Milky Way Galaxy

{{Infobox
 * name=Infobox Galaxy
 * title=Milky Way Galaxy

The Milky Way is the galaxy in which our Solar System (and thus Earth) is located, visible from Earth as a nebulous band of light in the sky.

{{Infobox }}
 * name=Infobox Galaxy
 * title=Milky Way Galaxy
 * image= [[Image:Milky Way IR Spitzer.jpg|300px]]
 * caption=Infrared image of the core of the Milky Way galaxy
 * titlestyle=background: #30D5C8;
 * headerstyle=background: #30D5C8;
 * belowstyle=background: #30D5C8;
 * labelstyle=background: inherit;
 * header1= Observation data
 * label2=Type
 * data2=SBbc (barred spiral galaxy)
 * label3=Diameter
 * data3=100,000 light years
 * label4=Thickness
 * data4=1,000 light years
 * label5=Number of stars
 * data5=100-400 billion (1–4&times;10{{sup|11}})
 * label6=Oldest known star
 * data6=13.2 billion years
 * label7=Mass
 * data7=5.8{{e|11}} M{{sub|☉}}
 * label8=Sun's distance to galactic center
 * data8=26,000 ± 1,400 light-years
 * label9=Sun's galactic rotation period
 * data9=220 million years (negative rotation)
 * label10=Spiral pattern rotation period
 * data10=50 million years
 * label11=Bar pattern rotation period
 * data11=15 to 18 million years
 * label12=Speed relative to CMB rest frame
 * data12=552 km/s
 * below= See also: Galaxy, List of galaxies

The Milky Way, or simply the Galaxy, is the galaxy in which the 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 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. However, it is unclear how widespread this convention is, and the term Milky Way is routinely used in either context.

Appearance from Earth
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 the Solar System lies close to the galactic plane. The Milky Way has a relatively low surface brightness, making it difficult to see from any urban or suburban location suffering from light pollution.

Size


The stellar disk of the Milky Way galaxy is approximately 100000 ly in diameter, and is concidered to be, on average, about 1000 ly thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, 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 12000 ly&mdash;twice the previously accepted value. As a guide to the relative physical scale of the Milky Way, if it were reduced to 10m in diameter, the Solar System, including the Oort cloud, would be no more than 0.1mm in width (0.001%).

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 ~180000 ly. 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 heavier than some previously thought. The mass 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 figure at about 254 km/s, significantly higher than the widely accepted value of 220 km/s. This in turn implies that the Milky Way has a total mass equivalent to around 3 trillion Suns, about 50% more massive than some previously thought.

Age


It is extremely difficult to define the age of the Milky Way 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.

This estimate is based on research by a team of astronomers in 2004 using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in globular cluster NGC 6397. 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.

Composition and structure


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. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1990s rather than an ordinary spiral galaxy. Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005 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 solar masses (M☉)  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.

Galactic center


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

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.

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 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. 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 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.

Observations presented in 2008 by Robert Benjamin of the University of Wisconsin-Whitewater 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.

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, 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, 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. With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the galactic disk.

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 light-years away.

Sun's location and neighborhood
The Sun (and therefore the Earth and Solar System) may be found close to the inner rim 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. The Sun is currently 5–30 parsecs from the central plane of the galactic disc. The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years. 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 light-year (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.

It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year), 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 light-year, or 8 days to travel 1 AU (astronomical unit).

Environment


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 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 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 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. 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.

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. 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 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.

Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s 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 Milky Way (decaying curve) is attributed to dark matter.

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. 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

In Sanskrit and several other Indo-Aryan languages, the Milky Way is called Akash Ganga (आकाशगंगा, Ganges of the heavens). The milky way is held to be sacred in the Hindu scriptures known as the Puranas, and the Ganges and the milky way are considered to be terrestrial-celestial analogs of each other. However, the term Kshira (क्षीर, milk) is also used as an alternative name for the milky way in Hindu texts.

