Milky Way
In the Babylonian epic poem Enūma Eliš, a primeval salt water dragon named Tiamat has her severed tail set in the sky by Marduk to create the Milky Way. This story was once thought to be based on an older Sumerian version where Enlil of Nippur slays Tiamat, but is now considered purely an invention of Babylonian propagandists. In Greek mythology, Zeus places his infant son Heracles on Hera's breast while she sleeps so the baby will drink divine milk and become immortal. Hera wakes up during breastfeeding and pushes the unknown baby away, causing some of her milk to spill and produce the band of light known as the Milky Way. The Ancient Greeks called this celestial feature the Milky Circle, one of 11 circles they identified in the sky alongside the zodiac and equator. Western culture derives the name Milky Way from its appearance as a dim unresolved milky glowing band arching across the night sky. Geoffrey Chaucer traced the English term back to a specific story that popularized the name among European speakers. Northern peoples observed that migratory birds follow the course of the galaxy while migrating at the Northern Hemisphere, leading to names like Birds' Path used in Uralic and Turkic languages. The Kaurna people of the Adelaide Plains of South Australia called the Milky Way wodliparri, meaning house river in their language. The Gomeroi people between New South Wales and Queensland named it Dhinawan, the giant Emu in the Sky stretching across the night sky. Pilgrims traveling to Compostela traditionally used the Milky Way as a guide, hence the use of Road to Santiago as a name for the galaxy. Sanskrit uses the name Akashaganga, River Ganga of the Sky, following Hindu belief systems. East Asia employs Silver River throughout China, Korea, and Vietnam, where the term also means galaxies in general. West Asia, Central Asia, and parts of the Balkans use names related to straw, such as Godfather's Straw in Serbo-Croatian. Scandinavian peoples call the galaxy Winter Street because it is most clearly visible during winter months at high northern latitudes.
Aristotle wrote in Meteorologica around 384 BC that Greek philosophers Anaxagoras and Democritus proposed the Milky Way was the glow of stars not directly visible due to Earth's shadow. Aristotle himself believed the Milky Way was part of Earth's upper atmosphere, composed of great circles formed by burning stars that did not dissipate. Neoplatonist philosopher Olympiodorus the Younger criticized this view before 570 AD, arguing that if the Milky Way were sublunary, it should appear different at various times and places on Earth. The Persian astronomer Al-Biruni between 973 and 1048 proposed that the Milky Way was a collection of countless fragments resembling nebulous stars. Avempace died in 1138 after proposing that the Milky Way consisted of many stars appearing continuous in Earth's atmosphere, citing his observation of Jupiter and Mars conjunctions in 1106 or 1107. Nasir al-Din al-Tusi wrote in Tadhkira from 1201 to 1274 that the galaxy is made up of very large numbers of small tightly clustered stars seeming like cloudy patches likened to milk color. Ibn Qayyim al-Jawziyya lived between 1292 and 1350 and proposed the Milky Way is a myriad of tiny stars packed together in the sphere of fixed stars. Galileo Galilei used a telescope in 1610 to study the Milky Way and discovered it was composed of huge numbers of faint stars. Immanuel Kant published a treatise in 1755 drawing on earlier work by Thomas Wright suggesting the Milky Way might be a rotating body held together by gravitational forces. William Herschel produced a diagram of the Milky Way shape in 1785 by carefully counting stars in different regions of visible sky. Lord Rosse constructed a new telescope in 1845 able to distinguish between elliptical and spiral-shaped nebulae. He observed M51, the Whirlpool Galaxy, on the 5th of March 1845, resolving it into stars with magnifying power of 548. Jacobus Kapteyn reported proper motions of stars in 1904 showing they were not random but divided into two streams moving nearly opposite directions. Heber Doust Curtis observed nova S Andromedae within Great Andromeda Nebula in 1917 finding 11 more novae averaging 10 magnitudes fainter than those occurring within the Milky Way. The Great Debate took place in 1920 between Harlow Shapley and Heber Curtis concerning nature of Milky Way and spiral nebulae. Edwin Hubble used Mount Wilson observatory Hooker telescope in early 1920s to produce astronomical photographs resolving outer parts of some spiral nebulae as collections of individual stars. Gaia spacecraft provides distance estimates by determining parallax of billion stars and mapping the Milky Way since its launch. A study in 2020 concluded that Gaia detected wobbling motion of galaxy possibly caused by torques from misalignment of disc rotation axis.
