# Milky Way <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Out to the nearest star (Solar System explorer)* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?view=neighborhood&embed=1" data-title="Out to the nearest star (Solar System explorer)"></div> *The Solar System explorer locked on this article's state (`?view=neighborhood`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: read the ruler from the heliopause tick at 121 AU to its last mark, Proxima Centauri at 4.25 light-years, the full reach of the explorer; set show to clouds and boundaries to see the Oort cloud points reach most of the way along it; press l to hide the labels and drag the view edge-on, remembering that the Milky Way's disc is roughly 20,000 times wider than this ruler.* The **Milky Way** is the galaxy that contains the [[Sun]] and the Solar System. Its name describes how it looks from [[Earth]]: a faint band of light across the night sky, made of stars in the galactic disc too distant and too crowded to be seen one by one.[^waller2003] It is a barred spiral galaxy with a disc about 26.8 kiloparsecs (87,000 light-years) across and only about a thousand light-years thick, holding some 100 to 400 billion stars and probably at least as many planets.[^goodwin1998][^nasa-stars][^cassan2012] The Sun lies about 8.3 kiloparsecs (27,000 light-years) from the centre, near the inner edge of the [[Orion_Arm|Orion Arm]]; the centre itself holds a black hole of about four million solar masses, Sagittarius A*.[^gillessen2017][^ghez2008] Galileo resolved the band into stars in 1610. Until the 1920s most astronomers took the Milky Way to be the whole universe, until Edwin Hubble showed that the spiral nebulae were galaxies far beyond it.[^mactutor-galileo][^hubble1929] The Milky Way is one of the two largest members of the Local Group, which belongs to the Laniakea supercluster.[^tully2014] The explorer at the top of this page stops at the nearest star. Its neighbourhood view is a ruler from the heliopause to Proxima Centauri, 4.25 light-years away; the galactic disc is about 20,000 times wider (derived), so none of the structure described here is drawn. ## Mythology In the Babylonian creation epic *Enūma Eliš*, the god Marduk slays the primeval sea dragon Tiamat and sets her severed tail in the sky as the Milky Way.[^brown2010] An older Sumerian version with Enlil as the slayer was once proposed as the source of the story, but the episode is now regarded as a Babylonian composition meant to raise Marduk above the older gods.[^lambert1964] Many other cultures read the band as a river, a road or a trail of spilled material, as the names below show. ## Etymology The English name translates the Latin *via lactea*, from the Greek *galaxias kyklos*, "milky circle"; the Greek word *gala*, milk, is also the root of "galaxy", a name first used for this system and later for all such systems.[^etymonline] Greek myth explained the milk: Zeus placed the infant Heracles at the breast of the sleeping Hera so that he would become immortal, and when she woke and pushed him away, her spilled milk became the band across the sky.[^waller2003][^leeming1998] The "milky circle" was one of several great circles the Greeks traced on the sky, alongside the zodiac, the meridian and the equator.[^eratosthenes1997] ### Other common names Pilgrims in medieval Europe followed it as a guide and called it the road to Santiago de Compostela.[^macleod1911] The Kaurna people of the Adelaide Plains name it *wodliparri*, "house river", and the Gomeroi see in its dark lanes Dhinawan, the Emu in the Sky.[^kaurna][^mandow2021] In Arabic and many neighbouring languages it is the "path of straw", a name that may have spread from Armenian.[^harutyunyan2003] ## Appearance The band has a low surface brightness, and light pollution or light from the [[Moon]] quickly hides it. It needs a sky darker than about 20.2 magnitudes per square arcsecond.[^crumey2014] According to the world atlas of artificial night-sky brightness, more than a third of humanity can no longer see it from home.[^falchi2016] Seen from Earth, the galactic plane runs through about 30 constellations. It is brightest toward Sagittarius, where the Galactic Centre lies, and passes through Auriga on the opposite side, the galactic anticentre.[^miller2015] The plane is tilted by about 60° to the [[Ecliptic|ecliptic]] and 63° to the celestial equator.