The Milky Way[c] is the
galaxy that includes the
Solar System, with the name describing the galaxy's appearance from
Earth: a hazy band of light seen in the
night sky formed from stars that cannot be individually distinguished by the
naked eye. The term Milky Way is a translation of the Latin via lactea, from the
Greekγαλακτικὸς κύκλος (galaktikòs kýklos), meaning "milky circle". From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within.
Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the
Universe. Following the 1920
Great Debate between the astronomers
Harlow Shapley and
Heber Doust Curtis, observations by
Edwin Hubble showed that the Milky Way is just one of many galaxies.
It is estimated to contain 100–400 billion stars and at least that number of
planets. The Solar System is located at a radius of about 27,000 light-years (8.3 kpc) from the
Galactic Center, on the inner edge of the
Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a
bulge and one or more bars that radiate from the bulge. The Galactic Center is an intense radio source known as
Sagittarius A*, a
supermassive black hole of 4.100 (± 0.034) million
solar masses. Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second (136 miles per second). The constant rotational speed appears to contradict the laws of
Keplerian dynamics and suggests that much (about 90%) of the
mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing
electromagnetic radiation. This conjectural mass has been termed "
dark matter". The rotational period is about 212 million years at the radius of the Sun.
The Milky Way as a whole is moving at a velocity of approximately 600 km per second (372 miles per second) with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the
Dark Ages of the
Babylonian epic poem Enūma Eliš, the Milky Way is created from the severed tail of the primeval salt water
dragonessTiamat, set in the sky by
Marduk, the Babylonian
national god, after slaying her. This story was once thought to have been based on an older
Sumerian version in which Tiamat is instead slain by
Nippur, but is now thought to be purely an invention of Babylonian propagandists with the intention to show Marduk as superior to the Sumerian deities.
Zeus places his son born by a mortal woman, the infant
Hera's breast while she is asleep so the baby will drink
her divine milk and thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way. In another Greek story, the abandoned Heracles is given by
Athena to Hera for feeding, but Heracles' forcefulness causes Hera to rip him from her breast in pain.
In western culture, the name "Milky Way" 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 Latinvia lactea, in turn derived from the
Hellenistic Greekγαλαξίας, short for γαλαξίας κύκλος (galaxías kýklos), meaning "milky circle". The
Ancient Greekγαλαξίας (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.
The Milky Way has a relatively low
surface brightness. Its visibility can be greatly reduced by background light, such as
light pollution or moonlight. The sky needs to be darker than about 20.2
magnitude per square arcsecond in order for the Milky Way to be visible. It should be visible if the
limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1. This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but very prominent when viewed from
rural areas when the Moon is below the horizon.[d] Maps of artificial night sky brightness show that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution.
The galactic plane is inclined by about 60° to the
ecliptic (the plane of
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 12h 49m,
declination +27.4° (
β Comae Berenices, and the south galactic pole is near
α Sculptoris. Because of this high inclination, depending on the time of night and year, the Milky Way arch may appear relatively low or relatively high in the sky. For observers from latitudes approximately 65° north to 65° south, the Milky Way passes
directly overhead twice a day.
The Milky Way arching at a high inclination across the night sky. The
Magellanic Clouds can be seen on the left. The bright object near top center is Jupiter in the constellation
Sagittarius. Galactic north is downward. This
composited panorama was taken at
Paranal Observatory in northern Chile; the orange glow at the horizon on the right is Antofagasta city with a jet trail above it.
Aristotle (384–322 BC) states that the
Greek philosophersAnaxagoras (
c. 500–428 BC) and
Democritus (460–370 BC) proposed that the Milky Way is the glow of stars not directly visible due to Earth's shadow, while other stars receive their light from the Sun (but have their glow obscured by solar rays). Aristotle himself believed that the Milky Way was part of the Earth's upper atmosphere (along with the stars), and that it was a byproduct of stars burning that did not dissipate because of its outermost location in the atmosphere (composing its
great circle). He also said that the milky appearance of the Milky Way
Galaxy is due to the refraction of the Earth's atmosphere. The
Olympiodorus the Younger (c. 495–570 AD) criticized this view, arguing that if the Milky Way were
sublunary, it should appear different at different times and places on Earth, and that it should have
parallax, which it does not. In his view, the Milky Way is celestial. This idea would be influential later in the
Al-Biruni (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of
nebulous stars". The
d 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image in the Earth's atmosphere, citing his observation of a
conjunction of Jupiter and Mars in 1106 or 1107 as evidence. The Persian astronomer
Nasir al-Din al-Tusi (1201–1274) in his Tadhkira wrote: "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."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".
