Milky Way

From Wikipedia, the free encyclopedia
  (Redirected from Milky Way Galaxy)
Jump to: navigation, search
Milky Way Galaxy
An image of the Milky Way in the night sky above Paranal Observatory showing a guide-star laser aimed at the Galactic Center
Observation data
Type SBc[1] (barred spiral galaxy)
Diameter 100–120 kly (31–37 kpc)[2]
Thickness of thin stellar disk ≈2 kly (0.6 kpc)[2][3]
Number of stars 200-400 billion (3×1011 ±1×1011)[4][5][6]
Oldest known star >13.6 Gyr[7]
Mass 0.8–1.5×1012 M[8][9]
Sun's distance to Galactic Center 27.2 ± 1.1 kly (8.34 ± 0.34 kpc)[10]
Sun's Galactic rotation period 240 Myr[11]
Spiral pattern rotation period 220–360 Myr[12]
Bar pattern rotation period 100–120 Myr[12]
Speed relative to CMB rest frame 552 ± 6 km/s[13]
See also: Galaxy, List of galaxies

The Milky Way is the galaxy that contains our Solar System.[14][15][16][nb 1] Its name “milky” is derived from its appearance as a dim 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 Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, "milky circle").[17][18][19] From the Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within the Galaxy. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Up until the early 1920s, most astronomers thought that all of the stars in the universe were contained inside of the Milky Way. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis,[20] observations by Edwin Hubble definitively showed that the Milky Way is just one of many billions of galaxies.[21]

The Milky Way is a barred spiral galaxy some 100,000–120,000 light-years in diameter, which contains 100–400 billion stars. It may contain at least as many planets as well.[22][23] The Solar System is located within the disk, about 27,000 light-years away from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. The stars in the inner ≈10,000 light-years form a bulge and one or more bars that radiate from the bulge. The very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole.

Stars and gases at a wide range of distances from the Galactic center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been given the name “dark matter”.[24] The rotational period is about 240 million years at the position of the Sun.[11] The Galaxy as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest known star in the Galaxy is at least 13.82 [25] billion years old and thus must have formed shortly after the Big Bang.[7]

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, which again forms a subcomponent of the Laniakea supercluster.[26][27]



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.[28] Although all of the individual stars that can be seen in the entire sky with the naked eye are part of the Milky Way Galaxy,[29] the light in this band originates from the accumulation of un-resolved stars and other material when viewed in the direction of 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.[30] 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]

As viewed from Earth, the visible region of the Milky Way’s Galactic plane occupies an area of the sky that includes 30 constellations. The 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 band divides the night sky into two roughly equal hemispheres.


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 mass[edit]

A photograph of galaxy NGC 6744, considered by scientists to greatly resemble the Milky Way Galaxy

The stellar disk of the Milky Way Galaxy is approximately 100,000 ly (30 kpc) in diameter, and is, on average, about 1,000 ly (0.3 kpc) thick.[2][3] As a guide to the relative physical scale of the Milky Way, if it were reduced to 100 m in diameter, the Solar System, including the hypothesized Oort cloud, would be no more than 1 mm in width, about the size of a grain of sand. The nearest star, Proxima Centauri, would be 4.2 mm distant.[nb 3] Alternatively visualized, if the Solar System out to Pluto were the size of a US quarter (25 mm in diameter), the Milky Way would have a diameter of 2,000 kilometers, an area approximately one third the size of the United States.[31]

Schematic illustration showing the galaxy in profile

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.[32][33][34] 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.[35] 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 160,000 ly (49 kpc) of its center.[36] A 2010 measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×1011 M.[37] 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 suggest that the total mass of the entire Galaxy lies in the range 1–1.5×1012 M.[8] More recent studies indicate a mass as large as 4.5×1012 M [38] and as small as 0.8×1012 M.[39]

Stars and planets[edit]

