Milky Way Galaxy

An image of the Milky Way's Galactic Center in the night sky above Paranal Observatory
Observation data
Type	SBc[1] (barred spiral galaxy)
Diameter	100–120 kly (31–37 kpc)[2]
Thickness	1.0 kly (0.31 kpc)[2]
Number of stars	300 ± 100 billion[3][4]
Oldest known star	13.2 Gyr[5]
Mass	1.0-1.5×1012 M☉[6]
Sun's distance to Galactic Center	27.2 ± 1.1 kly (8.3 ± 0.34 kpc)[7]
Sun's Galactic rotation period	250 Myr (negative rotation)[8]
Spiral pattern rotation period	50 Myr[9]
Bar pattern rotation period	15–18 Myr[9]
Speed relative to CMB rest frame	552 ± 6 km/s[10]
See also: Galaxy, List of galaxies

The Milky Way is the galaxy that contains the Earth.^{[11][nb 1]} This name derives from its appearance as a dim "milky" glowing band arching across the night sky, in which the naked eye cannot distinguish individual stars. The term "Milky Way" is a translation of the Classical Latin via lactea, from the Hellenistic Greek γαλαξίας κύκλος (pr. galaxías kýklos, "milky circle").^[12][13][14] The Milky Way appears like a band because it is a disk-shaped structure being viewed from inside. The fact that this faint band of light is made up of stars was proven in 1610 when Galileo Galilei used his telescope to resolve it into individual stars. In the 1920s, observations by astronomer Edwin Hubble showed that the Milky Way is just one of many galaxies.

The Milky Way is a barred spiral galaxy 100,000–120,000 light-years in diameter containing 200–400 billion stars. It may contain at least as many planets, with an estimated 10 billion of those orbiting in the habitable zone of their parent stars.^[15] The Solar System is located within the disk, around two thirds of the way out from the Galactic Center, on the inner edge of a spiral-shaped concentration of gas and dust called the Orion–Cygnus Arm. The stars in the inner ≈10,000 light-years are organized in a bulge and one or more bars. The very center is marked by an intense radio source named Sagittarius A* which is likely to be a supermassive black hole. The Galaxy rotates once every 15 to 50 million years. The Galaxy as a whole is moving at a velocity of 552 to 630 km per second, depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe. Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a subcomponent of the Virgo Supercluster.

Appearance[link]

A view of the Milky Way towards the Constellation Sagittarius (including the Galactic Center) as seen from a non-light polluted area (the Black Rock Desert, Nevada).

When observing the night sky, the term "Milky Way" is limited to the hazy band of white light some 30 degrees wide arcing across the sky^[16] (although all of the stars that can be seen with the naked eye are part of the Milky Way Galaxy). The light in this band originates from un-resolved stars and other material that lie within the Galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, correspond to areas where light from distant stars is blocked by interstellar dust.

The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light such as light pollution or stray light from the moon. It is readily visible when the limiting magnitude is +5.1 or better, while showing a great deal of detail at +6.1.^[17] This makes the Milky Way difficult to see from any brightly-lit urban or suburban location but very prominent when viewed from a rural area when the moon is below the horizon.^{[nb 2]}

The Milky Way passes through parts of roughly 30 constellations. The center of the Galaxy lies in the direction of the constellation Sagittarius; it is here that the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass westward to the Galactic anticenter in Auriga. The band then continues westward the rest of the way around the sky back to Sagittarius. The fact that the band divides the night sky into two roughly equal hemispheres indicates that the Solar System lies close to the Galactic plane.^{[citation needed]}

The Galactic plane is inclined by about 60 degrees to the ecliptic (the plane of the Earth's orbit). 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 north Galactic pole is situated at right ascension 12^h 49^m, declination +27.4° (B1950) near beta Comae Berenices, and the south Galactic pole is near alpha Sculptoris. Because of this high inclination, depending on the time of night and the year, the arc of Milky Way can appear relatively low or relatively high in the sky. For observers from about 65 degrees north to 65 degrees south on the Earth's surface the Milky Way passes directly overhead twice a day.

A fish-eye mosaic of the Milky Way arching at a high inclination across the night sky, shot from a dark sky location in Chile.