In a large area from Central Asia to Africa, the name for the Milky Way is related to the word for "straw". This may have originated in ancient Armenian mythology, (Յարդ զողի Ճանապարհ hard goghi chanaparh, or "Trail of the Straw Thief"), and been carried abroad by Arabs. 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 Qi Xi legend celebrated in many Asian cultures references a seasonal bridge across the Milky Way formed by birds, usually magpies or crows.) The Chinese name "Silver River" (銀河) 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"(天汉). In Japanese, "Silver River" (銀河 ginga) means galaxies in general and the Milky Way is called the "Silver River System" (銀河系 gingakei) or the "River of Heaven" (天の川 Amanokawa or Amanogawa). In Swedish, it is called Vintergatan, or "Winter Avenue", 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"

Discovery


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 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." The Arabian astronomer, Alhazen (965-1037 AD), refuted this by making the first attempt at observing and measuring the Milky Way's parallax, 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."

The Persian astronomer, Abū Rayhān al-Bīrūnī (973-1048), proposed the Milky Way galaxy to be a collection of countless nebulous stars. 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. 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.

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. 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.

The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Galaxy with the Solar System close to the center.

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.

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. 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.

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. In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.

Appearance from Earth
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 the Solar System lies close to the galactic plane. The Milky Way has a relatively low surface brightness, making it difficult to see from any urban or suburban location suffering from light pollution.

Size


The stellar disk of the Milky Way galaxy is approximately 100000 ly in diameter, and is concidered to be, on average, about 1000 ly thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, 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 12000 ly&mdash;twice the previously accepted value. As a guide to the relative physical scale of the Milky Way, if it were reduced to 10m in diameter, the Solar System, including the Oort cloud, would be no more than 0.1mm in width (0.001%).

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 ~180000 ly. 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 heavier than some previously thought. The mass 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 figure at about 254 km/s, significantly higher than the widely accepted value of 220 km/s. This in turn implies that the Milky Way has a total mass equivalent to around 3 trillion Suns, about 50% more massive than some previously thought.

Age


It is extremely difficult to define the age of the Milky Way 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.

This estimate is based on research by a team of astronomers in 2004 using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in globular cluster NGC 6397. 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.

Composition and structure


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. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1990s rather than an ordinary spiral galaxy. Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005 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 solar masses (M☉)  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.

Galactic center


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

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.

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 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. 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 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.

Observations presented in 2008 by Robert Benjamin of the University of Wisconsin-Whitewater 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.

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, 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, 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. With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the galactic disk.

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 light-years away.

Sun's location and neighborhood
The Sun (and therefore the Earth and Solar System) may be found close to the inner rim 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. The Sun is currently 5–30 parsecs from the central plane of the galactic disc. The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years. 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 light-year (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.

It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year), 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 light-year, or 8 days to travel 1 AU (astronomical unit).

Environment


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 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 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 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. 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.

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. 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 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.

Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s 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 Milky Way (decaying curve) is attributed to dark matter.

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. 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

In Sanskrit and several other Indo-Aryan languages, the Milky Way is called Akash Ganga (आकाशगंगा, Ganges of the heavens). The milky way is held to be sacred in the Hindu scriptures known as the Puranas, and the Ganges and the milky way are considered to be terrestrial-celestial analogs of each other. However, the term Kshira (क्षीर, milk) is also used as an alternative name for the milky way in Hindu texts.

In a large area from Central Asia to Africa, the name for the Milky Way is related to the word for "straw". This may have originated in ancient Armenian mythology, (Յարդ զողի Ճանապարհ hard goghi chanaparh, or "Trail of the Straw Thief"), and been carried abroad by Arabs. 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 Qi Xi legend celebrated in many Asian cultures references a seasonal bridge across the Milky Way formed by birds, usually magpies or crows.) The Chinese name "Silver River" (銀河) 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"(天汉). In Japanese, "Silver River" (銀河 ginga) means galaxies in general and the Milky Way is called the "Silver River System" (銀河系 gingakei) or the "River of Heaven" (天の川 Amanokawa or Amanogawa). In Swedish, it is called Vintergatan, or "Winter Avenue", 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"

Discovery


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 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." The Arabian astronomer, Alhazen (965-1037 AD), refuted this by making the first attempt at observing and measuring the Milky Way's parallax, 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."

