Sidereal Messenger (London, England: Rivingtons, 1880), pages 42 and 43. {{webarchive|url=https://web.archive.org/web/20121202215542weblink |date=December 2, 2012 }} In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright,Thomas Wright, An Original Theory or New Hypothesis of the Universe … (London, England: H. Chapelle, 1750).
  • On page 57, Wright stated that despite their mutual gravitational attraction, the stars in the constellations don't 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, … "
  • On page 48, 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 … "
  • On page 65, 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; ... "
  • On page 73, Wright called the Milky Way the Vortex Magnus (the great whirlpool) and estimated its diameter to be 8.64×1012 miles (13.9×1012 km).
  • On page 33, 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 … " speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales.Immanuel Kant, Allgemeine Naturgeschichte und Theorie des Himmels … [Universal 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: "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 hasn't 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.) The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae 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.Kant (1755), pages xxxiii–xxxvi of the Preface (Vorrede): "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.)The term Weltinsel (island universe) 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), pages 187, 189. From page 187: "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." (Thomas Wright of Durham, Kant, Lambert and first of all 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.)In the English translation—Alexander von Humboldt with E.C. Otté, trans., Cosmos … (New York City: Harper & Brothers, 1897), vols. 3–5—see page 147.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.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: In 1845, 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.File:Pic iroberts1.jpg|thumb|Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda GalaxyAndromeda GalaxyIn 1917, Heber Curtis had observed the nova S Andromedae within the Great Andromeda Nebula (Messier object 31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 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 actually independent galaxies. In 1920 the 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 shift.The controversy was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory {{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 astronomical photographs that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some Cepheid variables 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.

Mapping

The ESA spacecraft Gaia provides distance estimates by determining the parallax of a billion stars and is mapping the Milky Way with four planned releases of maps in 2022.WEB,weblink New Milky Way Map Is a Spectacular Billion-Star Atlas, September 14, 2016, September 15, 2016, no,weblink" title="web.archive.org/web/20160915183426weblink">weblink September 15, 2016, WEB,weblink Gaia > Gaia DR1, www.cosmos.esa.int, September 15, 2016, no,weblink" title="web.archive.org/web/20160915091251weblink">weblink September 15, 2016,

See also

Notes

{{reflist|group=nb|colwidth=30em|refs=Jay M. Pasachoff in his textbook Astronomy: From the Earth to the Universe states the term Milky Way should refer exclusively to the band of light that the galaxy forms in the night sky, while the galaxy should receive the full name Milky Way Galaxy; however, this does not reflect a firm consensus in the astronomical community. See:
  • BOOK, Pasachoff, Jay M., Jay Pasachoff, Astronomy: From the Earth to the Universe, Harcourt School, 1994, 978-0-03-001667-7, 500,
See also Bortle Dark-Sky Scale.For a photo see: WEB, Sagittarius A*: Milky Way monster stars in cosmic reality show, Chandra X-ray Observatory, Harvard-Smithsonian Center for Astrophysics, January 6, 2003,weblink May 20, 2012, no,weblink" title="web.archive.org/web/20080317061809weblink">weblink March 17, 2008, }}{{notes| notes ={{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 here, an absolute visual magnitude of −21.35 (−20.8 − 0.55 = −21.35) is obtained. Note that determining the absolute magnitude of the Milky Way is very difficult, because Earth is inside it.}}}}

References

Further reading

  • JOURNAL, Dambeck, Thorsten Dambeck, March 2008, Gaia's Mission to the Milky Way, Sky & Telescope, 36–39,
  • JOURNAL, Chiappini, Cristina, November–December 2001, The Formation and Evolution of the Milky Way,weblink American Scientist, 506–515,

External links

{{Commons category|Milky Way Galaxy}} {{Milky Way}}{{Earth's location}}{{Authority control}}

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