The Milky Way contains at least 100 billion planets[40] and between 200 and 400 billion stars.[41][42] 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.[43] 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,[44] 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.[45][46] 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,[22][47] while microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.[48][49] The Milky Way Galaxy contains at least one planet per star, resulting in 100–400 billion planets, according to a January 2013 study of the five-planet star system Kepler-32 with the Kepler space observatory.[23] A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in the Milky Way Galaxy.[50] On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy.[51][52][53] 11 billion of these estimated planets may be orbiting sun-like stars.[54] The nearest such planet may be 12 light-years away, according to the scientists.[51][52] Such Earth-sized planets may be more numerous than gas giants.[22] Besides exoplanets, "exocomets", comets beyond the Solar System, have also been detected and may be common in the Milky Way Galaxy.[50]

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 decreases with distance from the center of the Galaxy. For reasons that are not understood, beyond a radius of roughly 40,000 ly (13 kpc) from the center, the number of stars per cubic parsec drops much faster with radius.[55] 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).[56] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way. The integrated absolute visual magnitude of the Milky Way is estimated to be −20.9.[57]



This artist’s impression shows how the Milky Way galaxy would look from very different perspectives than we get from the Earth. From some angles the central bulge shows up as a peanut shaped glowing ball of stars and from above the central narrow bar appears clearly. The many spiral arms and their associated dust clouds are also clearly seen.
Artist's conception of the spiral structure of the Milky Way with two major stellar arms and a bar[58]
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 type SBc in the Hubble classification, which represents spiral galaxies 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.[59] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[60] that showed the Galaxy's central bar to be larger than previously suspected.

Galactic quadrants[edit]

Main article: Galactic quadrant

A galactic quadrant, or quadrant of the galaxy, refers to one of four circular sectors in the division of the Milky Way galaxy. In actual astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the pole of the mapping system.

Quadrants are described using ordinals—for example, "1st galactic quadrant",[61] "second galactic quadrant",[62] or "third quadrant of the Galaxy".[63] Viewing from the north galactic pole with 0 degrees (°) as the ray that runs starting from the Sun and through the galactic center, the quadrants are as follow:

  • 1st galactic quadrant – 0° ≤ longitude (ℓ) ≤ 90°[64]
  • 2nd galactic quadrant – 90° ≤ ℓ ≤ 180°[62]
  • 3rd galactic quadrant – 180° ≤ ℓ ≤ 270°[63]
  • 4th galactic quadrant – 270° ≤ ℓ ≤ 360° (0°)[61]

Galactic Center[edit]

Main article: Galactic Center

The Sun is 26,000–28,000 ly (8.0–8.6 kpc) from the Galactic Center. This value is estimated using geometric-based methods or by measuring selected astronomical objects that serve as standard candles, with different techniques yielding various values within this approximate range.[10][65][66][67][68] In the inner few kpc (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge.[69] It has been proposed that our galaxy lacks a bulge formed due to a collision and merger between previous galaxies and that instead has a pseudobulge formed by its central bar.[70]

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.[71] This concentration of mass is best explained as a supermassive black hole[nb 4][10][65] with an estimated mass of 4.1–4.5 million times the mass of the Sun.[65] Observations indicate that there are supermassive black holes located near the center of most normal galaxies.[72][73]

The nature of the Galaxy's bar is actively debated, with estimates for its half-length and orientation spanning from 1–5 kpc (3,000–16,000 ly) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.[67][68][74] Certain authors advocate that the Galaxy features two distinct bars, one nestled within the other.[75] In most galaxies, Wang et al. report, the rate of accretion of the supermassive black hole is slow, but the Milky Way seems to be an important exception. X-ray emission is aligned with the massive stars surrounding the central bar.[76] However, RR Lyr variables do not trace a prominent Galactic bar.[68][77][78] 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.[79]

In 2010, two gigantic spherical bubbles of high energy emission were detected 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.[80][81] Subsequently, observations with the Parkes Telescope at radio frequencies identified polarized emission which is associated with the Fermi bubbles. These observations are best interpreted as a magnetized outflow driven by star formation in the central 640 ly (200 pc) of the Galaxy.[82]

Spiral arms[edit]