The Persian astronomer, Abū Rayhān al-Bīrūnī (973-1048), proposed the Milky Way galaxy to be a collection of countless nebulous stars. 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. 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.

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. 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.

The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Galaxy with the Solar System close to the center.

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.

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. 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.

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. In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.

M31 (Andromeda Galaxy)
The Andromeda Galaxy has appeared in a number of science fiction works, as a source of alien life forms that invade the Milky Way galaxy, or as a setting for alien worlds and territories. Examples include the following:

Literature

 * The novel Andromeda Nebula by Ivan Yefremov is named after the Andromeda Galaxy, even though the galaxy itself is only mentioned once, towards the end of the book.
 * Much of the fighting in the Revelation Space universe by Alastair Reynolds is caused by one race attempting to prepare the Milky Way for the impending collision with Andromeda, and attempting to wipe out any other race (including humanity) to prevent them interfering.
 * The Andromeda Galaxy plays an important role in the German space opera Perry Rhodan, from the third cycle on.

Comics

 * In Superman: Birthright, writer Mark Waid describes Krypton as a megagravity planet circling an M3 star in the Andromeda Galaxy.
 * In Marvel Comics, the Skrull empire consists mainly of "about a thousand worlds" in the Andromeda galaxy. The Nova Corps of Xandar, and the Symbiotes of Klyntar are also from the Andromeda galaxy.
 * In Schlock Mercenary, the Fleetmind Artificial Intelligence character known as Petey (also P.D., Post-Dated Check Loan, Sword of Inevitable Justice, The Rogue) is currently prosecuting a war against the Paan'uri in their home galaxy of Andromeda.

Film and television

 * In the 1961 BBC television series A for Andromeda, co-authored by astronomer Fred Hoyle, a radio signal is received from the direction of the Andromeda Galaxy. The signal contained instructions for building a super-computer, as well as the code needed to make it run. A year later the sequel series The Andromeda Breakthrough was released.
 * In The Galaxy Being, the debut episode of The Outer Limits (1963 TV series), an engineer for a small radio station somehow makes contact with a peaceful alien creature from the Andromeda Galaxy, who gets transported to Earth by accident.
 * In the 1968 Star Trek episode "By Any Other Name", the starship Enterprise is hijacked by Kelvans, aliens from an empire spanning the Andromeda Galaxy; their home galaxy was approaching an uninhabitable state and thus they aimed to conquer our own. In another episode, I, Mudd, the lead android says their creators were humanoids from "Andromeda".
 * The second season of the 1970s anime series Space Battleship Yamato (known as Star Blazers to Western audiences) featured the Comet Empire, a massive empire contained in an artificial comet, which sought to conquer Earth after having enslaved the entire Andromeda Galaxy.
 * In Gene Roddenberry's Andromeda, the Systems Commonwealth had territory in the Andromeda Galaxy, the Triangulum Galaxy and the Milky Way.
 * Doctor Who episodes:
 * The Ark in Space — Star Pioneers from Earth reached Andromeda where they fought a war with the Wirrn.
 * Evolution (novel) — The Doctor takes a group of humans who have mutated into mermaids to live on a water planet in the Andromeda Galaxy.
 * In the seventh episode in the first series of the British comedy sketch show Monty Python's Flying Circus, blancmanges from the planet Skyron in the Andromeda galaxy were a major plot point. For instance, they are seen to convert people into stereotypical Scotsmen in order to win the Wimbledon tennis tournament, as it was well known that the Scots cannot play tennis (!).
 * In the Renkin 3-kyū Magical? Pokān episode "The Spell of Dreams is Magic That Lasts One Day," Aiko has Uma use the magical Happy Chalk to draw a galactic train that could take them to the Andromeda Galaxy as one of Aiko's attempts to gain a human body since she has heard about something like this happening there before. Uma draws up the galactic train and they end up riding it to the Andromeda Galaxy even getting there faster when Uma drew a turbo boose enough for a trip due to the effects of the Happy Chalk lasting until the end of the day's first use. Upon landing on a planet in the Andromeda Galaxy, Uma, Pachira, Liru, and Aiko head to the area near Andromeda Tanashi where a robot would gain a real body or vice versa. It turns out that it takes a full day in order for the body changing to occur. The princesses leave and Aiko now knows that what she heard about the Andromeda Galaxy was true all along.
 * The Guardians of the Galaxy films take place primarily in the Andromeda Galaxy.