The Milky Way is a barred spiral galaxy with D25 isophotal diameter estimated at unknown value but only about 1,000 light-years thick at spiral arms. Recent simulations suggest dark matter area containing visible stars may extend up to diameter of almost 2 million light-years or 613 kiloparsecs. The galaxy contains between 100 and 400 billion stars and at least that many planets according to January 2013 study of five-planet star system Kepler-32. In inner few kiloparsecs around 10,000 light-year radius lies dense concentration mostly old stars called bulge shaped roughly spheroidally. Galactic Center marked by intense radio source Sagittarius A* harbors massive compact object best explained as supermassive black hole with estimated mass 4.1 to 4.5 million times Sun's mass. Rate of accretion of supermassive black hole consistent with inactive galactic nucleus being estimated at per year. Outside gravitational influence of Galactic bar structure interstellar medium and stars disk organized into four spiral arms starting near center. Spiral arms typically contain higher density interstellar gas dust than Galactic average greater concentration star formation traced by H II regions molecular clouds. Possible scenario Sun within spur Local arm emphasizes features probably not unique existing elsewhere in Milky Way. Estimates pitch angle arms range from about 7 degrees to 25 degrees. Two smaller arms spurs include Orion-Cygnus Arm containing Sun Solar System. Scutum-Centaurus arm contains approximately 30% more red giants expected absence spiral arm detected by surveys near-infrared light sensitive primarily red giants unaffected dust extinction. Monoceros Ring outer ring gas stars torn other galaxies billions years ago initially believed remnant Canis Major Dwarf Galaxy. Disk warped along S curve following evidence supported more recent studies as 2021. Halo component does not contain dust obscuring view wavelengths unlike disk component. Active star formation takes place disk especially spiral arms representing areas high density but does not take place halo little cool gas collapse stars. Globular clusters follow rosette orbits about Milky Way contrasting elliptical orbit planet around star. Chandra X-ray Observatory XMM-Newton Suzaku provided evidence gaseous halo containing large amount hot gas extending hundreds thousands light-years farther stellar halo close distance Large Small Magellanic Clouds. Mass hot halo nearly equivalent mass Milky Way itself temperature between 1 and 2.5 million Kelvin or 1.8 and 4.5 million Fahrenheit.
Sun located radius about 27,000 light-years or 8.3 kiloparsecs from Galactic Center inner edge Orion Arm one spiral-shaped concentrations gas dust. Sun currently above north central plane Galactic disk according to studies of stellar orbits around Sagittarius A* by Gillessen et al. 2016. Distance local arm next arm out Perseus Arm about unknown value. Solar System located Milky Way galactic habitable zone. About 208 stars brighter absolute magnitude 8.5 within sphere radius from Sun giving density one star per 69 cubic parsecs or one star per 2,360 cubic light-years. On other hand 64 known stars any magnitude not counting four brown dwarfs within from Sun giving density about one star per 8.2 cubic parsecs or one per 284 cubic light-years. Apex Sun way solar apex direction Sun travels Local standard rest Milky Way. General direction Sun Galactic motion towards star Deneb near constellation Cygnus angle roughly 90 sky degrees direction Galactic Center. Sun orbit Milky Way expected roughly elliptical addition perturbations due to Galactic spiral arms non-uniform mass distributions. Sun passes through Galactic plane approximately 2.7 times per orbit similar simple harmonic oscillator works no drag force damping term. Takes Solar System about 240 million years complete one orbit Milky Way galactic year so Sun thought completed 18 to 20 orbits lifetime 1/1250 revolution since origin humans. Orbital speed Solar System center Milky Way approximately unknown value or 0.073% speed light. Sun moves heliosphere at unknown velocity takes around 1,400 years Solar System travel distance 1 light-year 8 days travel 1 AU astronomical unit. Solar System headed direction zodiacal constellation Scorpius follows ecliptic path. Galactic quadrant refers one four circular sectors division Milky Way based upon galactic coordinate system placing Sun origin mapping system. Quadrants described using ordinals first second third fourth with galactic longitude increasing counter-clockwise direction positive rotation viewed north Galactic Center.
