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Milky Way

Milky Way
Milky Way Galaxy

band of light formed by the galactic plane as seen from Earth (see etymology of galaxy). Some sources hold that, strictly speaking, 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, or alternatively the Galaxy.[4][5][6] However, it is unclear how widespread this convention is, and the term Milky Way is routinely used in either context.

Appearance from Earth
Infrared image of the core of the Milky Way galaxy Observation data Type Diameter Thickness Number of stars Oldest known star Mass Sun’s distance to galactic center Sun’s galactic rotation period Spiral pattern rotation period Bar pattern rotation period Speed relative to CMB rest frame SBbc (barred spiral galaxy) 100,000 light years 1,000 light years (stars) 200 to 400 billion 13.2 billion years[1] 5.8 × 1011 M? 26,000 ± 1,400 lightyears 220 million years (negative rotation) 50 million years[2] 15 to 18 million years[2] 552 km/s[3]

See also: Galaxy, List of galaxies

The Milky Way, sometimes called simply the Galaxy, is the galaxy in which our Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies. It is one of billions of galaxies in the observable universe. Its name is a translation of the Latin Via Lactea, in turn translated from the Greek Γαλαξίας (Galaxias), referring to the pale

The Milky Way galaxy, as viewed from Earth, itself situated on a spur off one of the spiral arms of the galaxy (see Sun’s location and neighborhood), appears as a hazy band of white light in the night sky arching across the entire celestial sphere and originating from stars and other material that lie within the galactic plane. The plane of the Milky Way is inclined by about 60° to the ecliptic (the plane of the Earth’s orbit), with the North Galactic Pole situated at right ascension 12h 49m, declination +27.4° (B1950) near beta Comae Berenices. The South Galactic Pole is near alpha Sculptoris. The center of the galaxy is in the direction of Sagittarius, and the Milky Way then "passes" (going westward) through Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion & Gemini, Taurus, Auriga, Perseus, Andromeda, Cassiopeia, Cepheus & Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius. The Milky Way looks brightest in the direction of the constellation of Sagittarius, toward the galactic center. 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 fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that our Solar System lies close to the galactic plane. The Milky Way has a relatively low surface brightness,

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making it difficult to see from any urban or suburban location suffering from light pollution.

Milky Way
figure at about 254 km/s, significantly higher than the widely accepted value of 220 km/ s.[12] This in turn implies that the Milky Way has a total mass equivalent to around 3 trillion Suns, about 50% more massive than previously thought.[13]

360-degree photographic panorThe Milky Way as seen ama of the from Death Valley, 2007. galaxy. This is a panoramic picture.

Age

Size
The stellar disk of the Milky Way galaxy is approximately 100,000 light-years (9.5×1017 km) in diameter, and is believed to be, on average, about 1,000 ly (9.5×1015 km) thick.[7] It is estimated to contain at least 200 billion stars[8] and possibly up to 400 billion stars,[9] the exact figure depending on the number of very low-mass stars, which is highly uncertain. Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12,000 ly (1.1×1017 km)—twice the previously accepted value.[10] As a guide to the relative physical scale of the Milky Way, if it were reduced to 100 m in diameter, the Solar System, including the Oort Cloud, would be no more than 1 mm in width. The Galactic Halo extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at ~180,000 ly (1.7×1018 km).[11] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky Way. Recent measurements by the Very Long Baseline Array (VLBA) have revealed that the Milky Way is much larger than previously thought. The size of our home galaxy is now considered to be roughly similar to that of our largest local neighbour, the Andromeda galaxy. By using the VLBA to measure the apparent shift of far-flung star-forming regions when the Earth is on opposite sides of the Sun, the researchers were able to measure the distance to those regions using fewer assumptions than prior efforts. The newer and more accurate estimate of the galaxy’s rotational speed (and in turn the amount of dark matter contained by the galaxy) puts the

A green and red Perseid meteor streaks across the sky just below the Milky Way in August 2007. It is extremely difficult to define the age at which the Milky Way formed, but the age of the oldest star in the Galaxy yet discovered, HE 1523-0901, is estimated to be about 13.2 billion years, nearly as old as the Universe itself.[1] This estimate is based on research by a team of astronomers in 2004 using the UVVisual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in globular cluster NGC 6397.[14] From this research, the elapsed time between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster was deduced to be 200 million to 300 million years. By including the estimated age of the stars in the globular cluster (13.4 ± 0.8 billion years), they estimated the age of the oldest stars in the Milky Way at 13.6 ± 0.8 billion years. Based upon this emerging science, the Galactic thin disk is estimated to have been formed between 6.5 and 10.1 billion years ago.