Games

 * In the Star Fleet Universe games, the Andromedans (who may or may not have been Kelvans; see Star Trek above) launch a devastating war on the Milky Way, using the Lesser Magellanic Cloud as a stepping-stone.
 * In the Sega video game series Phantasy Star, the instruction booklet of the first game states that the Algol Solar System is located deep in the Andromeda Galaxy (while the real Algol is only about 90 light years away from us)
 * In Star Trek Online, the alien race known as the Iconians are revealed to have a Gateway network and a Dyson Sphere in the Andromeda Galaxy, which is where they have been hiding since fleeing their homeworld in the Milky Way. Once their invasion of the Milky Way begins, the sphere teleports itself there.
 * In Ben 10 Ultimate Alien: Cosmic Destruction, the five Andromeda aliens (listed above) are playable forms for Ben as Water Hazard, Terraspin, NRG, Armodrillo, and AmpFibian.
 * In the Space Quest series, there are recurring characters, who are the alter egos of the creators, called the Two Guys from Andromeda.
 * In Star Ocean: Till the End of Time, the Andromeda Galaxy is referenced when a message regarding the "closing" of the Milky Way Galaxy server is issued to all Eternal Sphere players. Due to the player party's emergence into 4D space from the Milky Way server of the Eternal Sphere MMO, all current players in that server are advised to relocate to the Andromeda server, which houses its titular galaxy.
 * Mass Effect: Andromeda (2017), a video game developed by BioWare Montreal and published by Electronic Arts. The game is a spin off of the original Mass Effect trilogy, set 600 years following the events of Mass Effect 2. In the game, each race participating in the initiative sends a large ship, called an Ark, on a 600-year voyage to Andromeda. Each ship contains the races' best and brightest volunteers, suspended in cryosleep. The player takes control of one of the customizable Ryder twins and becomes the "Pathfinder", responsible for making decisions in how the humans will settle the new galaxy, and how humanity will help find the other races' missing arks.

Music

 * The Andromeda galaxy has also featured heavily in the central theme of Enigma's sixth album A Posteriori, in which a collision involving the Milky Way is prophesied to occur in the distant future. The most explicit reference to this appears at the album's close with a doom-laden yet mysterious statement, suggesting a collision of the galaxies will give rise to a "new, gigantic cosmic world" being born.
 * M31 appears in the song "Out of the White Hole" of the album Universal Migrator Part 2: Flight of the Migrator by Ayreon.
 * In the story of the ship named Iron Savior, as told by the band Iron Savior, in the Andromeda Galaxy there is a region of space called the Realm Of Steel, with a Machine World at its center. Both the Realm Of Steel and the Machine World are feared by the rest of the galaxy.