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 is composed of a huge number of faint stars. Galileo also concluded that the appearance of the Milky Way was due to
refraction of the Earth's atmosphere . 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. Wright and 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 the Milky Way and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.
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 Milky Way with the Solar System close to the center.
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 1904, studying the
proper motions of stars,
Jacobus Kapteyn reported that these were not random, as it was believed in that time; stars could be divided into two streams, moving in nearly opposite directions. It was later realized that Kapteyn's data had been the first evidence of the rotation of our galaxy, which ultimately led to the finding of galactic rotation by
Bertil Lindblad and
Jan Oort.
Heber Curtis had observed the nova
S Andromedae within the
Great Andromeda Nebula (
Messier object 31). Searching the photographic record, he found 11 more
novae. Curtis noticed that these novae were, on average, 10
magnitudes fainter than those that occurred within the Milky Way. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were independent galaxies. 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 is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant
ESA spacecraft Gaia provides distance estimates by determining the
parallax of a billion stars and is mapping the Milky Way with four planned releases of maps in 2016, 2018, 2021 and 2024.
Data from Gaia has been described as "transformational". It has been estimated that Gaia has expanded the number of observations of stars from about 2 million stars as of the 1990s to 2 billion. It has expanded the measurable volume of space by a factor of 100 in radius and a factor of 1,000 in precision.
A study in 2020 concluded that Gaia detected a wobbling motion of the galaxy, which might be caused by "
torques from a misalignment of the disc's rotation axis with respect to the principal axis of a non-spherical halo, or from
accreted matter in the halo acquired during late infall, or from nearby, interacting satellite galaxies and their consequent tides".
Diagram of the Milky Way with the position of the Solar System marked by a yellow arrow and a red dot in the
Orion Arm. The dot roughly covers the larger surroundings of the Solar System, the space between the
Radcliffe wave and Split linear structures (formerly the
Sun is near the inner rim of the
Orion Arm, within the
Local Fluff of the
Local Bubble, between the
Radcliffe wave and Split linear structures (formerly
Gould Belt). Based upon studies of stellar orbits around Sgr A* by Gillessen et al. (2016), the Sun lies at an estimated distance of 27.14 ± 0.46 kly (8.32 ± 0.14 kpc) from the Galactic Center. Boehle et al. (2016) found a smaller value of 25.64 ± 0.46 kly (7.86 ± 0.14 kpc), also using a star orbit analysis. The Sun is currently 5–30 parsecs (16–98 ly) above, or north of, the central plane of the Galactic disk. The distance between the local arm and the next arm out, the
Perseus Arm, is about 2,000 parsecs (6,500 ly). The Sun, and thus the Solar System, is located in the Milky Way's
galactic habitable zone.
There are about 208 stars brighter than
absolute magnitude 8.5 within a sphere with a radius of 15 parsecs (49 ly) from the Sun, giving a density of one star per 69 cubic parsecs, or one star per 2,360 cubic light-years (from
List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4
brown dwarfs) within 5 parsecs (16 ly) of the Sun, giving a density of about one star per 8.2 cubic parsecs, or one per 284 cubic light-years (from
List of nearest stars). This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about 500 stars brighter than
apparent magnitude 4 but 15.5 million stars brighter than apparent magnitude 14.
The apex of the Sun's way, or the
solar apex, is the direction that the Sun travels through 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 about the Milky Way 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 passes through the Galactic plane approximately 2.7 times per orbit. This is very similar to how a
simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with
mass lifeform extinction periods on Earth. A reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation.