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Milky Way

Composition and structure

The galactic center in the direction of Sagittarius. The primary stars of Sagittarius are indicated in red. center is now estimated at 26,000 ± 1400 light-years, while older estimates could put the Sun as far as 35,000 light-years from the central bulge. The galactic center harbors a compact object of very large mass as determined by the motion of material around the center.[23] The intense radio source named Sagittarius A*, thought to mark the center of the Milky Way, is newly confirmed to be a supermassive black hole. For a photo see Chandra X-ray Observatory; Jan. 6, 2003 Most galaxies are believed to have a supermassive black hole at their center.[24] The Galaxy’s bar is thought to be about 27,000 light-years long, running through its center at a 44 ± 10 degree angle to the line between the Sun and the center of the Galaxy. It is composed primarily of red stars, believed to be ancient (see red dwarf, red giant). The bar is surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the Galaxy, as well as most of the Milky Way’s star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of our own galaxy.[25]

The Milky Way is thought to be a barred spiral galaxy. Messier 109 is one possible analog.[15] The Galaxy consists of a bar-shaped core region surrounded by a disk of gas, dust and stars forming four distinct arm structures spiralling outward in a logarithmic spiral shape (see Spiral arms). The mass distribution within the Galaxy closely resembles the Sbc Hubble classification, which is a spiral galaxy with relatively loosely-wound arms.[16] Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1990s[17] rather than an ordinary spiral galaxy. Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[18] which showed the Galaxy’s central bar to be larger than previously suspected. The Milky Way’s mass is thought to be about 5.8 × 1011 solar masses (M?)[19][20][21] comprising 200 to 400 billion stars. Its integrated absolute visual magnitude has been estimated to be −20.9. Most of the mass of the Galaxy is thought to be dark matter, forming a dark matter halo of an estimated 600–3000 billion M? which is spread out relatively uniformly.[21]

Spiral arms
Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. There are believed to be four major spiral arms which all start near the Galaxy’s center. These are named as follows, according to the image at right: Outside of the major spiral arms is the Outer Ring or Monoceros Ring, a ring of stars around the Milky Way proposed by

Galactic center
The galactic disc, which bulges outward at the galactic center, has a diameter of between 70,000 and 100,000 light-years.[22] The distance from the Sun to the galactic

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color cyan purple green pink orange arm(s) 3-kpc and Perseus Arm Norma and Cygnus Arm (Along with a newly discovered extension) Scutum-Crux Arm Carina and Sagittarius Arm Orion Arm (which contains our own Solar System and Sun)

Milky Way

There are at least two smaller arms or spurs, including:

what is observed in spiral galaxies; instead, astronomers propose that the spiral arms form as a result of a matter-density wave emanating from the galactic center. This can be likened to a moving traffic jam on a highway — the cars are all moving, but there is always a region of slow-moving cars. Thus this results in several spiral arms where there are a lot of stars and gas. This model also agrees with enhanced star formation in or near spiral arms; the compressional waves increase the density of molecular hydrogen and protostars form as a result.

Observed and extrapolated structure of the spiral arms astronomers Brian Yanny and Heidi Jo Newberg, which consists of gas and stars torn from other galaxies billions of years ago. As is typical for many galaxies, the distribution of mass in the Milky Way Galaxy is such that the orbital speed of most stars in the Galaxy does not depend strongly on its distance from the center. Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240 km/s.[26] 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 are expected to have significantly different velocities associated with them. This difference is one of the major pieces of evidence for the existence of dark matter. Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If one believes that the inner parts of the arms rotate faster than the outer part, then the Galaxy will wind up so much that the spiral structure will be thinned out. But this is not

Artist’s conception of the spiral structure of the Milky Way with two major stellar arms and a bar.[27] Observations presented in 2008 by Robert Benjamin of the University of WisconsinWhitewater suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum-Centaurus arm. The rest of the arms are minor or adjunct arms.[27]