Large Magellanic Cloud

 * In the comedy film My Stepmother Is an Alien (1988), the Clouds of Megallan is a galaxy 92 light-years from Earth, and the home world of the alien Celeste Martin.
 * In Olaf Stapledon's 1937 science fiction novel Star Maker, there is a group of aliens that live in the Large Magellanic Cloud called the Symbiotics, a super-intelligent symbiotic race composed of telepathically linked arachnoid beings and whale-like ichthyoid beings. The "Symbiotics" are the most advanced intelligent life in the Milky Way Galaxy and its satellite galaxies. The reason is because they combine the manipulative tool-using intelligence of the arachnoids with the contemplative meditative and mathematical intelligence of the ichthyoids. The "Symbiotics" travel in starships equipped with tanks of water for the ichthyoids and piloted by the arachnoids. They conduct vast terraforming projects, terraforming planets in numerous different planetary systems. They also construct large artificial planetoids, hollow spheres filled with water inhabited on the inside by the icthyoids and on the surface by the arachnoids. They have the most advanced and powerful telepathic powers of any race in our galaxy and its satellites and are thus able to stop the War of Galactic Empires within the Milky Way Galaxy by telepathically attacking the military forces of the various Galactic Empires and causing them to have doubts about imperialism. This causes the various Imperial military forces to become totally disorganized. The Galaxy is led into a new era of galactic peace supervised by the democratic and communistic Galactic Community of Worlds, which emerges after the fall of the Galactic Empires.
 * The Large Magellanic Cloud is the destination of the spacecraft Yamato in the first season of the 1970s anime series Space Battleship Yamato (known as Space Cruiser Yamato or Star Blazers to Western audiences). The LMC is the host galaxy for the destination, Iscandar, and the home planet of the alien force attacking Earth, Gamilas. The Yamato must use an untested faster-than-light drive to make the 296,000 light year round trip in one Earth year. (Note: in the 1974 original, the one-way distance was given as 148,000 LY; in the reimagined Space Battleship Yamato 2199, this is corrected to 168,000 LY to reflect more-recent measurements.)
 * In Scientology, these are said to be literal clouds. They are involved in the fictional story of Helatrobus.
 * In Marvel Comics, the Large Magellanic Cloud is home to the Kree empire but its territory may extend into other galaxies.
 * In Arthur C Clarke's novel Rendezvous with Rama, the giant spaceship Rama is at the end directed towards the Large Magellanic Cloud. Its journey through the solar system was apparently just to use the Sun to refuel and as a gravitational slingshot.
 * In The Forever War, a 1974 science fiction novel by Joe Haldeman, a significant battle takes place on a 'collapsar' portal planet in the Large Magellanic Cloud.
 * In James Blish's series Cities in Flight, hero John Amalfi and the New Earth planet escape to the Greater Magellenic Cloud to avoid unjust prosecution for an atrocity.
 * In the Robert Silverberg novel Collision Course, a wayward space ship trying to fly back to Earth gets lost in "null-space" and ends up emerging into normal space within the Large Magellanic Cloud; the captain suggests they find a permanent home there as their chances of getting home are extremely remote and they are fortunate to have dropped into normal space within a galaxy. Ultimately, they are returned to the Milky Way by sympathetic, superior aliens. The Milky Way galaxy is easily visible, evoking a feeling of loneliness and separation when the crew realize they're looking at their home galaxy.
 * In the game Infinite Space, the main character flees to the Large Magellanic Cloud after the civilizations of the Small Magellanic Cloud are taken over by an invading empire. Most of the second half of the game is spent in this galaxy.
 * In the Deadlands: Lost Colony roleplaying game, the Faraway system is located in the Large Magellanic Cloud.
 * In Halo, the parasitic Flood was said to have come from Large Magellanic Cloud on ancient cargo ships. In the novel Halo: Silentium, Forerunners (who called it "Path Kethona") were revealed to have chased the surviving Precursors to the Magellanic Cloud during a Forerunner war and ruthlessly killed off nearly the entire species. The Forerunners that committed this act were ashamed and lived the rest of their lives on a planet in the Magellanic Cloud.
 * In the soundtrack of the game Starbound, there is a song titled "Large Magellanic Cloud".
 * In The Mixed Men fix-up by A. E. van Vogt, the culture clash between imperial Earth and the local colonies propelling the plot happens in the Large Magellanic Cloud.
 * In Beyond the Aquila Rift by Alastair Reynolds a faster than light travel accident sends the protagonist not merely beyond the titular "Aquila Rift" (~400 light years off course), but to the Large Magellanic Cloud (~150,000 light years off course). Dealing with the reality that hundreds of years have passed outside of FTL travel (and that they have no way to send him home) is a key point of the story.