Outside the gravitational influence of the Galactic bars, astronomers generally organize the structure of the interstellar medium and stars in the disk of the Milky Way into four spiral arms.[83] Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation, as traced by H II regions[84][85] and molecular clouds.[86]

Maps of the Milky Way's spiral structure are notoriously uncertain and exhibit striking differences.[58][83][85][87][88][89][90][91] Some 150 years after Alexander (1852)[92] 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 only crudely describe features near the Sun;[85][90] namely since galaxies commonly exhibit arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.[68][90][91] The possible scenario of the Sun within a spur / Local arm[85] emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Galaxy.[90]

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 about 7° to 25°.[93][94] There are thought to be four 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:

Color Arm(s)
purple Norma and Outer arm (Along with extension discovered in 2004[95])
green Scutum–Centaurus Arm
pink Carina–Sagittarius Arm
There are at least two smaller arms or spurs, including:

Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giant stars and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum-Centaurus Arm contains approximately 30% more red giant stars than would be expected in the absence of a spiral arm.[93][96] 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 old stars.[58] In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way.[97][98][99] Thus, the Galaxy appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear.[99]

The Near 3 kpc Arm (also called Expanding 3 kpc Arm or simply 3 kpc Arm) was discovered in the 1950s by astronomer van Woerden and collaborators through 21-centimeter radio measurements of HI (atomic hydrogen).[100][101] It was found to be expanding away from the center of the galaxy at more than 50 km/s. It is located in the fourth galactic quadrant at a distance of about 5.2 kpc from the Sun and 3.3 kpc from the galactic center. The Far 3 kpc Arm was discovered in 2008 by astronomer Tom Dame (Harvard-Smithsonian CfA). It's located in the first galactic quadrant at a distance of 3 kpc (about 10,000 ly) from the galactic center.[101][102]





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.[106] 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.[107] The globular clusters can follow rosette orbits about the Galaxy, in contrast to the elliptical orbit of a planet around a star.[108]

While the disk contains dust which obscures the view in some wavelengths, the halo component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little gas cool enough to collapse into stars.[11] Open clusters are also located primarily in the disk.[109]

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,[110] 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.[95][111] 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.


Gaseous halo[edit]

In addition to the stellar halo, the Chandra X-ray Observatory, XMM-Newton, and Suzaku have provided evidence that there is a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousand of light years, much further than the stellar halo and close to the distance of the Large and Small Magellanic Clouds. The mass of this hot halo is nearly equivalent to the mass of the galaxy itself.[113][114][115] The temperature of this halo gas is between 1 million and 2.5 million kelvin, a few hundred times hotter than the surface of the sun.[116]

Observations of distant galaxies indicate that the Universe had about one-sixth as much baryonic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way.[117] If the finding that the mass of the halo is comparable to the mass of the galaxy is confirmed, it could be the identity of the missing baryons around the Milky Way.[117]

Sun’s location and neighborhood[edit]

Diagram of the Sun’s 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 is near the inner rim of the Galaxy's Orion Arm, within the Local Fluff of 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.[10][65][118] The Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the Galactic disk.[119] The distance between the local arm and the next arm out, the Perseus Arm, is about 2,000 parsecs (6,500 ly).[120] The Sun, and thus the Solar System, is found in the Galactic habitable zone.



It takes the Solar System about 240 million years to complete one orbit of the Galaxy (a Galactic year),[11] 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).[124]


Galactic rotation[edit]

The stars and gas in the Galaxy rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Galaxy does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 and 240 km/s.[127] 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 have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation. Toward the center of the galaxy the orbit speeds are too low while beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation.