[^cosmos-plane] ### Naked eye viewing The band marks the plane of the disc: looking along the plane means looking through far more stars than looking out of it, although stars in every direction belong to the galaxy. Because the plane is steeply inclined to [[Earth|Earth's]] [[Orbit|orbit]], the band crosses the sky at an angle and its best-placed part, toward Sagittarius and Scorpius, is up at night only in part of the year; for the rest it lies in the daytime sky. In the northern hemisphere the central region is best seen in summer, low in the south, while from the southern hemisphere it passes high overhead.[^miller2015] Dark-adapted eyes, a moonless sky and dry, clear air all help.[^crumey2014] ## Astronomical history ### Ancient, naked eye observations Aristotle reports that Anaxagoras and Democritus took the Milky Way to be the light of stars not drowned out by the [[Sun]] because they lie in [[Earth|Earth's]] shadow; Aristotle himself placed it in the upper atmosphere.[^aristotle-meteor] Olympiodorus the Younger objected that anything so near would show parallax, which the band does not.[^heidarzadeh2008] Medieval astronomers in the Islamic world, among them Nasir al-Din al-Tusi, concluded that it is made of a vast number of small, tightly clustered stars.[^ragep1993] ### Telescopic observations Galileo confirmed this in 1610, when his telescope broke the band into countless faint stars.[^mactutor-galileo] In 1755 Immanuel Kant, building on Thomas Wright, proposed that the Milky Way is a rotating disc of stars held together by [[Gravity|gravity]], and that some nebulae might be similar systems, "island universes".[^revealing2013] William Herschel counted stars in many directions in 1785 and drew the first map of the galaxy's shape, with the Sun near the centre.[^revealing2013] In 1904 Jacobus Kapteyn found that stellar motions fall into two streams, the first sign of galactic rotation later explained by Bertil Lindblad and Jan Oort.[^kapteyn1906] The scale of the universe was debated in 1920 by Harlow Shapley and Heber Curtis. Curtis had found novae in the Andromeda nebula ten magnitudes fainter than those in the Milky Way and argued that it was a separate galaxy.[^curtis1917][^shapley1921] Hubble settled the question with the 100-inch telescope at Mount Wilson: he found Cepheid variable stars in Andromeda and derived a distance far beyond the Milky Way.[^hubble1929] ### Satellite observations The European Space Agency's Gaia spacecraft, launched in 2013, measures positions, parallaxes and motions of more than a billion stars, and its catalogues have enlarged the volume of the galaxy that can be mapped in detail by orders of magnitude.[^gaia2016][^skibba2021] Its data show, for example, that the warp in the outer disc is precessing, a sign that the galaxy's shape is still being disturbed.[^poggio2020] ## Astrography ### Sun's location and neighborhood The Sun lies about 8.3 kiloparsecs from the Galactic Centre, a distance measured by following individual stars as they orbit the central black hole, and some 5 to 30 parsecs north of the disc's midplane.[^gillessen2017][^majaess2009] Its surroundings are the [[Local_Bubble|Local Bubble]] and, inside it, the [[Local_Interstellar_Cloud|Local Interstellar Cloud]], next to the Radcliffe Wave of star-forming clouds.[^alves2020] The Sun orbits the centre at about 220 to 240 km/s and completes a circuit, a [[Galactic_year|galactic year]], in roughly 220 to 240 million years; it has therefore made about 20 circuits since it formed.[^sparke2007] As it orbits it also bobs up and down through the disc, crossing the plane about 2.7 times per circuit.[^moore2014] Periodic links between these crossings and mass extinctions on Earth have been proposed, but a reanalysis using the distribution of molecular gas in the spiral arms found no correlation.[^overholt2009] ### Galactic quadrants The galactic coordinate system adopted by the International Astronomical Union in 1958 places the [[Sun]] at the origin, measures galactic longitude ℓ from the direction of the Galactic Centre, and divides the sky into four quadrants: the first from 0° to 90°, the second from 90° to 180°, the third from 180° to 270° and the fourth from 270° to 360°.[^blaauw1960] The first and fourth quadrants look inward, across the Galactic Centre; the second and third look outward toward the anticentre. ## General characteristics ### Size Size depends on definition. The standard isophotal diameter, where the disc's blue surface brightness falls to 25 magnitudes per square arcsecond, is estimated at 26.8 ± 1.1 kiloparsecs, which makes the Milky Way an ordinary spiral, smaller than the Andromeda Galaxy.