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Milky Way
of the Galaxy’s Orion Arm, in the Local Fluff inside the Local Bubble, and in the Gould Belt, at a distance of 7.62±0.32 kpc (~25,000±1,000 ly) from the Galactic Center.[31][32][33][34] The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years.[35] The Sun, and thus the Solar System, is found in the galactic habitable zone. There are about 208 stars brighter than absolute magnitude 8.5 within 15 parsecs of the Sun, giving a density of 0.0147 such stars per cubic parsec, or 0.000424 per cubic lightyear (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs of the Sun, giving a density of 0.122 stars per cubic parsec, or 0.00352 per cubic light-year (from List of nearest stars), illustrating the fact that most stars are less bright than absolute magnitude 8.5. 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 around the Galaxy 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 oscillates up and down relative to 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 often coincide with mass extinction periods on Earth; presumably the higher density of stars close to the galactic plane leads to more impact events.[36] It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year),[37] so it is thought to have completed 20–25 orbits during the lifetime of the Sun and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Galaxy is approximately 220 km/s. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 lightyear, or 8 days to travel 1 AU (astronomical unit).[38]

Halo
The galactic disk is surrounded by a spheroid halo of old stars and globular clusters, of which 90% lie within 100,000 light-years,[28] suggesting a stellar halo diameter of 200,000 light-years. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the galactic center. While the disk contains gas and dust which obscure the view in some wavelengths, the spheroid component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk. Recent discoveries have added dimension to the knowledge of the Milky Way’s structure. With the discovery that the disc of the Andromeda Galaxy (M31) extends much further than previously thought,[29] the possibility of the disk of our own Galaxy extending further is apparent, and this is supported by evidence of the newly discovered Outer Arm extension of the Cygnus Arm.[30] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of Sagittarius 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. On January 9, 2006, Mario Jurić and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky found 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 Galaxy. 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 30,000 lightyears away.

Sun’s location and neighborhood
The Sun (and therefore the Earth and Solar System) may be found close to the inner rim

Environment
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Milky Way
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 circle the Galaxy, causing vibrations at certain frequencies when they pass through its edges.[39] Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, by taking into account dark matter, the movement of these two galaxies creates a wake that influences the larger Milky Way. Taking dark matter into account results in an approximately twenty-fold increase in mass for the Galaxy. This calculation is according to a computer model made by Martin Weinberg of the University of Massachusetts, Amherst. In this model, the dark matter is spreading out from the galactic disc with the known gas layer. As a result, the model predicts that the gravitational effect of the Magellanic Clouds is amplified as they pass through the Galaxy. Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 kilometers per second. The Milky Way may collide with it in 3 to 4 billion years, depending on the importance of unknown lateral components to the galaxies’ relative motion. If they collide, individual stars within the galaxies would not collide, but instead the two galaxies will merge to form a single elliptical galaxy over the course of about a billion years.[40]

Broad infrared view of our Milky Way Galaxy from the Spitzer Space Telescope created from more than 800,000 frames. This is the most detailed infrared picture of our galaxy to date. 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, itself being part of the Virgo Supercluster. 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 20,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a peculiar streamer of neutral hydrogen gas connecting these two small galaxies. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Galaxy. 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 Milky Way dwarf galaxies are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies, which are dynamically bound to the Milky Way, as well as some that have already been cannibalized by the Milky Way, such as Omega Centauri. Observations through the zone of avoidance are frequently detecting new distant and nearby galaxies. Some galaxies consisting mostly of gas and dust may also have evaded detection so far. In January 2006, researchers reported that the heretofore unexplained warp in the

Velocity
In the general sense, the absolute velocity of any object through space is not a meaningful question according to Einstein’s special theory of relativity, which declares that there is no "preferred" inertial frame of reference in space with which to compare the Galaxy’s motion. (Motion must always be specified with respect to another object.) Astronomers believe the Milky Way is moving at approximately 630 km per second relative to the local co-moving frame of reference that moves with the Hubble flow.[44] If the Galaxy is moving at 600 km/s, Earth travels 51.84 million km per day, or more than 18.9 billion km per year, about 4.5 times its closest distance from Pluto. The Milky Way is thought to be moving in the direction of the Great Attractor. The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the