Small Magellanic Cloud



 * The Small Magellanic Cloud is the site of the climax of Robert A. Heinlein's novel Have Space Suit—Will Travel, in which the spectacular view of the Milky Way is admired.
 * In two Star Trek books from Pocket Books – The Lost Era: The Sundered and Titan: The Red King, the Small Magellanic Cloud is home to the Neyel, an offshoot of humanity who dominated several species native to the Cloud.
 * The Small Magellanic Cloud is the location of an Alderson disk onto which the surface of Cold War-era Earth is transplanted in Charles Stross's short story "Missile Gap."
 * In the Star Fleet Universe, the Small Magellanic Cloud (referred to in that setting as the Lesser Magellanic Cloud) is home to a number of starfaring empires; such as the Baduvai Imperium, the Eneen Protectorates and the Maghadim Hives. All of these empires were conquered by the Andromedans, who then used the Cloud as a jumping off point into the Milky Way. The Cloud was eventually liberated from Andromedan rule in Operation Unity; a coalition of task forces sent by the Federation, Klingons and other Alpha Octant powers.
 * Iain M. Banks' novel The Player of Games is partially set in the Empire of Azad in the Small Magellanic Cloud.
 * In Dan Simmons' Hyperion series, the planet Earth is thought swallowed up by a black hole but is actually transported to somewhere within the Lesser Magellanic Cloud.
 * In Edmond Hamilton's Star Kings series, a race of powerful telepaths called H'Harn originated in the Small Magellanic Cloud has made two attempts to conquer the Milky Way Galaxy.
 * Salo, the space traveler stranded on Titan in Kurt Vonnegut's The Sirens of Titan, hails from the fictional planet Tralfamadore, located in the Small Magellanic Cloud.

M33 ('Triangulum Galaxy')

 * In the novel Voyage of the Space Beagle by A.E. Van Vogt, the starship Space Beagle is sent out by Earth to investigate alien life forms. When it reaches the M33 galaxy in Triangulum, it encounters an enormous disembodied life form called The Anabis that covers the entire galaxy.  "The Anabis" turns planets into jungle planets because it lives off the life force of living beings when they die and jungle planets have the highest rate of ecological energetics.  Since "The Anabis" has destroyed civilized planets in this way, the crew of the starship devises a way to defeat this being.
 * In the fictional Star Trek universe, the Triangulum Galaxy is where the USS Enterprise-D is warped to after a being known as The Traveler uses his knowledge to influence the ship's warp drive. M33 is said to be approximately 2.7 million light years away from the United Federation of Planets territories, or about three hundred years' travel time for the Federation's most advanced vessels. The galaxy was first encountered in the Star Trek: The Next Generation episode "Where No One Has Gone Before".
 * In the Star Fleet Universe, the Triangulum Galaxy was once dominated by a now-defunct precursor empire; the civil war, which tore the Old Empire asunder, left two major successor states (the loyalist Imperium and the rebel Frigian Kingdom) picking up the pieces. Another elder species, the Helgardians, appeared in M33 at the same time; in the modern era, an array of younger powers (such as the Mallaran Empire) would emerge on the back of Helgardian-traded warp technology.
 * The second season finale of seaQuest DSV featured the seaQuest being transported to the planet Hyperion, over eleven million light years away from Earth in the Triangulum Galaxy.
 * On Gene Roddenberry's Andromeda the Systems Commonwealth had territory in the Triangulum Galaxy in addition to the Andromeda and Milky Way Galaxies. The new homeworld of which, Tarazed, was located in this Galaxy.
 * In the Belgian comic book series Yoko Tsuno written by Roger Leloup, M33 is the home of the blue-skinned Vineans, whom the main characters befriend. About a third of the stories take place in this galaxy.
 * In the Orion's Arm universe, the Triangulum galaxy is home to an extra-galactic civilization, having sent a powerful warning message about a massive artificial object heading towards the local group several million years ago.
 * In the 2004 Iain M. Banks novel The Algebraist, the Triangulum Nebula was the starting point of the Long Crossing, a 30-million-year journey undertaken by the Dweller civilization to return to the world of Nasqueron where much of the novel is set.
 * In the Crysis video game series, the alien antagonists, named the "Ceph", originate from this galaxy.
 * The 2000 computer game Gunman Chronicles takes place in M33, where humanity has established multiple colonies and has the ability to trigger supernovae.
 * In March 2012 the news satire website The Onion reported that a fleet of warships from the planet Zarklan 12 in the Triangulum Galaxy has arrived at Earth to intervene in the Syrian War on the side of the Syrian rebel alliance. "Supreme Emperor and Dynastic Overlord Thuu'l", the Onion reported, is disturbed by Earth's apathy toward the massacres of innocent Syrians.
 * The promotional posters for the movie E.T. the Extra-Terrestrial give the phrase "three million lightyears from home", leading many to believe that the title character hails from M33. Furthermore, the Easter egg of E.T.'s race appearing in Star Wars: Episode I – The Phantom Menace have many Star Wars fans pointing to M33 as the "galaxy far, far away".
 * In the 2016 Al DesHotel novel 235, The Triangulum Galaxy was a destination for exploration by the Alaytion Empire by the Emperor Kaajii Morrell.