[^goodwin1998] Disc stars have since been found 26 to 31.5 kiloparsecs from the centre, and the dark matter halo extends much farther: Deason and colleagues place its edge at about 292 kiloparsecs.[^lopezcorredoira2018][^deason2020] ### Mass Most of the mass is unseen. Estimates of the total, which depend on how far out the galaxy is taken to extend, range from about 0.6 to 1.5 trillion solar masses; a 2023 review adopts about 0.88 trillion within 200 kiloparsecs, and the motions of 46 halo globular clusters measured by Gaia and Hubble imply a virial mass of about 1.5 trillion.[^bobylev2023][^watkins2019] Stars account for only 46 to 64 billion solar masses, and the [[Interstellar_medium|interstellar gas]] for a small fraction more; the rest is attributed to dark matter.[^licquia2013][^mcmillan2011] A simple check uses the Sun's orbit: by [[Newton's_law_of_universal_gravitation|Newton's law of gravitation]], a circular speed v at radius R requires an enclosed mass M = v²R/G, and with v ≈ 230 km/s and R ≈ 8.3 kiloparsecs this gives about 10¹¹ solar masses inside the Sun's orbit alone (derived).[^openstax-grav] ### Rotation curve The galaxy rotates differentially, but the orbital speed of most stars stays near 200 to 220 km/s over a wide range of radii instead of falling with distance as the planets' speeds fall in the Solar System under [[Kepler's_laws_of_planetary_motion|Kepler's laws]].[^camarillo2018] If only the visible stars and gas were present, speeds would drop outside the bulk of the disc. The flat curve is the main evidence for a dark matter halo; a minority view explains it by modifying the law of gravity instead.[^koupelis2007][^schneider2006] ### Peculiar velocity Besides rotating, the galaxy as a whole moves. Relative to the cosmic microwave background the Local Group travels at about 630 km/s, a motion measured by the COBE satellite as a dipole in the background's temperature, warmer ahead and cooler behind.[^kogut1993] The motion points toward the Great Attractor region and the Shapley Supercluster beyond, and it is pulled in part by the Virgo Cluster.[^kocevski2006] ## Contents The galaxy contains between 100 and 400 billion stars; the uncertainty comes mainly from faint red dwarfs, which are hard to count beyond a few hundred light-years.[^nasa-stars] Microlensing surveys indicate at least one bound planet per star on average, and Kepler data suggest that about one in five Sun-like stars has an Earth-sized planet in its [[Habitable_zone|habitable zone]], implying billions of such worlds.[^cassan2012][^petigura2013] The nearest known one orbits Proxima Centauri, the closest star to the Sun, in [[Alpha_Centauri|Alpha Centauri]].[^anglada2016] Between the stars lies the [[Interstellar_medium|interstellar medium]], a disc of gas and dust at least as wide as the stellar disc, a few hundred light-years thick for cold gas and thousands for warm gas.[^ferriere2001] The stellar disc has no sharp edge: beyond about 13 kiloparsecs the density of stars falls off faster.[^sale2010] In 2023 the IceCube observatory detected high-energy neutrinos from the galactic plane, a first view of the galaxy in neutrinos.[^icecube2023] ## Structure The Milky Way has a bar-shaped central region surrounded by a warped disc of gas, dust and stars. Suspected since the 1960s, the bar was confirmed as larger than expected by the Spitzer Space Telescope's GLIMPSE survey in 2005.[^benjamin2005] The spiral arms are less certain than those of many external galaxies, since they must be mapped from inside. Gas and young stars trace four major arms, while counts of old stars in the infrared show only two, the Perseus and Scutum–Centaurus arms; the [[Sun|Sun's]] [[Orion_Arm|Orion Arm]] is a smaller segment between them.[^urquhart2014][^benjamin2005][^reid2019] ### Galactic Center At the centre is the compact radio source Sagittarius A*. Stars orbiting it within light-days show that it contains about four million solar masses in a tiny volume, which is best explained by a supermassive black hole; the Event Horizon Telescope imaged its shadow in 2022.[^ghez2008][^gillessen2017][^eht2022] Around it lies the bulge of mostly old stars, a few kiloparsecs across. In 2010 the Fermi Gamma-ray Space Telescope found two giant lobes of gamma-ray emission, the Fermi bubbles, extending about 25,000 light-years above and below the centre, likely the product of past outbursts from the centre.