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Milky Way

History
Etymology and beliefs
There are many creation myths around the world which explain the origin of the Milky Way and give it its name. The English phrase is a translation from Greek Γαλαξίας, Galaxias, which is derived from the word for milk (γάλα, gala). This is also the origin of the word galaxy. Indians call it the Akashganga or a celestial form of the holy river, Ganga. In Greek myth, the Milky Way was caused by milk spilt by Hera when suckling Heracles. The term Milky Way first appeared in English literature in a poem by Chaucer. "See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt." —Geoffrey Chaucer, Geoffrey Chaucer The House of Fame, c. 1380.[47] In a large area from Central Asia to Africa, the name for the Milky Way is related to the word for straw. It has been claimed that this was spread by Arabs who in turn borrowed the word from Armenian.[48] In several Uralic, Turkic languages, Fenno-Ugric languages and in the Baltic languages the Milky Way is called the "Birds’ Path" (Linnunrata in Finnish), since the route of the migratory birds appear to follow the Milky Way. The Chinese name "Silver River" (??) is used throughout East Asia, including Korea and Japan. An alternative name for the Milky Way in ancient China, especially in poems, is "Heavenly Han River"(??). In Japanese, "Silver River" (?? ginga) means galaxies in general and the Milky Way is called the "Silver River System" (??? gingakei) or the "River of Heaven" (??? Amanokawa or Amanogawa). In Swedish, it is called Vintergatan, or "Winter Street", because the stars in the belt were used to predict time of the approaching winter. In some of the Iberian languages, the name refers to "Road of Saint James"

Galaxy rotation curve for the Milky Way. Vertical axis is speed of rotation about the galactic center. Horizontal axis is distance from the galactic center in kpcs. The sun is marked with a yellow ball. The observed curve of speed of rotation is blue. The predicted curve based upon stellar mass and gas in the Milky Way is red. Scatter in observations roughly indicated by gray bars. The difference is due to dark matter or perhaps a modification of the law of gravity.[41][42][43] 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 967 km/s as part of the Hubble flow, the 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.[45] Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s[3] 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. The galaxy rotates about its center according to its galaxy rotation curve as shown in the figure. The discrepancy between the observed curve (relatively flat) and the curve based upon the known mass of the stars and gas in the Milk Way (decaying cure) is attributed to dark matter.[46]

Discovery
See also: Galaxy—Observation history As Aristotle (384-322 BC) informs us in Meteorologica (DK 59 A80), the Greek philosophers Anaxagoras (ca. 500–428 BC) and Democritus (450–370 BC) proposed that the Milky Way might consist of distant stars. However, Aristotle himself believed the Milky

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Milky Way
stars in different regions of the visible sky. He produced a diagram of the shape of the Galaxy with the Solar System close to the center.

The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the Solar System was assumed near center. Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" 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."[49] The Arabian astronomer, Alhazen (965-1037 AD), refuted this by making the first attempt at observing and measuring the Milky Way’s parallax,[50] and he thus "determined that because the Milky Way had no parallax, it was very remote from the earth and did not belong to the atmosphere."[51] The Persian astronomer, Abū Rayhān alBīrūnī (973-1048), proposed the Milky Way galaxy to be a collection of countless nebulous stars.[52] Avempace (d. 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 the Earth’s atmosphere.[49] Ibn Qayyim Al-Jawziyya (1292-1350) proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars" and that that these stars are larger than planets.[53] 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 was composed of a huge number of faint stars.[54] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, 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 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.[55] 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

Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy 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.[56] In 1917, Heber Curtis had observed the nova S Andromedae within the "Great Andromeda Nebula" (Messier object M31). 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 our galaxy. 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.[57] 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 was 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.[58] The matter was conclusively settled by Edwin Hubble in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.[59] In 1936 Hubble produced a classification

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system for galaxies that is used to this day, the Hubble sequence.[60]

Milky Way

See also
• • • • • Galactic coordinate system Dark matter halo Smith’s Cloud Oort Constants The Great Rift - A molecular dust cloud located between the solar system and the Sagittarius Arm of the which appears to split the Milky Way into two lanes over a third of its length.

Notes
[1] ^ Frebel, Anna (2007). "Discovery of HE 1523-0901, a Strongly r-Processenhanced Metal-poor Star with Detected Uranium". The Astrophysical Journal 660: L117. doi:10.1086/518122. arΧiv:astro-ph/0703414. [2] ^ Bissantz, Nicolai (2003). "Gas dynamics in the Milky Way: second pattern speed and large-scale morphology". Monthly Notices of the Royal Astronomical Society 340: 949. doi:10.1046/j.1365-8711.2003.06358.x. arΧiv:astro-ph/0212516. [3] ^ Kogut, A.; Lineweaver, C.; Smoot, G. F.; Bennett, C. L.; Banday, A.; Boggess, N. W.; Cheng, E. S.; de Amici, G.; Fixsen, D. J.; Hinshaw, G.; Jackson, P. D.; Janssen, M.; Keegstra, P.; Loewenstein, K.; Lubin, P.; Mather, J. C.; Tenorio, L.; Weiss, R.; Wilkinson, D. T.; Wright, E. L. (1993). "Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps". Astrophysical Journal 419: 1. doi:10.1086/173453. http://adsabs.harvard.edu/cgi-bin/nphbib_query?bibcode=1993ApJ...419....1K. Retrieved on 2007-05-10. [4] Freedman, Roger A.; Kaufmann, William J. (2007). Universe. WH Freeman & Co.. p. 605. ISBN 0-7167-8584-6. [5] "Galaxies — Milky Way Galaxy". Encyclopedia Britannica. 19. Encyclopedia Britannica, Inc.. 1998. pp. p. 618. [6] Pasachoff, Jay M. (1994). Astronomy: From the Earth to the Universe. Harcourt School. p. 500. ISBN 0-03-001667-3.

