Milky Way

In the Babylonian epic poem Enūma Eliš, the Milky Way is created from the severed tail of the primeval salt water dragon Tiamat, set in the sky by Marduk, the Babylonian national god, after slaying her. This story was once thought to have been based on an older Sumerian version in which Tiamat is instead slain by Enlil of Nippur, but is now thought to be purely an invention of Babylonian propagandists with the intention of showing Marduk as superior to the Sumerian deities. == Etymology ==

In Greek mythology, Zeus places Heracles, his infant son born to Alcmene, on Hera's breast while she is asleep so the baby will drink her divine milk and become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way. In another Greek story, the abandoned Heracles is given by Athena to Hera for feeding, but Heracles' forcefulness causes Hera to rip him from her breast in pain. In Western culture, the name "Milky Way" derives from its appearance as a dim, unresolved, "milky" glowing band arching across the night sky. The term is a translation of the Classical Latin via lactea, in turn derived from the Hellenistic Greek , short for ('), meaning "milky circle". The Ancient Greek (') – from root -, ("milk") + (forming adjectives) – is also the root of "galaxy", the name for our, and later all such, collections of stars. while migrating in the Northern Hemisphere. The name "Birds' Path" (in Finnish, Estonian, Latvian, Lithuanian, Bashkir, and Kazakh) has some variations in other languages, e.g., "Way of the grey (wild) goose" in Chuvash, Mari, and Tatar, and "Way of the Crane" in Erzya and Moksha. • The Kaurna people of the Adelaide Plains of South Australia called the Milky Way wodliparri in the Kaurna language, meaning "house river". • The Gomeroi people between New South Wales and Queensland called the Milky Way Dhinawan, the giant "Emu in the Sky" that it stretches across the night sky. • The Milky Way was traditionally used as a guide by pilgrims traveling to the holy site at Santiago de Compostela, hence the use of "The Road to Santiago" as a name for the Milky Way. Curiously, La Voje Ladee ("The Milky Way") was also used to refer to the pilgrimage road. • River Ganga of the Sky: this Sanskrit name ( Ākāśagaṃgā) is used in many Indian languages following a Hindu belief. • The Chinese name "Silver River" () is used throughout East Asia, including Korea and Vietnam (Ngân hà). In Japan and Korea, "Silver River" (; ) refers to any galaxy. • The Japanese name for the Milky Way is the , as well as an alternative name in Chinese (). In Vietnamese, "River of Heaven" (Thiên hà) refers to any galaxy. • In West Asia, Central Asia, and parts of the Balkans, the name for the Milky Way is related to the word for straw. Today, Persians, Pakistanis, and Turks use it alongside Arabs. It has been suggested that the term was spread by medieval Arabs who in turn borrowed it from Armenians. • In Serbo-Croatian it is interchangeably called "Kumova slama" () along the "Mliječni put" (). • Scandinavian peoples, such as Swedes, have called the galaxy "Winter Street" (Vintergatan) as the galaxy is most clearly visible during the winter in the northern hemisphere, especially at high latitudes where the glow of the Sun late at night can obscure it during the summer. == Appearance ==

The Milky Way is visible as a hazy band of white light, some 30° wide, arching in the night sky. The light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Brighter regions around the band appear as soft visual patches known as star clouds. The most conspicuous of these is the Large Sagittarius Star Cloud, a portion of the central bulge of the galaxy. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars. Peoples of the southern hemisphere, including the Inca and Australian Aboriginals, identified these regions as dark cloud constellations. 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 for the Milky Way to be visible. It should be visible if the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1. 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 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 path of the Sun in the sky). It is tilted at an angle of 63° to the celestial equator. Naked eye viewing Naked eye viewing of the Milky Way as a band across the sky is based on several considerations: • Because the galaxy is structured as a plane, a high density of stars is seen by looking into the plane, toward the Galactic Center. However, the other stars outside the band are also in the Milky Way. • The galactic plane is 60 degrees tilted from the ecliptic of the Earth's orbit of the Sun. Thus, the band appears in the night sky at an angle to the horizon. • As with all celestial object viewing, the Earth's orbit around the Sun affects the direction of view. The night sky is only facing the plane's center for a portion of the orbit; thus, the seasonality of viewing varies. The band is always there, but for a portion of the year, it is in the direction of the sun (daytime), so its light is washed out. • As with all celestial object viewing, the Milky Way appears to rise over the horizon, rotate higher, and then fall over the horizon. This is caused by Earth's rotation, which brings the band into and out of our line of sight. Viewing the Milky Way band of light and its Galactic Center requires the following conditions. • Eyes adapted to the dark sky (30 minutes) • Low light pollution (Bortle 4 or lower) • Low or no moonlight (during a 5-day window of a new moon) • Low level of haze and humidity, which may not be obvious • Correct season for the hemisphere from which you are viewing (May-August is best in North America) • Correct time of night; as with other celestial objects, it appears to rise and fall in the night sky. • Correct viewing direction: in the Northern Hemisphere, it is in the south, between Sagittarius and Scorpius. In the southern hemisphere, it is near Carina and Centaurus. == Astronomical history ==