If the Galaxy contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter.[24] Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve.[128]



Since the first stars began to form, the Milky Way has grown through both galaxy mergers (particularly early in the Galaxy's growth) and accretion of gas directly from the Galactic halo.[130] The Milky Way is currently accreting material from two of its 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.[131][132] However, properties of the Milky Way such as stellar mass, angular momentum, and metallicity in its outermost regions suggest it has undergone no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies; its neighbour the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies.[133][134]

According to recent studies, the Milky Way as well as Andromeda lie in what in the galaxy color–magnitude diagram is known as the green valley, a region populated by galaxies in transition from the blue cloud (galaxies actively forming new stars) to the red sequence (galaxies that lack star formation). Star-formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both our galaxy and the Andromeda Galaxy.[135] In fact, measurements of other galaxies similar to our own suggest it is among the reddest and brightest spiral galaxies that are still forming new stars and it is just slightly bluer than the bluest red sequence galaxies.[136]


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 12.5 ± 3 billion years for CS 31082-001[137] and 13.8 ± 4 billion years for BD +17° 3248.[138] 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 billion years. Globular clusters are among the oldest objects in the Milky Way Galaxy, which thus set a lower limit on the age of the galaxy. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.[139]

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.[140] 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.[140]



The position of the Local Group within the Virgo Supercluster
Main article: Local Group


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 14,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a stream of neutral hydrogen gas extending from these two small galaxies across 100° of the sky. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way.[142] 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.




Although special relativity states that there is no "preferred" inertial frame of reference in space with which to compare the Milky Way, the Galaxy does have a velocity with respect to cosmological frames of reference.



Etymology and mythology[edit]

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.[17][148][149][150] 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.[151]

Astronomical history[edit]



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".[154] 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.[152] Ibn Qayyim Al-Jawziyya (1292–1350) proposed that the Milky Way is "a myriad of tiny stars packed together in the sphere of the fixed stars" and that these stars are larger than planets.[155]


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.[157][158] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright,[159] 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.[160] 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.[161][162][163]






See also[edit]


  2. ^ See also Bortle Dark-Sky Scale.
  3. ^ The scale is 1 mm equals 1 ly.