It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a
galactic year), 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 Milky Way is approximately 220 km/s (490,000 mph) or 0.073% of the
speed of light. The Sun moves through the heliosphere at 84,000 km/h (52,000 mph). 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). The Solar System is headed in the direction of the zodiacal constellation
Scorpius, which follows the ecliptic.
Quadrants are described using
ordinals – for example, "1st galactic quadrant", "second galactic quadrant", or "third quadrant of the Milky Way". Viewing from the
north galactic pole with 0°
(zero degrees) as the
ray that runs starting from the Sun and through the Galactic Center, the quadrants are:
The Milky Way is one of the two largest galaxies in the
Local Group (the other being the
Andromeda Galaxy), although the size for its
galactic disc and how much it defines the isophotal diameter is not well understood. It is estimated that the significant bulk of stars in the galaxy lies within the 26 kiloparsecs (80,000 light-years) diameter, and that the number of stars beyond the outermost disc dramatically reduces to a very low number, with respect to an extrapolation of the exponential disk with the scale length of the inner disc.
There are several methods being used in astronomy in defining the size of a galaxy, and each of them can yield different results with respect to one another. The most commonly employed method is the
D25 standard – the
isophote where the photometric brightness of a galaxy in the B-band (445 nm wavelength of light, in the blue part of the
visible spectrum) reaches 25 mag/arcsec2. An estimate from 1997 by Goodwin and others compared the distribution of
Cepheid variable stars in 17 other spiral galaxies to the ones in the Milky Way, and modelling the relationship to their surface brightnesses. This gave an
isophotal diameter for the Milky Way at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,600 light-years), by assuming that the galactic disc is well represented by an exponential disc and adopting a central surface brightness of the galaxy (µ0) of 22.1±0.3B-mag/arcsec−2 and a disk scale length (h) of 5.0 ± 0.5 kpc (16,300 ± 1,600 ly). This is significantly smaller than the Andromeda Galaxy's isophotal diameter, and slightly below the mean isophotal sizes of the galaxies being at 28.3 kpc (92,000 ly). The paper concludes that the Milky Way and Andromeda Galaxy were not overly large spiral galaxies and as well as
one of the largest known (if the former not being the largest) as previously widely believed, but rather average ordinary spiral galaxies. To compare the relative physical scale of the Milky Way, if the
Solar System out to
Neptune were the size of a
US quarter (24.3 mm (0.955 in)), the Milky Way would be approximately at least the greatest north–south line of the
contiguous United States. An even older study from 1978 gave a lower diameter for Milky Way about 23 kpc (75,000 ly).
A 2015 paper discovered that there is a ring-like filament of stars called
Triangulum–Andromeda Ring (TriAnd Ring) rippling above and below the relatively flat
galactic plane, which alongside
Monoceros Ring were both suggested to be primarily the result of disk oscillations and wrapping around the Milky Way, at a diameter of at least 50 kpc (160,000 ly), which may be part of the Milky Way's outer disk itself, hence making the stellar disk larger by increasing to this size. However, a more recent 2018 paper later somewhat ruled out this hypothesis, and supported a conclusion that the Monoceros Ring,
A13 and TriAnd Ring were stellar overdensities rather kicked out from the main stellar disk, with the velocity dispersion of the RR Lyrae stars found to be higher and consistent with halo membership. Another 2018 study revealed the very probable presence of disk stars at 26–31.5 kpc (84,800–103,000 ly) from the Galactic Center or perhaps even farther, significantly beyond approximately 13–20 kpc (40,000–70,000 ly), in which it was once believed to be the abrupt drop-off of the stellar density of the disk, meaning that few or no stars were expected to be above this limit, save for stars that belong to the old population of the galactic halo.
A 2020 study predicted the edge of the Milky Way's
dark matter halo being around 292 ± 61
kpc (952,000 ± 199,000
ly), which translates to a diameter of 584 ± 122
kpc (1.905 ± 0.3979
Mly). The Milky Way's stellar disk is also estimated to be approximately up to 1.35 kpc (4,000 ly) thick.