Size and composition[link]

The stellar disk of the Milky Way Galaxy is approximately 100,000 light-years (30 kiloparsecs) in diameter, and is, on average, about 1,000 ly (0.3 kpc) thick.^[2] As a guide to the relative physical scale of the Milky Way, if it were reduced to 100 meters (110 yd) in diameter, the Solar System, including the hypothesized Oort cloud, would be no more than 1 millimeter (0.039 in) in width, or a grain of sand in a sports field. The nearest star, Proxima Centauri, would be 4.2 mm (0.17 in) distant.^{[nb 3]}

The Milky Way contains at least 100 billion stars^[18] and may have up to 400 billion stars.^[19][20] The exact figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (10¹²) stars.^[21] Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars,^[22] while the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas.^[23][24] Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way,^[15][25] while microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.^[26][27] Earth-sized planets may be more numerous than gas giants.^[15]

The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars drops smoothly with distance from the center of the Galaxy. Beyond a radius of roughly 40,000 ly (12 kpc), the number of stars per cubic parsec drops much faster with radius,^[28] for reasons that are not understood. Surrounding the Galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach to the Galactic center is about 180,000 ly (55 kpc).^[29] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would likely be ejected from the vicinity of the Milky Way. The integrated absolute visual magnitude of the Milky Way is estimated to be −20.9.^[30]

360-degree panorama view of the Milky Way Galaxy (an assembled mosaic of photographs).

Estimates for the mass of the Milky Way vary, depending upon the method and data used. At the low end of the estimate range, the mass of the Milky Way is 5.8×10¹¹ solar masses (M_☉), somewhat smaller than the Andromeda Galaxy.^[31][32][33] Measurements using the Very Long Baseline Array in 2009 found velocities as large as 254 km/s for stars at the outer edge of the Milky Way, higher than the previously accepted value of 220 km/s.^[34] As the orbital velocity depends on the total mass inside the orbital radius, this suggests that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7×10¹¹ M_☉ within 50 kiloparsecs (160,000 ly) of its center.^[35] A 2010 measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×10¹¹ M_☉.^[36] Most of the mass of the Galaxy appears to be matter of unknown form which interacts with other matter through gravitational but not electromagnetic forces; this is dubbed dark matter. A dark matter halo is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center. Mathematical models of the Milky Way suggests that the total mass of the entire Galaxy lies in the range 1-1.5×10¹² M_☉.^[6]

Structure[link]

Artist's conception of the spiral structure of the Milky Way with two major stellar arms and a bar.^[37]

A false-color infrared image of the core of the Milky Way Galaxy taken by NASA's Spitzer Space Telescope. Older cool stars are blue, dust features lit up by large hot stars are shown in a reddish hue, and the bright white spot in the middle marks the site of Sagittarius A*, the super-massive black hole at the center of the Galaxy.

The Galaxy consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. The gas, dust and stars are organized in roughly logarithmic spiral arm structures (see Spiral arms below). The mass distribution within the Galaxy closely resembles the SBc Hubble classification, which is a spiral galaxy with relatively loosely wound arms.^[1] Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s.^[38] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005^[39] that showed the Galaxy's central bar to be larger than previously suspected.

Galactic Center[link]

The Sun is 8.0–8.7 kpc (26,000–28,000 ly) from the Galactic Center. This value is estimated based upon geometric-based methods or using selected astronomical objects that serve as standard candles, with different techniques yielding different values within this approximate range.^{[7][40][41][42][43]} In the inner few kpc (≈10,000 light-years) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge.^[44]

The Galactic Center is marked by an intense radio source named Sagittarius A*. The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object.^[45] This concentration of mass is best explained as a supermassive black hole^{[nb 4][7][40]} with an estimated mass of 4.1–4.5 million times the mass of the Sun.^[40] Observations indicate that there are supermassive black holes located near the center of most normal galaxies.^[46][47]

The nature of the Galaxy's bar is actively debated, with estimates for its half-length and orientation spanning from 1–5 kpc (3,300–16,000 ly) (short or a long bar) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.^[42][43][48] Certain authors advocate that the Galaxy features two distinct bars, one nestled within the other.^[49] The bar is delineated by red clump stars. However, RR Lyr variables do not trace a prominent Galactic bar.^[43][50][51] The bar may be 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.^[52]

Spiral arms[link]