  2. ^ a b c Christian, Eric; Safi-Harb, Samar. "How large is the Milky Way?". NASA: Ask an Astrophysicist. Retrieved 2007-11-28. 
  3. ^ a b Rix, Hans-Walter and Bovy, Jo (2013). "The Milky Way's Stellar Disk". The Astronomy and Astrophysics Review. in press. arXiv:1301.3168. Bibcode:2013A&ARv..21...61R. doi:10.1007/s00159-013-0061-8. 
  4. ^ "NASA – Galaxy". NASA and World Book. November 29, 2007. Archived from the original on 2009-04-12. Retrieved 2012-12-06. 
  5. ^ Staff (December 16, 2008). "How Many Stars are in the Milky Way?". Universe Today. Retrieved 2010-08-10. 
  6. ^ Odenwald, S. (March 17, 2014). "Counting the Stars in the Milky Way". The Huffington Post. Retrieved 2014-06-09. 
  7. ^ a b H.E. Bond; E. P. Nelan; D. A. VandenBerg; G. H. Schaefer; D. Harmer (February 13, 2013). "HD 140283: A Star in the Solar Neighborhood that Formed Shortly After the Big Bang". The Astrophysical Journal 765 (1): L12. arXiv:1302.3180. Bibcode:2013ApJ...765L..12B. doi:10.1088/2041-8205/765/1/L12. 
  10. ^ a b c d Gillessen, S. et al. (2009). "Monitoring stellar orbits around the massive black hole in the Galactic Center". Astrophysical Journal 692 (2): 1075–1109. arXiv:0810.4674. Bibcode:2009ApJ...692.1075G. doi:10.1088/0004-637X/692/2/1075.  edit
  12. ^ a b Gerhard, O. Pattern speeds in the Milky Way. arXiv:1003.2489v1. 
  13. ^ a b c Kogut, A. et al. (1993). "Dipole anisotropy in the COBE differential microwave radiometers first-year sky maps". The Astrophysical Journal 419: 1. arXiv:astro-ph/9312056. Bibcode:1993ApJ...419....1K. doi:10.1086/173453.  edit
  14. ^ "Milky Way". Oxford University Press. Retrieved 2012-10-31. 
  15. ^ "Milky Way Galaxy". Merriam-Webster Incorportated. Retrieved 2012-10-31. 
  16. ^ "Milky Way Galaxy". Encyclopædia Britannica, Inc. Retrieved 2012-10-31. 
  17. ^ a b Harper, Douglas. "galaxy". Online Etymology Dictionary. Retrieved 2012-05-20. 
  23. ^ a b Staff (January 2, 2013). "100 Billion Alien Planets Fill Our Milky Way Galaxy: Study". Retrieved January 3, 2013. 
  25. ^
  26. ^ Laniakea: Our home supercluster
  27. ^ 4 September 2014, The Laniakea supercluster of galaxies
  29. ^ H. A. Rey (1976). The Stars. Houghton Mifflin Harcourt. p. 145. ISBN 0395248302. 
  33. ^ Vayntrub, Alina (2000). "Mass of the Milky Way". The Physics Factbook. Retrieved 2007-05-09. 
  35. ^ Finley, Dave; Aguilar, David (January 5, 2009). "Milky Way a Swifter Spinner, More Massive, New Measurements Show". National Radio Astronomy Observatory. Retrieved 2009-01-20. 
  38. ^ "The mass of the Milky Way and M31 using the method of least action". Retrieved February 6, 2014. 
  39. ^ "On the shoulders of Giants: Properties of the Stellar Halo and the Milky Way mass distribution". Retrieved August 8, 2014. 
  40. ^ Villard, Ray (January 11, 2012). "The Milky Way Contains at Least 100 Billion Planets According to Survey". Retrieved 2012-01-11. 
  41. ^ Frommert, H.; Kronberg, C. (August 25, 2005). "The Milky Way Galaxy". SEDS. Retrieved 2007-05-09. 
  42. ^ Wethington, Nicholos. "How Many Stars are in the Milky Way?". Retrieved 2010-04-09. 
  43. ^ Young, Kelly (June 6, 2006). "Andromeda Galaxy hosts a trillion stars". NewScientist. Retrieved 2006-06-08. 
  47. ^ Borenstein, Seth (February 19, 2011). "Cosmic census finds crowd of planets in our galaxy". The Washington Post. Associated Press. Archived from the original on 2011-02-21. 
  49. ^ "Free-Floating Planets May be More Common Than Stars". Pasadena, CA: NASA's Jet Propulsion Laboratory. February 18, 2011. Archived from the original on 2011-05-25. "The team estimates there are about twice as many of them as stars." 
  50. ^ a b Staff (January 7, 2013). "17 Billion Earth-Size Alien Planets Inhabit Milky Way". Retrieved January 8, 2013. 
  53. ^ Borenstein, Seth (2013-11-04). "8.8 billion habitable Earth-size planets exist in Milky Way alone". The Associated Press. NBC News. 