Ancient, naked eye observations In Meteorologica, Aristotle (384–322 BC) states that the Greek philosophers Anaxagoras (–428 BC) and Democritus (460–370 BC) proposed that the Milky Way is the glow of stars not directly visible due to Earth's shadow, while other stars receive their light from the Sun, but have their glow obscured by solar rays. Aristotle himself believed that the Milky Way was part of the Earth's upper atmosphere, along with the stars, and that it was a byproduct of stars burning that did not dissipate because of its outermost location in the atmosphere, composing its great circle. He said that the milky appearance of the Milky Way Galaxy is due to the refraction of the Earth's atmosphere. The Neoplatonist philosopher Olympiodorus the Younger (–570 AD) criticized this view, arguing that if the Milky Way were sublunary, it should appear different at different times and places on Earth, and that it should have parallax, which it does not. In his view, the Milky Way is celestial. This idea would be influential later in the Muslim world. In a treatise in 1755, Immanuel Kant, misinterpreting Thomas Wright suggestion that the Milky Way was shaped like balloon, 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. The resulting disk of stars would be seen as a band in the sky from our perspective inside the disk. Wright and Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own. In 1904, while studying the proper motions of stars, Jacobus Kapteyn reported that these were not random, as was believed at the time; stars could be divided into two streams, moving in nearly opposite directions. It was later realized that Kapteyn's data had been the first evidence of the rotation of the Milky Way, which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort. In 1917, Heber Doust Curtis had observed the nova S Andromedae within the Great Andromeda Nebula (Messier object 31). Searching the photographic record, he found 11 more 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 independent galaxies. Data from Gaia has been described as "transformational". It has been estimated that Gaia has expanded the number of observed stars from about 2 million in the 1990s to 2 billion. It has expanded the measurable volume of space by a factor of 100 in radius and 1,000 in precision. A study in 2020 concluded that Gaia detected a wobbling motion of the galaxy, which might be caused by "torques from a misalignment of the disc's rotation axis with respect to the principal axis of a non-spherical halo, or from accreted matter in the halo acquired during late infall, or from nearby, interacting satellite galaxies and their consequent tides". In April 2024, initial studies and related maps, involving the magnetic fields of the Milky Way were reported. == Astrography ==

Sun's location and neighborhood The Sun is near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, between the Radcliffe wave and Split linear structures (formerly Gould Belt). Based upon studies of stellar orbits around Sgr A* by Gillessen et al. (2016), the Sun lies at an estimated distance of There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of 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 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. The apex of the Sun's way, or the solar apex, is the direction that the Sun travels through the Local standard of rest in the Milky Way. The general direction of the Sun's Galactic motion is towards the star Deneb near the constellation of Cygnus, at an angle of roughly 90 sky degrees to the direction of the Galactic Center. The Sun's orbit about the Milky Way is expected to be roughly elliptical, with perturbations from the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun passes through the Galactic plane approximately 2.7 times per orbit. 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. 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 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. Quadrants are described using ordinalsfor example, "1st galactic quadrant", "second galactic quadrant", or "third quadrant of the Milky Way". Viewing from the north galactic pole with 0° (zero degrees) as the ray that runs starting from the Sun and through the Galactic Center, the quadrants are: :: with the galactic longitude (ℓ) increasing in the counter-clockwise direction (positive rotation) as viewed from north of the Galactic Center (a view-point several hundred thousand light-years distant from Earth in the direction of the constellation Coma Berenices); if viewed from south of the Galactic Center (a view-point similarly distant in the constellation Sculptor), ℓ would increase in the clockwise direction (negative rotation). == General characteristics ==