Beyond the gravitational influence of the Galactic bars, astronomers generally organize the interstellar medium and stars in the disk of the Milky Way in four spiral arms.^[53] All of these arms contain more interstellar gas and dust than the Galactic average as well as a high concentration of star formation, traced by H II regions^[54][55] and molecular clouds.^[56] Counts of stars in near infrared light indicate that two arms contain approximately 30% more red giant stars than would be expected in the absence of a spiral arm, while two do not contain more red giant stars than regions outside of arms.^[57][58]

Maps of the Milky Way's spiral structure are notoriously uncertain and exhibit striking differences.^{[37][53][55][59][60][61][62][63]} Some 150 years after Alexander (1852)^[64] first suggested that the Milky Way was a spiral, there is currently no consensus on the nature of the Galaxy's spiral arms. Perfect logarithmic spiral patterns ineptly describe features near the Sun,^[55][62] namely since galaxies commonly exhibit arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.^[43][62][63] The possible scenario of the Sun within a spur / Local arm^[55] emphasizes that point and indicates that such features are likely not unique, and exist elsewhere in the Galaxy.^[62]

As in most spiral galaxies, each spiral arm can be described as a logarithmic spiral. Estimates of the pitch angle of the arms range from ≈7° to ≈25°.^[57][65] Until recently, there were thought to be four major spiral arms which all start near the Galaxy's center. These are named as follows, with the positions of the arms shown in the image at right:

Observed and extrapolated structure of the spiral arms. The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations.

Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit around the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giant stars and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm.^[57][58] In 2008, Robert Benjamin of the University of Wisconsin–Whitewater used this observation to suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess stars.^[37] This would mean that the Milky Way is similar in appearance to NGC 1365.^{[citation needed]}

Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), proposed by astronomers Brian Yanny and Heidi Jo Newberg, a ring 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 their distance from the center. Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240 km/s.^[67] 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 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 pattern is a 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. 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.

Halo[link]

The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center,^[68] 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. About 40% of the galaxy's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation.^[69] The globular clusters can follow rosette orbits about the Galaxy, in contrast to the elliptical orbit of a planet around a star.^[70]

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.

Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought,^[71] the possibility of the disk of the Milky Way Galaxy extending further is apparent, and this is supported by evidence from the 2004 discovery of the Outer Arm extension of the Cygnus Arm.^[66][72] 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 (9 kpc) away.^[73]

Illustration of the two gigantic X-ray/gamma-ray bubbles (blue-violet) of the Milky Way (center).

Gamma-ray bubbles[link]

On November 9, 2010, Doug Finkbeiner of the Harvard–Smithsonian Center for Astrophysics announced that he had detected two gigantic spherical bubbles of high energy emission that are erupting to the north and the south of the Milky Way core, using data of the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc); they stretch up to Grus and to Virgo on the night-sky of the southern hemisphere. Their origin remains unclear, so far.^[74][75]

Sun's location and neighborhood[link]

Diagram of the Sun location in the Milky Way Galaxy. The angles represent longitudes in the galactic coordinate system.

Diagram of the stars in the Solar neighborhood.

The Sun (and therefore the Earth and the 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 8.33 ± 0.35 kiloparsecs (27,200 ± 1,100 ly) from the Galactic Center.^[7][40][76] The Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the Galactic disk.^[77] The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years (2.0 kpc).^[78] 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 (49 ly) 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 (16 ly) 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.^{[citation needed][original research?]}

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 were until recently thought to coincide with mass extinction periods on Earth.^[79] However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find these correlations.^[80]

It takes the Solar System about 225–250 million years to complete one orbit around the Galaxy (a Galactic year),^[81] so the Sun is thought to have completed 18–20 orbits during its lifetime 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 or 0.073% of the speed of light. 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).^[82]

Formation[link]

This section requires expansion.

The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang. Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. These stars and clusters now comprise the stellar halo of the Galaxy. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk.^[83][84]

Since the first stars began to form, the Milky Way has grown through both galaxy mergers and accretion of gas directly from the Galactic halo.^[84] The Milky Way is currently in the process of stripping material from its two nearest satellite galaxies, the Large and Small Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in high velocity clouds like the Smith Cloud.^[85][86]

Age[link]

The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238, then comparing the results to estimates of their original abundance, a technique called nucleocosmochronology. These yield values of about 14.0 ± 2.4 Ga for CS 31082-001 and 13.8 ± 4 Ga for BD+17° 3248. Once a white dwarf star is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as 12.7 ± 0.7 Ga. Globular clusters are among the oldest objects in the Milky Way Galaxy, which thus set a lower limit on the Galaxy age. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 Ga, and a 95% confidence upper limit of 16 Ga.^[87]

In 2007, a star in the Galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old, ≈0.5 billion years less than the age of the universe. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way.^[5] This estimate was determined using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of Thorium and other elements created by the R-process. The line strengths yield abundances of different elemental isotopes, from which an estimate of the age of the star can be derived using nucleocosmochronology.^[5]

The age of stars in the Galactic thin disk has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed between 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the Galactic halo and the thin disk.^[88]

Environment[link]

Diagram of the galaxies in the Local Group relative to the Milky Way.