[7] Christian, Eric; Samar, Safi-Harb. "How large is the Milky Way?". http://imagine.gsfc.nasa.gov/docs/ ask_astro/answers/980317b.html. Retrieved on 2007-11-28. [8] Sanders, Robert (January 9, 2006). "Milky Way galaxy is warped and vibrating like a drum". UCBerkeley News. http://www.berkeley.edu/news/ media/releases/2006/01/09_warp.shtml. Retrieved on 2006-05-24. [9] Frommert, H.; Kronberg, C. (August 25, 2005). "The Milky Way Galaxy". SEDS. http://www.seds.org/messier/more/ mw.html. Retrieved on 2007-05-09. [10] "Milky Way fatter than first thought". The Sydney Morning Herald. Australian Associated Press. 2008-02-20. http://news.smh.com.au/milky-way-fatterthan-first-thought/20080220-1tbv.html. Retrieved on 2008-04-24. [11] Connors, et al. (2007). "N-body simulations of the Magellanic stream". Monthly Notices of the Royal Astronomical Society 371: 108. doi:10.1111/j.1365-2966.2006.10659.x. http://cdsads.u-strasbg.fr/cgi-bin/nphbib_query?bibcode=2006MNRAS.371..108C&db_key Retrieved on 2007-01-26. [12] "Milky Way a Swifter Spinner, More Massive, New Measurements Show". 2009-01-05. http://www.nrao.edu/pr/ 2009/mwrotate/. Retrieved on 2009-01-20. [13] Ron Cowen (January 5, 2009). "This just in: Milky Way as massive as 3 trillion suns". Society for Science & the Public. http://www.sciencenews.org/view/ generic/id/39709/title/ This_just_in_Milky_Way_as_massive_as_3_trillion_sun Retrieved on 2009-01-14. [14] Del Peloso, E. F. (2005). "The age of the Galactic thin disk from Th/Eu nucleocosmochronology". Astronomy and Astrophysics 440: 1153. doi:10.1051/ 0004-6361:20053307. arΧiv:astro-ph/ 0506458. Bibcode: 2005A&A...440.1153D. [15] "The Milky Way: A New Galactic SelfPortrait". Planetary Radio. June 23, 2008. http://www.planetary.org/radio/ show/00000294/. Contains an interview with Robert Benjamin and Thomas Dame. [16] Ortwin, Gerhard (2002). "Mass distribution in our Galaxy". Space