  57. ^ Coffey, Jerry. "Absolute Magnitude". Retrieved 2010-04-0. 
  58. ^ a b c Benjamin, R. A. (2008). "The Spiral Structure of the Galaxy: Something Old, Something New...". In Beuther, H.; Linz, H.; Henning, T. (ed.). Massive Star Formation: Observations Confront Theory 387. Astronomical Society of the Pacific Conference Series. p. 375. Bibcode:2008ASPC..387..375B. 
    See also Bryner, Jeanna (June 3, 2008). "New Images: Milky Way Loses Two Arms". Retrieved 2008-06-04. 
  60. ^ McKee, Maggie (August 16, 2005). "Bar at Milky Way's heart revealed". New Scientist. Retrieved 2009-06-17. 
  65. ^ a b c d Ghez, A. M. et al. (December 2008). "Measuring distance and properties of the Milky Way's central supermassive black hole with stellar orbits". The Astrophysical Journal 689 (2): 1044–1062. arXiv:0808.2870. Bibcode:2008ApJ...689.1044G. doi:10.1086/592738.  edit
  69. ^ Grant, J.; Lin, B. (2000). "The Stars of the Milky Way". Fairfax Public Access Corporation. Retrieved 2007-05-09. 
  72. ^ Blandford, R. D. (1999). "Origin and Evolution of Massive Black Holes in Galactic Nuclei". Galaxy Dynamics, proceedings of a conference held at Rutgers University, 8–12 August 1998, ASP Conference Series vol. 182. Bibcode:1999ASPC..182...87B. 
  79. ^ Staff (September 12, 2005). "Introduction: Galactic Ring Survey". Boston University. Retrieved 2007-05-10. 
  81. ^ "Rätselhafte Blasen im All". Süddeutsche Zeitung. Retrieved 2010-11-10. 
  87. ^ Nakanishi, Hiroyuki; Sofue, Yoshiaki (2003). "Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: I. The H I Disk". Publications of the Astronomical Society of Japan 55: 191. arXiv:astro-ph/0304338. Bibcode:2003PASJ...55..191N. doi:10.1093/pasj/55.1.191. 
  90. ^ a b c d Majaess, D. J.; Turner, D. J.; Lane (2009). "Searching Beyond the Obscuring Dust Between the Cygnus-Aquila Rifts for Cepheid Tracers of the Galaxy's Spiral Arms". The Journal of the American Association of Variable Star Observers 37: 179. arXiv:0909.0897. Bibcode:2009JAVSO..37..179M. 
  91. ^ a b Lépine, J. R. D. et al. (2011). "The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance". Monthly Notices of the Royal Astronomical Society 414 (2): 1607. arXiv:1010.1790. Bibcode:2011MNRAS.414.1607L. doi:10.1111/j.1365-2966.2011.18492.x.  edit
  97. ^ "Massive stars mark out Milky Way's 'missing' arms", University of Leeds. 17 Dec 2013. Retrieved 18 Dec 2013.
  98. ^ Russell Westerholm, "Milky Way Galaxy Has Four Arms, Reaffirming Old Data and Contradicting Recent Research", University Herald. 18 Dec 2013. Retrieved 18 Dec 2013.
  99. ^ a b J. S. Urquhart, C. C. Figura, T. J. T., Moore, M. G. Hoare, S. L. Lumsde, J. C. Mottram, M. A. Thompson, R. D. Oudmaijer (2013). "The RMS Survey: Galactic distribution of massive star formation". Monthly Notices of the Royal Astronomical Society. in press. arXiv:1310.4758. Bibcode:2014MNRAS.437.1791U. doi:10.1093/mnras/stt2006. 
  101. ^ a b A New Spiral Arm of the Galaxy: The Far 3-Kpc Arm, T. M. Dame, P. Thaddeus, ApJ Letters, 2008
  102. ^ Milky Way's Inner Beauty Revealed, Press Release Harvard-Smithsonian Center for Astrophysics, 2008
  103. ^ "Star-Crossed: Milky Way's Spiral Shape May Result from a Smaller Galaxy's Impact". Scientific American. 14 September 2011. 
  106. ^ Harris, William E. (February 2003). "Catalog of Parameters for Milky Way Globular Clusters: The Database" (text). SEDS. Retrieved 2007-05-10. 
  111. ^ "Outer Disk Ring?". SolStation. Retrieved 2007-05-10. 
  117. ^ a b J.D. Harrington, Janet Anderson, and Peter Edmonds (September 24, 2012). "NASA's Chandra Shows Milky Way is Surrounded by Halo of Hot Gas". NASA. 
  120. ^ English, Jayanne (January 14, 2000). "Exposing the Stuff Between the Stars". Hubble News Desk. Retrieved 2007-05-10. 
  121. ^ "Magnitude". National Solar Observatory—Sacramento Peak. Archived from the original on 2008-02-06. Retrieved 2013-08-09. 