The Milky Way is approximately 890 billion to 1.54 trillion times the mass of the
Sun in total (8.9×1011 to 1.54×1012 solar masses), although stars and planets make up only a small part of this. Estimates of the mass of the Milky Way vary, depending upon the method and data used. The low end of the estimate range is 5.8×1011solar masses (M☉), somewhat less than that of the
Andromeda Galaxy. Measurements using the
Very Long Baseline Array in 2009 found velocities as large as 254 km/s (570,000 mph) for stars at the outer edge of the Milky Way. Because 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 7×1011M☉ within 160,000 ly (49 kpc) of its center. In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kilo
parsecs is 7×1011M☉. According to a study published in 2014, the mass of the entire Milky Way is estimated to be 8.5×1011M☉, but this is only half the mass of the Andromeda Galaxy. A recent 2019 mass estimate for the Milky Way is 1.29×1012M☉.
Much of the mass of the Milky Way seems to be
dark matter, an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A
dark matter halo is conjectured to spread out relatively uniformly to a distance beyond one hundred kiloparsecs (kpc) from the Galactic Center. Mathematical models of the Milky Way suggest that the mass of dark matter is 1–1.5×1012M☉. 2013 and 2014 studies indicate a range in mass, as large as 4.5×1012M☉ and as small as 8×1011M☉. By comparison, the total mass of all the stars in the Milky Way is estimated to be between 4.6×1010M☉ and 6.43×1010M☉. In addition to the stars, there is also interstellar gas, comprising 90%
hydrogen and 10%
helium by mass, with two thirds of the hydrogen found in the
atomic form and the remaining one-third as
molecular hydrogen. The mass of the Milky Way's interstellar gas is equal to between 10% and 15% of the total mass of its stars.
Interstellar dust accounts for an additional 1% of the total mass of the gas.
In March 2019, astronomers reported that the
virial mass of the Milky Way Galaxy is 1.54 trillion
solar masses within a
radius of about 39.5 kpc (130,000 ly), over twice as much as was determined in earlier studies, suggesting that about 90% of the mass of the galaxy is
The Milky Way contains between 100 and 400 billion stars and at least that many planets. An exact figure would depend on counting the number of very-low-mass stars, which are difficult 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. The Milky Way may contain ten billion
white dwarfs, a billion
neutron stars, and a hundred million stellar
black holes.[f] 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, whereas the thickness of the gas layer ranges from hundreds of light-years for the colder gas to thousands of light-years for warmer gas.
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 Milky Way. For reasons that are not understood, beyond a radius of roughly 40,000 light years (13 kpc) from the center, the number of stars per cubic
parsec drops much faster with radius. Surrounding the galactic disk is a spherical
galactic halo of stars and
globular clusters that extends farther outward, but is limited in size by the orbits of two Milky Way satellites, the Large and Small
Magellanic Clouds, whose
closest approach to the Galactic Center is about 180,000 ly (55 kpc). 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 around −20.9.[g]
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, and microlensing measurements indicate that there are more
rogue planets not bound to host stars than there are stars. The Milky Way 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 by the
Kepler space observatory. A different January 2013 analysis of Kepler data estimated that at least 17 billion
Earth-sizedexoplanets reside in the Milky Way. 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 dwarfs within the Milky Way. 11 billion of these estimated planets may be orbiting Sun-like stars. The nearest exoplanet may be 4.2 light-years away, orbiting the
red dwarfProxima Centauri, according to a 2016 study. Such Earth-sized planets may be more numerous than gas giants, though harder to detect at great distances given their small size. Besides exoplanets, "
comets beyond the Solar System, have also been detected and may be common in the Milky Way. More recently, in November 2020, over 300 million habitable exoplanets are estimated to exist in the Milky Way Galaxy.
UGC 6093 and
UGC 12158, believed to closely resemble the Milky Way in their structure and appearances.
The Milky Way consists of a bar-shaped core region surrounded by a warped disk of
gas, dust and stars. The mass distribution within the Milky Way closely resembles the type Sbc in the
Hubble classification, which represents spiral galaxies with relatively loosely wound arms. Astronomers first began to conjecture that the Milky Way is a
barred spiral galaxy, rather than an ordinary
spiral galaxy, in the 1960s. These conjectures were confirmed by the
Spitzer Space Telescope observations in 2005 that showed the Milky Way's central bar to be larger than previously thought.