Size , including the Milky Way The Milky Way is one of the two largest galaxies in the Local Group (the other being the Andromeda Galaxy). However, the size of its galactic disc and the extent to which it defines the isophotal diameter are not well understood. An estimate from 1997 by Goodwin and others compared the distribution of Cepheid variable stars in 17 other spiral galaxies to the ones in the Milky Way, and modelling the relationship to their surface brightnesses. This gave an isophotal diameter for the Milky Way at , by assuming that an exponential disc well represents the galactic disc and adopting a central surface brightness of the galaxy (μ0) of B-mag/arcsec−2 and a disk scale length (h) of . This is significantly smaller than the Andromeda Galaxy's isophotal diameter, and slightly below the mean isophotal sizes of the galaxies, being at . To compare the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter (), the Milky Way would be approximately at least the greatest north–south line of the contiguous United States. An even older study from 1978 gave a lower diameter for Milky Way of about . which may be part of the Milky Way's outer disk itself, hence making the stellar disk larger by increasing to this size. Another 2018 study revealed the very probable presence of disk stars at from the Galactic Center or perhaps even farther, significantly beyond approximately , in which it was once believed to be the abrupt drop-off of the stellar density of the disk, meaning that few or no stars were expected to be above this limit, save for stars that belong to the old population of the galactic halo. A 2020 study predicted the edge of the Milky Way's dark matter halo being around , which translates to a diameter of . Mass The Milky Way is approximately 0.88 trillion times the mass of the Sun in total (8.8 solar masses), using a cutoff of 200kpc to define the galaxy. Estimates of the mass of the Milky Way vary, depending upon the method and data used. The low end of the estimate range is 5.8 solar masses (), somewhat less than that of the Andromeda Galaxy. but this is only half the mass of the Andromeda Galaxy. Much of the mass of the Milky Way seems to be dark matter, an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A dark matter halo is conjectured to spread out relatively uniformly to a distance beyond one hundred kiloparsecs (kpc) from the Galactic Center. Mathematical models of the Milky Way suggest that the mass of dark matter is 1–1.5 . 2013 and 2014 studies indicate a range in mass, as large as 4.5  and as small as 8 . and 6.43 . and 15% of the total mass of its stars. Interstellar dust accounts for an additional 1% of the total mass of the gas. In September 2023, astronomers reported that the virial mass of the Milky Way Galaxy is only , only a tenth of the mass of previous studies. The mass was determined from data of the Gaia spacecraft. Rotation curve for the Milky Way. The vertical axis is the rotation speed about the galactic center. The horizontal axis is the distance from the galactic center. The Sun is marked in yellow. Data points mark the observed rotation speed curve. The predicted curve based on the stellar mass and gas of the Milky Way is in black. The difference is due to dark matter or possibly a modification of gravity like MOND. The data shown can be found here. The 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 200 and 220 km/s. Hence the orbital period of the typical star is approximately proportional to the length of the path traveled. This is unlike the situation in the Solar System, where two-body gravitational dynamics dominate, and different orbits have significantly different velocities. The rotation curve (shown in the figure) describes this rotation. If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotational speed would decrease with distance from the center. However, the observed curve is relatively flat, suggesting additional mass that cannot be directly detected with electromagnetic radiation. This inconsistency is attributed to dark matter. Peculiar 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 with respect to this local co-moving frame of reference. The Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters, including the Shapley Supercluster, behind it. 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 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. Another reference frame is provided by the cosmic microwave background (CMB), in which the CMB temperature is least distorted by Doppler shift (zero dipole moment). The Milky Way is moving at with respect to this frame, 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. == Contents ==

, as seen by one of the 2MASS infrared telescopes, is located in the bright upper left portion of the image. The Milky Way contains between 100 and 400 billion stars 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, 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, In November 2013, astronomers reported, based on Kepler space telescope 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. The nearest exoplanet may be 4.2 light-years away, orbiting the red dwarf Proxima Centauri, according to a 2016 study. Such Earth-sized planets may be more numerous than gas giants, More recently, in November 2020, over 300 million habitable exoplanets are estimated to exist in the Milky Way Galaxy. When compared to other more distant galaxies in the universe, the Milky Way galaxy has a below average amount of neutrino luminosity making our galaxy a "neutrino desert". == Structure ==