Milky Way began one several small overdensities mass distribution Universe shortly after Big Bang 13.61 billion years ago. Some overdensities seeds globular clusters oldest remaining stars now Milky Way formed nearly half matter Milky Way may come other distant galaxies. These stars clusters now comprise stellar halo Milky Way. Within few billion years birth first stars mass Milky Way large enough spinning relatively quickly due conservation angular momentum led gaseous interstellar medium collapse roughly spheroidal shape disk. Later generations stars formed spiral disk most younger stars including Sun observed disk. Since first stars began form Milky Way grown through galaxy mergers particularly early Milky Way growth accretion gas directly Galactic halo. Currently accreting material several small galaxies including two largest satellite galaxies Large Small Magellanic Clouds through Magellanic Stream. Direct accretion gas observed high-velocity clouds like Smith Cloud. Cosmological simulations indicate 11 billion years ago merged particularly large galaxy labeled Kraken. Properties Milky Way such stellar mass angular momentum metallicity outermost regions suggest undergone no mergers large galaxies last 10 billion years. Lack recent major mergers unusual among similar spiral galaxies neighbor Andromeda Galaxy appears typical history shaped more recent mergers relatively large galaxies. Milky Way Andromeda Galaxy lie galaxy color-magnitude diagram known green valley region populated galaxies transition blue cloud actively forming new stars red sequence galaxies lack star formation. Star formation activity green valley galaxies slowing run out star-forming gas interstellar medium. Simulated galaxies similar properties star formation typically extinguished within about five billion years now accounting expected short-term increase rate star formation collision between both Milky Way Andromeda Galaxy. Measurements other galaxies similar Milky Way suggest among reddest brightest spiral galaxies still forming new stars slightly bluer bluest red sequence galaxies. Ages individual stars Milky Way estimated measuring abundance long-lived radioactive elements thorium-232 uranium-238 comparing results estimates original abundance technique called nucleocosmochronology. Globular clusters oldest objects Milky Way thus set lower limit age Milky Way. Age globular cluster M4 estimated as unknown value best fit estimate 12.6 billion years 95% confidence upper limit 16 billion years. November 2018 astronomers reported discovery one oldest stars universe 13.5 billion-years-old 2MASS J18082002-5104378 B tiny ultra metal-poor UMP star made almost entirely materials released Big Bang possibly first stars. Discovery star Milky Way suggests galaxy at least 3 billion years older previously thought. Several individual stars found Milky Way halo measured ages very close 13.80-billion-year age Universe. Star HE 1523-0901 galactic halo estimated about 13.2 billion years old 2007 oldest known object Milky Way time measurement placed lower limit age Milky Way.
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Common questions
What is the origin of the name Milky Way in Western culture?
Western culture derives the name Milky Way from its appearance as a dim unresolved milky glowing band arching across the night sky. Geoffrey Chaucer traced the English term back to a specific story that popularized the name among European speakers.
When did Galileo Galilei discover the composition of the Milky Way using a telescope?
Galileo Galilei used a telescope in 1610 to study the Milky Way and discovered it was composed of huge numbers of faint stars. This observation resolved the galaxy into individual stars rather than a continuous cloud.
How old is the oldest known star found within the Milky Way halo?
Astronomers reported the discovery of one of the oldest stars in the universe on November 2018 named 2MASS J18082002-5104378 B which is 13.5 billion-years-old. This ultra metal-poor UMP star suggests the galaxy is at least 3 billion years older than previously thought.
Where is the Sun located relative to the Galactic Center of the Milky Way?
The Sun is located at a radius of about 27,000 light-years or 8.3 kiloparsecs from the Galactic Center inside the inner edge of the Orion Arm. Studies by Gillessen et al. 2016 confirm the Sun currently sits above the north central plane of the Galactic disk.
What is the estimated mass of the supermassive black hole at the center of the Milky Way?
The Galactic Center marked by intense radio source Sagittarius A* harbors a massive compact object with an estimated mass between 4.1 and 4.5 million times the Sun's mass. This object best explains the nature of the inactive galactic nucleus observed today.