'Pegasus Galaxy'

 * The Pegasus Dwarf Irregular Galaxy is the "Pegasus Galaxy" in which the television series Stargate Atlantis takes place.

M51 ('Whirlpool Galaxy')

 * The Whirlpool Galaxy or M51 was used as the basis for the galaxy in which the story of the Homeworld series of games takes place. The original Homeworld game takes the player on a journey from the galaxy's outer rim to its core; this is accurately illustrated by the background images as the game campaign progresses.
 * The Whirlpool Galaxy appears in the video game, Ōkami where it is summoned by the main character, Amaterasu, to create a whirlpool in the ocean that she uses to travel to an aquatic palace.
 * The three teenage alien protagonists of the Roswell (TV series)--itself a spin-off of the Roswell High novels—are said to come from the Five Worlds of the Whirlwind Galaxy.
 * The Whirlpool Galaxy or M51 appears in the novels The Genesis Quest and Second Genesis by Donald Moffitt.  In the novels, future humans beam the sum of human knowledge towards the Virgo supercluster, which is received and understood by aliens in the intervening M51 galaxy. The information, 37 million years after transmission, is used for the aliens to reconstruct mankind long after they ceased to exist in the Milky Way galaxy.
 * In Mighty Morphin' Power Rangers, Master Vile, father of Rita Repulsa, is the ruler of the M51 galaxy. As such, it is likely that Rita herself originates there, though that is not confirmed.

M81

 * In the fictional Star Fleet Universe, M81 is the home galaxy for a number of star-faring species, including the Tholians (who conquered that galaxy and ruled it for many centuries), the Nebuline (one of the Tholians' former enemies, whose remaining nebula-bound colonies laid the groundwork for M81 piracy. ) and the Seltorians (the last of a series of enforcer species for the Tholians; their Revolt virtually exterminated the Tholians as a people, and their subsequent Tribunal fleets pursued exile Tholians to other galaxies, such as the Milky Way ) and the Bolosco (some of whom established themselves in the Omega Octant of the Milky Way).

M83

 * The French synthpop band M83 are named after this galaxy.

"Hey kid, are you going my way? Hop in, we'll have ourselves a field day. We'll find us some spacegrass, Lay low, watch the universe expand. Skyway, permanent Saturday. Oh, by the way, Saturn is my rotary. Hop in, it'll be eternity Till we make it to M83."
 * The band Clutch makes reference to M83 in their song Spacegrass off their self titled album.

M86

 * In the TV show Andromeda, M86 is cited as being the origin of the Magog invasion of the three galaxies in the season 2 opener "The Widening Gyre".

Wolf-Lundmark-Melotte Galaxy

 * Part of the Lensman series by E. E. Smith takes place in Lundmark's Nebula. This is a clear reference to astronomer Knut Lundmark whose particular area of specialization was the study of galaxies, and may refer to the Wolf-Lundmark-Melotte Galaxy. — albeit this hypothesis would be contradicted by the series establishing material stating that "Lundmark's Nebula" and the Milky Way Galaxy collided "edge-on" during the "planet-forming era, as well as by stated discrepancies in the books regarding distances and structures compared to the real Wolf-Lundmark-Melotte."
 * The Doctor Who novel Synthespians by Craig Hinton refers to the New Earth Republic of the 101st Century and beyond, which spearheads a programme of colonisation, sending sleeper ships to the Wolf-Lundmark-Melotte galaxy and Andromeda.