The Sun is 25,000–28,000 ly (7.7–8.6 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. In the inner few kiloparsecs (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called
the bulge. It has been proposed that the Milky Way lacks a
bulge due to a
collision and merger between previous galaxies, and that instead it only has a
pseudobulge formed by its central bar. However, confusion in the literature between the (peanut shell)-shaped structure created by instabilities in the bar, versus a possible bulge with an expected half-light radius of 0.5 kpc, abounds.
The nature of the Milky Way's bar is actively debated, with estimates for its half-length and orientation spanning from 1 to 5 kpc (3,000–16,000 ly) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center. Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other. However,
RR Lyrae-type stars do not trace a prominent Galactic bar. 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 Milky Way, as well as most of the Milky Way's
star formation activity. Viewed from the
Andromeda Galaxy, it would be the brightest feature of the Milky Way. X-ray emission from the core is aligned with the massive stars surrounding the central bar and the
In June 2023, astronomers reported using a new cascade neutrino technique to detect, for the first time, the release of
neutrinos from the
galactic plane of the Milky Way
galaxy, creating the first neutrino view of the Milky Way.
Gamma rays and x-rays
Since 1970, various gamma-ray detection missions have discovered 511-
keVgamma rays coming from the general direction of the Galactic Center. These gamma rays are produced by
positrons (antielectrons) annihilating with
electrons. In 2008 it was found that the distribution of the sources of the gamma rays resembles the distribution of low-mass
X-ray binaries, seeming to indicate that these X-ray binaries are sending positrons (and electrons) into interstellar space where they slow down and annihilate. The observations were both made by
ESA's satellites. In 1970 gamma ray detectors found that the emitting region was about 10,000 light-years across with a luminosity of about 10,000 suns.
In 2010, two gigantic spherical bubbles of high energy gamma-emission were detected to the north and the south of the Milky Way core, using data from the
Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc) (or about 1/4 of the galaxy's estimated diameter); they stretch up to
Grus and to
Virgo on the night-sky of the southern hemisphere. Subsequently, observations with the
Parkes Telescope at radio frequencies identified polarized emission that 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 Milky Way.
Later, on January 5, 2015,
NASA reported observing an
X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of an
asteroid falling into the black hole or by the entanglement of
magnetic field lines within gas flowing into Sagittarius A*.
Outside the gravitational influence of the Galactic bar, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms. 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 and
The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's arms. Perfect logarithmic spiral patterns only crudely describe features near the Sun, because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity. The possible scenario of the Sun within a spur / Local arm emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way. Estimates of the pitch angle of the arms range from about 7° to 25°. There are thought to be four spiral arms that all start near the Milky Way Galaxy's center. These are named as follows, with the positions of the arms shown in the image below:
Spitzer reveals what cannot be seen in visible light: cooler stars (blue), heated dust (reddish hue), and
Sgr A* as bright white spot in the middle.
Artist's conception of the spiral structure of the Milky Way with two major stellar arms and a bar.
Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum–Centaurus Arm contains approximately 30% more
red giants than would be expected in the absence of a spiral arm. This observation suggests 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. 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. Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear.
It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer
pseudoring, and the two patterns would be connected by the Cygnus arm.
Outside of the major spiral arms is the
Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped
thick disk of the Milky Way. The structure of the Milky Way's disk is warped along an
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. However, a few globular clusters have been found farther, such as PAL 4 and AM 1 at more than 200,000 light-years from the Galactic Center. About 40% of the Milky Way's clusters are on
retrograde orbits, which means they move in the opposite direction from the Milky Way rotation. The globular clusters can follow
rosette orbits about the Milky Way, in contrast to the
elliptical orbit of a planet around a star.
Although the disk contains dust that 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 cool gas to collapse into stars.Open clusters are also located 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 farther than previously thought, the possibility of the disk of the Milky Way extending farther is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the
Cygnus Arm and of a similar extension of the
Scutum–Centaurus 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.