The Milky Way consists of a bar-shaped core region surrounded by a warped 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. These conjectures were confirmed by the Spitzer Space Telescope observations in 2005 that showed the Milky Way's central bar to be larger than previously thought. The Galactic Center is marked by an intense radio source named (pronounced Sagittarius A-star). The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object. and the Galactic ridge. In June 2023, astronomers led by Naoko Kurahashi Neilson reported using a new cascade neutrino technique to detect, for the first time, the release of neutrinos from the galactic plane of the Milky Way galaxy, creating the first neutrino view of the Milky Way. Gamma rays and X-rays Since 1970, various gamma-ray detection missions have discovered 511-keV gamma rays coming from the general direction of the Galactic Center. These gamma rays are produced by positrons (antielectrons) annihilating with electrons. In 2008, it was found that the distribution of gamma-ray sources resembles that of low-mass X-ray binaries, suggesting that these X-ray binaries are sending positrons (and electrons) into interstellar space, where they slow and annihilate. The observations were made by both NASA and ESA's satellites. In 1970, gamma-ray detectors found that the emitting region was about 10,000 light-years across, with a luminosity of about 10,000 times that of the Sun. 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*. Spiral arms Outside the gravitational influence of the Galactic bar, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms. The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of its arms. These are named as follows, with the positions of the arms shown in the image: 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. 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. 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 (Center for Astrophysics Harvard & Smithsonian). It is located in the first galactic quadrant at a distance of 3 kpc (about 10,000 ly) from the Galactic Center. 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. 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, The structure of the Milky Way's disk is warped along an "S" curve. Halo The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within of the Galactic Center. Discoveries in the early 21st century have added new dimensions to our understanding of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much farther than previously thought, With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Upon the 2004 discovery of a ring of galactic debris in an in-plane orbit around the Milky Way, it was initially believed that the debris was the remnant of a system dubbed the Canis Major Dwarf Galaxy. Other scholars believed it to be due to the Galactic warp, a view which has been supported by more recent evidence as of 2021. The Sloan Digital Sky Survey of the northern sky shows a huge, diffuse structure (spread across an area about 5,000 times the size of a full moon) within the Milky Way that does not seem to fit current models. The collection of stars rises nearly perpendicular to the plane of the Milky Way's spiral arms. 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 away. The temperature of this halo gas is between 1 and 2.5 million K (1.8 and 4.5 million °F). 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. == Formation ==

History showing the red sequence (old galaxies, typically elliptical galaxies), the green valley (where the Milky Way is believed to be in), and the blue cloud (young galaxies, typically spiral galaxies). The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang 13.61 billion years ago. 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. 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 spiral galaxies of a similar type. Its neighbor, the Andromeda Galaxy, appears to have a more typical history, shaped by more recent mergers with relatively large galaxies. 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. 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. Age and cosmological history Globular clusters are among the oldest objects in the Milky Way, thereby setting 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 for CS 31082-001 and for BD +17° 3248. Once a white dwarf forms, it begins to cool radiatively, and its surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperatures, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as . Age estimates for the oldest of these clusters yield a best-fit age 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. or 12.0 ± 0.5 billion years. According to observations using adaptive optics to correct for Earth's atmospheric distortion, stars in the galaxy's bulge are about 12.8 billion years old. 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. The satellite galaxies surrounding the Milky Way are not randomly distributed but appear to result from the breakup of a larger system, producing a ring structure 500,000 light-years in diameter and 50,000 light-years wide. Close encounters between galaxies, like that expected in 4 billion years with the Andromeda Galaxy, can rip off huge tails of gas, which, over time can coalesce to form dwarf galaxies in a ring at an arbitrary angle to the main disc. == Intergalactic neighborhood ==

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 Local Sheet and in turn the Virgo Supercluster. Surrounding the Virgo Supercluster are several voids, devoid of many galaxies, the Microscopium Void to the "north", the Sculptor Void to the "left", the Boötes Void to the "right", and the Canes-Major Void to the "south". These voids change shape over time, forming filamentary structures in galaxies. The Virgo Supercluster, for instance, is being drawn towards the Great Attractor, which in turn forms part of a greater structure, called Laniakea. Two smaller galaxies and several dwarf galaxies in the Local Group orbit the Milky Way. The largest of these is the Large Magellanic Cloud with a diameter of 32,200 light-years. 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. 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 Milky Way has already absorbed some dwarf galaxies, such as the progenitor of Omega Centauri. In 2005 with further confirmation in 2012 researchers reported that most satellite galaxies of the Milky Way lie in a very large disk and orbit in the same direction. This came as a surprise: according to standard cosmology, satellite galaxies should form in dark matter halos and be widely distributed, moving in random directions. This discrepancy is still not explained. 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, which together account for about 2% of the Milky Way's mass, 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. over the course of about six billion years. == See also ==