Fictional galaxies

 * Murray Leinster's novel The Last Spaceship has escapees from our galaxy founding a new civilization in a remote galaxy.
 * The Stargate movie and television series feature several fictional galaxies, including the Kaliem galaxy (a vague reference to the constellation Caelum, perhaps), in which the planet Abydos is located in the film (this was later retconned to be in the Milky Way), and the Ida galaxy, home of the Asgard alien species. The Ori in Stargate SG-1 come from another unnamed galaxy that's in a different supercluster of Galaxies, and use the Supergate to travel to the Milky Way galaxy.
 * In Marvel Comics, the Shi'ar Imperium is within a region referred to as the "Shi'ar Galaxy" and is controlled by the Shi'ar race. As this region is said to be situated close to the Skrull and Kree Empires mentioned above, and is one of the three main alien empires of the Marvel Universe, the home of the Shi'ar empire may actually be the Triangulum Galaxy or one of the many dwarf galaxies within the Local Group. The Black Galaxy is the home of Ego the Living Planet.
 * Star Wars is set "A long time ago, in a galaxy far, far away...".
 * In the Superman film series, Krypton is said to be located in the "Xeno Galaxy".
 * In the Doctor Who series, the Face of Boe is said to be the oldest living inhabitant of the Isop galaxy. The insectoid inhabitants of the planet Vortis were also from the Isop galaxy.  Representatives of the six "Outer Galaxies" meet in The Daleks' Master Plan.
 * In Power Rangers: Lost Galaxy, Terra Venture and the Lost Galaxy Rangers travel to the 'Lost Galaxy', a galaxy that seems to exist outside our dimension and which can only be reached by portal. After many adventures in this galaxy Terra Venture and the Rangers escape the Lost Galaxy through a portal opened by reciting the Lost Galaxy Spell backwards.
 * In the book Skylark of Valeron (part of the Skylark of Space series by E. E. Smith), the protagonists visit another galaxy after being rotated through the fourth dimension and visit a number of different galaxies in the last book in the series, Skylark DuQuesne.
 * The finale of the 2007 anime series Tengen Toppa Gurren Lagann features the titular mecha as being larger than a galaxy, and capable of wielding small galaxies as deadly projectiles.
 * In the Mel Brooks film Spaceballs Captain Lone Starr hails from the 'Ford' galaxy, in reference both to Harrison Ford and the automobile Ford Galaxie.
 * Insomniac Games' popular video game franchise 'Ratchet & Clank' features three fictional galaxies called the Solana Galaxy, the Bogon Galaxy, and the Polaris Galaxy.
 * In the 1956 book Islands of Space (part of the Arcot, Wade, and Morey stories by John W. Campbell) the protagonists are lost in space and ask the astronomers of a planet in another galaxy to help them find the Milky Way by looking for a spiral galaxy with two small satellite galaxies, the Large and Small Magellanic Clouds.
 * In Episode 1.21 of The Outer Limits, "The Children of Spider County," the alien/human hybrids are said to be from the Krell Galaxy (an allusion to the fictional extinct extraterrestrial race, native to Altair IV, in the s.f. film classic Forbidden Planet).
 * In Space Battleship Yamato (also known as Star Blazers), the final major battle before reaching Gamilas/Iscandar is said to occur in what the characters call "The Rainbow Galaxy". However, onscreen, this looked more like a group of planetary masses than an actual galaxy. Presumably, the name comes from the different color of the different planets. This designation was only used in the English language Star Blazers. In the original Japanese Yamato, it was properly called the Rainbow System.
 * In the lore of space MMORPG EVE Online humanity has colonized a distant galaxy named "New Eden", where the game takes place.
 * Dimension X in the Teenage Mutant Ninja Turtles stories is in some TMNT versions thought to be a galaxy. This is especially featured in the TMNT Adventures comic book.