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Milky Way

proceedings of a conference held at Science Reviews 100 (1/4): 129–138. Rutgers University, 8–12 Aug 1998,ASP doi:10.1023/A:1015818111633. Conference Series vol. 182. http://adsabs.harvard.edu/abs/ [25] Staff (September 12, 2005). 2002astro.ph..3110G. Retrieved on "Introduction: Galactic Ring Survey". 2007-03-14. Boston University. http://www.bu.edu/ [17] Chen, W.; Gehrels, N.; Diehl, R.; galacticring/new_introduction.htm. Hartmann, D. (1996). "On the spiral arm Retrieved on 2007-05-10. interpretation of COMPTEL ^26^Al map [26] Imamura, Jim (August 10, 2006). "Mass features". Space Science Reviews 120: of the Milky Way Galaxy". University of 315–316. http://adsabs.harvard.edu/abs/ Oregon. http://zebu.uoregon.edu/ 1996A&AS..120C.315C. Retrieved on ~imamura/123/lecture-2/mass.html. 2007-03-14. Retrieved on 2007-05-10. [18] McKee, Maggie (August 16, 2005). "Bar [27] ^ Benjamin, R. A. (2008). "The Spiral at Milky Way’s heart revealed". New Structure of the Galaxy: Something Old, Scientist. Something New...". Beuther, H.; Linz, H.; http://www.newscientistspace.com/ Henning, T. (ed.) Massive Star article.ns?id=dn7854. Retrieved on Formation: Observations Confront 2007-05-09. Theory 387: 375, Astronomical Society [19] Karachentsev, I. D.; Kashibadze, O. G. of the Pacific Conference Series. (2006). "Masses of the local group and of See also Bryner, Jeanna (2008-06-03). the M81 group estimated from "New Images: Milky Way Loses Two distortions in the local velocity field". Arms". Space.com. Astrophysics 49 (1): 3–18. doi:10.1007/ http://www.space.com/scienceastronomy/ s10511-006-0002-6. 080603-aas-spiral-arms.html. Retrieved http://adsabs.harvard.edu/cgi-bin/nphon 2008-06-04. bib_query?bibcode=2006Ap.....49....3K. [28] Harris, William E. (February 2003). [20] Vayntrub, Alina (2000). "Mass of the "Catalog of Parameters for Milky Way Milky Way". The Physics Factbook. Globular Clusters: The Database" (text). http://hypertextbook.com/facts/2000/ SEDS. http://www.seds.org/messier/xtra/ AlinaVayntrub.shtml. Retrieved on data/mwgc.dat.txt. Retrieved on 2007-05-09. 2007-05-10. [21] ^ Battaglia, G.; Helmi, A.; Morrison, H.; [29] Ibata, R.; Chapman, S.; Ferguson, A. M. Harding, P.; Olszewski, E. W.; Mateo, M.; N.; Lewis, G.; Irwin, M.; Tanvir, N. Freeman, K. C.; Norris, J.; Shectman, S. (2005). "On the accretion origin of a vast A. (2005). "The radial velocity dispersion extended stellar disk around the profile of the Galactic halo: Constraining Andromeda galaxy". Astrophysical the density profile of the dark halo of the Journal 634 (1): 287–313. doi:10.1086/ Milky Way" (abstract). Monthly Notices 491727. http://adsabs.harvard.edu/abs/ of the Royal Astronomical Society 364: 2005ApJ...634..287I. Retrieved on 433–442. http://arxiv.org/abs/astro-ph/ 2007-05-10. 0506102. Retrieved on 2007-05-09. [30] "Outer Disk Ring?". SolStation. [22] Grant. J.; Lin, B. (2000). "The Stars of the http://www.solstation.com/x-objects/galMilky Way". Fairfax Public Access ring.htm. Retrieved on 2007-05-10. Corporation. http://members.fcac.org/ [31] Reid, Mark J. (1993). "The distance to ~sol/chview/chv5.htm. Retrieved on the center of the Galaxy". Annual review 2007-05-09. of astronomy and astrophysics 31: [23] Mark H. Jones, Robert J. Lambourne, 345–372. doi:10.1146/ David John Adams (2004). An annurev.aa.31.090193.002021. Introduction to Galaxies and Cosmology. http://adsabs.harvard.edu/cgi-bin/nphCambridge University Press. pp. 50–51. bib_query?bibcode=1993ARA%26A..31..345R&amp. ISBN 0521546230. Retrieved on 2007-05-10. http://books.google.com/ [32] Eisenhauer, F.; Schödel, R.; Genzel, R.; books?id=36K1PfetZegC&pg=PA50&dq=Milky+Way+%22black+hole%22&lr=&as_brr=0&as_pt=AL Ott, T.; Tecza, M.; Abuter, R.; Eckart, A.; [24] Blandford, R.D. (1999). "Origin and Alexander, T. (2003). "A Geometric evolution of massive black holes in Determination of the Distance to the galactic nuclei". Galaxy Dynamics,