[^su2010] ### Halo A roughly spheroidal halo of old stars and globular clusters surrounds the disc and contains most of the dark matter. Many globular clusters move on eccentric, some on retrograde, orbits.[^dauphole1996] X-ray absorption measured by Chandra, XMM-Newton and Suzaku revealed a hot gaseous halo at 1 to 2.5 million K extending hundreds of thousands of light-years, which may hold a mass comparable to that of the stars and could account for some of the galaxy's missing ordinary matter.[^gupta2012][^nasa2012] ## Formation ### History The Milky Way grew from small density fluctuations that collapsed soon after the Big Bang. The oldest stars, made almost entirely of the [[Hydrogen|hydrogen]] and [[Helium|helium]] left by the Big Bang, formed in these early clumps, which now make up the stellar halo; as the system gained mass and spin, [[Angular_momentum|conservation of angular momentum]] flattened the remaining gas into a disc, where later generations of stars, including the Sun, formed.[^buser2000] Growth continued through mergers and accretion of gas, and each generation of stars enriched the gas with heavier [[Chemical_element|elements]]. Simulations of the globular cluster population suggest a major early merger, with a galaxy dubbed Kraken, about 11 billion years ago, and the galaxy appears to have had no large merger in the last 10 billion years, which is unusual for a spiral of its mass, unlike Andromeda.[^kruijssen2020][^yin2009] It still accretes gas today, from the Magellanic Stream and from high-velocity clouds.[^putman2003] ### Age and cosmological history The oldest stars set a lower limit on the galaxy's age. The halo star HE 1523-0901, dated from the [[Radioactive_decay|radioactive decay]] of [[Thorium|thorium]] and [[Uranium|uranium]], is about 13.2 billion years old, and globular clusters give a best-fit age of about 12.6 billion years.[^frebel2007][^krauss2003] Stars of the thin disc point to its formation about 8.8 billion years ago, several billion years after the halo.[^delpeloso2005] ## Intergalactic neighborhood The Milky Way and the Andromeda Galaxy dominate the Local Group, a gravitationally bound group of dozens of galaxies within the Laniakea supercluster.[^tully2014] Its largest satellites are the Large and Small Magellanic Clouds, which trail the Magellanic Stream of neutral [[Hydrogen|hydrogen]] across about 100° of the sky.[^putman2003] Dozens of smaller dwarf satellites are known, and new ultra-faint ones are still being found; nine were discovered in one small patch of sky near the Magellanic Clouds in 2015.[^koposov2015] Andromeda is approaching at about 110 km/s. Van der Marel and colleagues find that the two galaxies should collide in about four billion years and merge into a single elliptical galaxy; stars themselves will almost never collide.[^vandermarel2012][^schiavi2020] ## See also - [[Orion_Arm]] · [[Local_Bubble]] · [[Local_Interstellar_Cloud]] - [[Galactic_year]] - [[Interstellar_medium]] - [[Alpha_Centauri]] - Galactic Center · Andromeda Galaxy · Local Group · Dark matter ## Notes Distances are given in parsecs (1 parsec ≈ 3.26 light-years). Values marked "derived" are computed in this article from the cited quantities. ## References [^waller2003]: Waller, W. H.; Hodge, P. W. (2003). *Galaxies and the Cosmic Frontier*. Harvard University Press. ISBN 978-0-674-01079-6. [^goodwin1998]: Goodwin, S. P.; Gribbin, J.; Hendry, M. A. (1998). "The relative size of the Milky Way". *The Observatory* 118: 201–208. Bibcode 1998Obs...118..201G. [^nasa-stars]: NASA Goddard Space Flight Center, Blueshift (22 July 2015). 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"The Milky Way's secrets: our galaxy's night-sky spectacle sparked scientific revolutions". *Scientific American* 329 (4): 86–87. - Sparke, L. S.; Gallagher, J. S. (2007). *Galaxies in the Universe: An Introduction*, 2nd ed. Cambridge University Press. ## External links - NASA Science, "The Milky Way galaxy": https://science.nasa.gov/resource/the-milky-way-galaxy/ - ESA, Gaia mission: https://www.esa.int/Science_Exploration/Space_Science/Gaia - SEDS, "The Milky Way Galaxy": http://messier.seds.org/more/mw.html ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Milky_Way) : [Wikitube](https://en.wikitube.io/wiki/Milky_Way) · pinned revision [1375546691](https://en.wikipedia.org/w/index.php?oldid=1375546691) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. 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