  127. ^ Imamura, Jim (August 10, 2006). "Mass of the Milky Way Galaxy". University of Oregon. Archived from the original on 2007-03-01. Retrieved 2007-05-10. 
  133. ^ Yin, J.; Hou, J.L; Prantzos, N.; Boissier, S.; Chang, R. X.; Shen, S. Y.; Zhang, B. (2009). "Milky Way versus Andromeda: a tale of two disks". Astronomy and Astrophysics 505 (2): 497–508. arXiv:0906.4821. Bibcode:2009A&A...505..497Y. doi:10.1051/0004-6361/200912316. 
  134. ^ Hammer, F.; Puech, M.; Chemin, L.; Flores, H.; Lehnert, M. D. (2007). "The Milky Way, an Exceptionally Quiet Galaxy: Implications for the Formation of Spiral Galaxies". The Astrophysical Journal 662 (1): 322–334. arXiv:astro-ph/0702585. Bibcode:2007ApJ...662..322H. doi:10.1086/516727. 
  135. ^ Mutch, S.J.; Croton, D.J.; Poole, G.B. (2011). "The Mid-life Crisis of the Milky Way and M31". The Astrophysical Journal 736 (2). arXiv:1105.2564. Bibcode:2011ApJ...736...84M. doi:10.1088/0004-637X/736/2/84. 
  137. ^ Cayrel et al (2001). "Measurement of stellar age from uranium decay". Nature 409: 691. arXiv:astro-ph/0104357. Bibcode:2001Natur.409..691C. 
  141. ^ Del Peloso, E. F. (2005). "The age of the Galactic thin disk from Th/Eu nucleocosmochronology". Astronomy and Astrophysics 440 (3): 1153. arXiv:astro-ph/0506458. Bibcode:2005A&A...440.1153D. doi:10.1051/0004-6361:20053307. 
  143. ^ "Milky Way Galaxy is warped and vibrating like a drum" (Press release). University of California, Berkeley. January 9, 2006. Retrieved 2007-10-18. 
  144. ^ Wong, Janet (April 14, 2000). "Astrophysicist maps out our own galaxy's end". University of Toronto. Archived from the original on 2007-01-08. Retrieved 2007-01-11. 
  145. ^ Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An Introduction to Galaxies and Cosmology. Cambridge University Press. p. 298. ISBN 0-521-54623-0. 
  147. ^ Peirani, S; Defreitaspacheco, J (2006). "Mass determination of groups of galaxies: Effects of the cosmological constant". New Astronomy 11 (4): 325. arXiv:astro-ph/0508614. Bibcode:2006NewA...11..325P. doi:10.1016/j.newast.2005.08.008. 
  152. ^ a b Montada, Josep Puig (September 28, 2007). "Ibn Bajja". Stanford Encyclopedia of Philosophy. Retrieved 2008-07-11. 
  158. ^ O'Connor, J. J.; Robertson, E. F. (November 2002). "Galileo Galilei". University of St. Andrews. Retrieved 2007-01-08. 
    • On page 65, Wright speculated that the central body of the Milky Way, around which the rest of the galaxy revolves, might not be visible to us: " ... the central body A, being supposed as incognitum [i.e., an unknown], without [i.e., outside of] the finite view; ... "
  162. ^ Evans, J. C. (November 24, 1998). "Our Galaxy". George Mason University. Retrieved 2007-01-04. 
    In the English translation — Alexander von Humboldt with E.C. Otté, trans., Cosmos … (New York, New York: Harper & Brothers, 1897), vols. 3-5 — see page 147.
  164. ^ William Herschel (1785) "On the Construction of the Heavens," Philosophical Transactions of the Royal Society of London, 75 : 213-266. Herschel's diagram of the galaxy appears immediately after the article's last page. See:
  165. ^ Abbey, Lenny. "The Earl of Rosse and the Leviathan of Parsontown". The Compleat Amateur Astronomer. Retrieved 2007-01-04. 
  167. ^ Weaver, Harold F. "Robert Julius Trumpler". National Academy of Sciences. Retrieved 2007-01-05. 

Cite error: A list-defined reference named "sachau2001" is not used in the content (see the help page).
Cite error: A list-defined reference named "jackson1989" is not used in the content (see the help page).
Cite error: A list-defined reference named "spencer1965" is not used in the content (see the help page).
Cite error: A list-defined reference named "fison1899" is not used in the content (see the help page).
Cite error: A list-defined reference named "waller_hodge2003" is not used in the content (see the help page).

Further reading[edit]

External links[edit]