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Galactic Center". The Astrophysical Journal 597: L121–L124. doi:10.1086/ 380188. http://adsabs.harvard.edu/abs/ 2003astro.ph..6220E. Retrieved on 2007-05-10. [33] Horrobin, M.; Eisenhauer, F.; Tecza, M.; Thatte, N.; Genzel, R.; Abuter, R.; Iserlohe, C.; Schreiber, J.; Schegerer, A.; Lutz, D.; Ott, T.; Schödel, R. (2004). "First results from SPIFFI. I: The Galactic Center" (PDF). Astronomische Nachrichten 325: 120–123. doi:10.1002/ asna.200310181. http://www.mpe.mpg.de/SPIFFI/ preprints/first_result_an1.pdf. Retrieved on 2007-05-10. [34] Eisenhauer, F. et al. (2005). "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central LightMonth". The Astrophysical Journal 628 (1): 246–259. doi:10.1086/430667. http://adsabs.harvard.edu/abs/ 2005ApJ...628..246E. Retrieved on 2007-08-12. [35] English, Jayanne (1991-07-24). "Exposing the Stuff Between the Stars". Hubble News Desk. http://www.ras.ucalgary.ca/ CGPS/press/aas00/pr/pr_14012000/ pr_14012000map1.html. Retrieved on 2007-05-10. [36] Gillman, M. and Erenler, H. (2008). "The galactic cycle of extinction". International Journal of Astrobiology 7. doi:10.1017/S1473550408004047. http://journals.cambridge.org/action/ displayAbstract?aid=1804088. Retrieved on 2008-04-11. [37] Leong, Stacy (2002). "Period of the Sun’s Orbit around the Galaxy (Cosmic Year)". The Physics Factbook. http://hypertextbook.com/facts/2002/ StacyLeong.shtml. Retrieved on 2007-05-10. [38] Garlick, Mark Antony (2002). The Story of the Solar System. Cambridge University. p. 46. ISBN 0521803365. [39] University of California, Berkeley (2006-01-09). Milky Way galaxy is warped and vibrating like a drum. Press release. http://www.berkeley.edu/news/ media/releases/2006/01/09_warp.shtml. Retrieved on 2007-10-18. [40] Wong, Janet (April 14, 2000). "Astrophysicist maps out our own galaxy’s end". University of Toronto.

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http://www.news.utoronto.ca/bin/ 000414b.asp. Retrieved on 2007-01-11. [41] Peter Schneider (2006). Extragalactic Astronomy and Cosmology. Springer. p. 4, Figure 1.4. ISBN 3540331743. http://books.google.com/ books?id=uP1Hz-6sHaMC&pg=PA100&dq=rotation+ [42] Theo Koupelis, Karl F Kuhn (2007). In Quest of the Universe. Jones & Bartlett Publishers. p. 492; Figure 16-13. ISBN 0763743879. http://books.google.com/ books?id=6rTttN4ZdyoC&pg=PA491&dq=Milky+Wa [43] Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An Introduction to Galaxies and Cosmology. Cambridge University Press. p. 21; Figure 1.13. ISBN 0521546230. http://books.google.com/ books?id=36K1PfetZegC&pg=PA20&dq=Milky+Way [44] Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An Introduction to Galaxies and Cosmology. Cambridge University Press. p. 298. ISBN 0521546230. http://books.google.com/ books?id=36K1PfetZegC&pg=PA4&dq=movement+ [45] Peirani, S (2006). "Mass determination of groups of galaxies: Effects of the cosmological constant". New Astronomy 11: 325. doi:10.1016/ j.newast.2005.08.008. [46] Theo Koupelis, Karl F. Kuhn (2007). In Quest of the Universe. Jones & Bartlett Publishers. pp. 492, Figure 16–13. ISBN 0763743879. http://books.google.com/ books?id=6rTttN4ZdyoC&pg=PA491&dq=Milky+Wa [47] "Online Etymology Dictionary". http://www.etymonline.com/ index.php?term=galaxy. Retrieved on 2007-01-03. [48] Harutyunyan, Hayk (2003-08-29). "The Armenian name of the Milky Way" ( – Scholar search). ArAS News (Armenian Astronomical Society (ArAS)) 6. http://www.aras.am/ARASNEWS/ arasnews06.html. Retrieved on 2007-01-05. [49] ^ Josep Puig Montada (September 28, 2007). "Ibn Bajja". Stanford Encyclopedia of Philosophy. http://plato.stanford.edu/entries/ibnbajja. Retrieved on 2008-07-11. [50] Mohamed, Mohaini (2000), Great Muslim Mathematicians, Penerbit UTM, pp. 49–50, ISBN 9835201579