The position of the Local Group within the Virgo Supercluster.

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 absorbed 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.^[89] 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 twentyfold 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 disk 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.^[90]

Velocity[link]

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.^[91][92][93]

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 object's motion. (Motion must always be specified with respect to another object.) This must be kept in mind when discussing the Galaxy's motion.

Astronomers believe the Milky Way is moving at approximately 630 km per second relative to the average velocity of galaxies taken over a large enough volume so that the expansion of the Universe dominates over local, random motions: the local co-moving frame of reference that moves with the Hubble flow.^{[94][further explanation needed]} The Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters, including the Shapley supercluster, behind it.^[95] 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.^[96]

Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at 552 ± 6 km/s^[10] 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.^[10]

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.^[92]

Etymology and mythology[link]

In western culture the name "Milky Way" is derived from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term is a translation of the Classical Latin via lactea, in turn derived from the Hellenistic Greek γαλαξίας, short for γαλαξίας κύκλος (pr. galaktikos kyklos, "milky circle"). The Ancient Greek γαλαξίας (galaxias), from root γαλακτ-, γάλα (milk) + -ίας (forming adjectives), is also the root of "galaxy", the name for our, and later all such, collections of stars.^{[12][97][98][99]} The Milky Way "milk circle" was just one of 11 circles the Greeks identified in the sky, others being the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, Arctic and Antarctic circles, and two colure circles passing through both poles.^[100]

There are many creation myths around the world which explain the origin of the Milky Way and give it its name. In Greek myth, the Milky Way was caused by milk spilt by Hera when suckling Heracles.^[101] It is also described as the road to mount Olympus, and the path of ruin made by the chariot of the Sun god Helios.^[102]

In Sanskrit and several other Indo-Aryan languages, the Milky Way is called Akash Ganga (आकाशगंगा, Ganges of the heavens); it is held to be sacred in the Hindu Puranas (scriptures), and the Ganges and the Milky Way are considered to be terrestrial and celestial analogs.^[103][104] Kshira (क्षीर, milk) is an alternative name for the Milky Way in Hindu texts in Sanskrit.^[105]

Astronomical history[link]

The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the Solar System was assumed near center.

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 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."^[106] The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 A.D.) criticized this view, arguing that if the Milky Way were sublunary it should appear different at different times and places on the Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic world.^[107]

According to Mohaini Mohamed, the Arabian astronomer, Alhazen (965–1037 AD), refuted this by making the first attempt at observing and measuring the Milky Way's parallax.^{[108][verification needed]} He determined that the Milky Way has no parallax and concluded that it must be remote from the Earth, not part of Earth's atmosphere.^[109]

The Persian astronomer Abū Rayhān al-Bīrūnī (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars".^[110] The Andalusian astronomer 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, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence.^[106] 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 these stars are larger than planets.^[111]

According to Jamil Ragep, the Persian astronomer Naṣīr al-Dīn al-Ṭūsī (1201,1274) in his Tadhkira writes: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly-clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. because of this, it was likened to milk in color."^[112]

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.^[113] 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. Kant referred to both our Galaxy and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.^[114]

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.^[115]

Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy.

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.^[116] 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.^[117]

The matter was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory 100 inch (2.5 m) Hooker telescope. With the light-gathering power of this new telescope he was able to produce astronomical photographs that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some Cepheid variables that he could use as a benchmark to estimate the distance to the nebulae: proving they were far too distant to be part of the Milky Way.^[118] In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.^[119]

See also[link]

	Astronomy portal
	Star portal
	Space portal

• Baade's Window
• Galactic coordinate system
• MilkyWay@Home, a distributed computing project that attempts to generate highly accurate three-dimensional dynamic models of stellar streams in the immediate vicinity of our Milky Way Galaxy.
• Oort constants

Notes[link]

References[link]