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[51] Hamid-Eddine Bouali, Mourad Zghal, Zohra Ben Lakhdar (2005). "Popularisation of Optical Phenomena: Establishing the First Ibn Al-Haytham Workshop on Photography" (PDF). The Education and Training in Optics and Photonics Conference. http://spie.org/ etop/ETOP2005_080.pdf. Retrieved on 2008-07-08. [52] O’Connor, John J.; Robertson, Edmund F., "Abu Rayhan Muhammad ibn Ahmad al-Biruni", MacTutor History of Mathematics archive [53] Livingston, John W. (1971), "Ibn Qayyim al-Jawziyyah: A Fourteenth Century Defense against Astrological Divination and Alchemical Transmutation", Journal of the American Oriental Society 91 (1): 96–103 [99], doi:10.2307/600445 [54] J. J. O’Connor, E. F. Robertson (November 2002). "Galileo Galilei". University of St Andrews. http://wwwgap.dcs.st-and.ac.uk/~history/ Biographies/Galileo.html. Retrieved on 2007-01-08. [55] Evans, J. C. (November 24, 1998). "Our Galaxy". George Mason University. http://physics.gmu.edu/~jevans/astr103/ CourseNotes/ECText/ch20_txt.htm. Retrieved on 2007-01-04. [56] Abbey, Lenny. "The Earl of Rosse and the Leviathan of Parsontown". The Compleat Amateur Astronomer. http://labbey.com/ Telescopes/Parsontown.html. Retrieved on 2007-01-04. [57] Heber D. Curtis (1988). "Novae in Spiral Nebulae and the Island Universe Theory". Publications of the Astronomical Society of the Pacific 100: 6. doi:10.1086/132128. http://adsabs.harvard.edu/abs/ 1988PASP..100....6C. [58] Weaver, Harold F.. "Robert Julius Trumpler". National Academy of Sciences. http://www.nap.edu/ readingroom/books/biomems/ rtrumpler.html. Retrieved on 2007-01-05. [59] Hubble, E. P. (1929). "A spiral nebula as a stellar system, Messier 31". Astrophysical Journal 69: 103–158. doi:10.1086/143167. http://adsabs.harvard.edu/cgi-bin/ bib_query?1929ApJ....69..103H. [60] Sandage, Allan (1989). "Edwin Hubble, 1889–1953". The Journal of the Royal

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Astronomical Society of Canada 83 (6). http://antwrp.gsfc.nasa.gov/ diamond_jubilee/1996/ sandage_hubble.html. Retrieved on 2007-01-08.

Further reading
• Thorsten Dambeck in Sky and Telescope, "Gaia’s Mission to the Milky Way", March 2008, p. 36–39.

External links
• The Milky Way Galaxy from An Atlas of the Universe • A 3D map of the Milky Way Galaxy • Milky Way – IRAS (infrared) survey wikisky.org • Milky Way – H-Alpha survey wikisky.org • Running Rings Around the Galaxy Spitzer Space Telescope News • The Milky Way Galaxy, SEDS Messier pages • MultiWavelength Milky Way, NASA site with images and VRML models • Milky Way Explorer, detailed images in infrared with radio, microwave and hydrogen-alpha as well • Face-on Milky Way maps, within about 10 thousand parsecs • The Milky Way at the Astro-Photography Site Of Mister T. Yoshida. • Widefield Image of the Summer Milky Way • Proposed Ring around the Milky Way • Milky Way spiral gets an extra arm, New Scientist.com • Possible New Milky Way Spiral Arm, Sky and Telescope.com • The Milky Way spiral arms and a possible climate connection • Galactic center mosaic via sun-orbiting Spitzer infrared telescope • Milky Way Plan Views, The University of Calgary Radio Astronomy Laboratory • Our Growing, Breathing Galaxy, Scientific American Magazine (January 2004 Issue) • Deriving The Shape Of The Galactic Stellar Disc, SkyNightly (March 17, 2006) • Digital Sky LLC, Digital Sky’s Milky Way Panorama and other images • A new view of the Milky Way galaxy obtained by the Diffuse Infrared Background Experiment (DIRBE) on NASA’s Cosmic Background Explorer satellite (COBE).

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From Wikipedia, the free encyclopedia
• Image of Milky Way galaxy arms, Chandra X-ray Observatory Center • The 1920 Shapley – Curtis Debate on the size of the Milky Way • Milky Way Voyage – India’s First & Largest Star Party • Astronomy Picture of the Day: • Composite image of the Milky Way • Milky Way Illustrated

Milky Way
• Barred Spiral Milky Way (Illustrated) • Radioactive Clouds in the Milky Way • Milky Way Molecule Map • The Milky Way’s Gamma-Ray Halo • Moving Milkyway seen from Teneriffe without any lightpollution • Multi-Gigapixel Infrared Milky Way A zoomable, annotated version of the Spitzer Space Telescope GLIMPSE survey.

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