Sloan Digital Sky Survey of the northern sky shows 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 Milky Way. 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.
In addition to the stellar halo, the
Chandra X-ray Observatory,
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 farther 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 Milky Way itself. The temperature of this halo gas is between 1 and 2.5 million K (1.8 and 4.5 million °F).
Observations of distant galaxies indicate that the Universe had about one-sixth as much
baryonic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way. If the finding that the mass of the halo is comparable to the mass of the Milky Way is confirmed, it could be the identity of the missing baryons around the Milky Way.
The stars and gas in the Milky Way rotate about its center
differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 ± 10 km/s (470,000 ± 22,000 mph). 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 Milky Way the orbit speeds are too low, whereas beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation.
If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotational speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter. The rotation curve of the Milky Way agrees with the
universal rotation curve of spiral galaxies, the best evidence for the existence of
dark matter in galaxies. Alternatively, a minority of astronomers propose that a
modification of the law of gravity may explain the observed rotation curve.
The Milky Way began as one or several small overdensities in the mass distribution in the
Universe shortly after the
Big Bang 13.61 billion years ago. Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. Nearly half the matter in the Milky Way may have come from other distant galaxies. Nonetheless, these stars and clusters now comprise the stellar halo of the Milky Way. 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.
Since the first stars began to form, the Milky Way has grown through both
galaxy mergers (particularly early in the Milky Way's growth) and accretion of gas directly from the Galactic halo. The Milky Way is currently accreting material from several small galaxies, including two of its largest satellite galaxies, the
Small Magellanic Clouds, through the
Magellanic Stream. Direct accretion of gas is observed in
high-velocity clouds like the
Smith Cloud. Cosmological simulations indicate that, 11 billion years ago, it merged with a particularly large galaxy that has been labeled the
Kraken. 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.
According to recent studies, the Milky Way as well as the Andromeda Galaxy lie in what in the
galaxy color–magnitude diagram is known as the "green valley", a region populated by galaxies in transition from the "blue cloud" (galaxies actively forming new stars) to the "red sequence" (galaxies that lack star formation). Star-formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both the Milky Way and the Andromeda Galaxy. In fact, measurements of other galaxies similar to the Milky Way suggest it is among the reddest and brightest spiral galaxies that are still forming new stars and it is just slightly bluer than the bluest red sequence galaxies.
Age and cosmological history
Globular clusters are among the oldest objects in the Milky Way, which thus set a lower limit on the age of the Milky Way. The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived
radioactive elements such as
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 and 13.8 ± 4 billion years for
BD +17° 3248. Once a
white dwarf 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. 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.
In November 2018, astronomers reported the discovery of one of the oldest stars in the universe. About 13.5 billion-years-old,
2MASS J18082002-5104378 B is a tiny ultra metal-poor (UMP) star made almost entirely of materials released from the
Big Bang, and is possibly one of the first stars. The discovery of the star in the Milky Way
Galaxy suggests that the galaxy may be at least 3 billion years older than previously thought.
Several individual stars have been found in the Milky Way's halo with measured ages very close to the 13.80-billion-year
age of the Universe. In 2007, a star in the galactic halo,
HE 1523-0901, was estimated to be about 13.2 billion years old. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way. This estimate was made 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. Another star,
HD 140283, is 14.5 ± 0.7 billion years old.
According to observations utilizing
adaptive optics to correct for Earth's atmospheric distortion, stars in the galaxy's bulge date to about 12.8 billion years old.
The age of stars in the galactic
thin disk has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the
galactic halo and the thin disk. Recent analysis of the chemical signatures of thousands of stars suggests that stellar formation might have dropped by an order of magnitude at the time of disk formation, 10 to 8 billion years ago, when interstellar gas was too hot to form new stars at the same rate as before.
The satellite galaxies surrounding the Milky Way are not randomly distributed but seem to be the result of a breakup of some larger system producing a ring structure 500,000 light-years in diameter and 50,000 light-years wide. Close encounters between galaxies, like that expected in 4 billion years with the Andromeda Galaxy rips off huge tails of gas, which, over time can coalesce to form dwarf galaxies in a ring at an arbitrary angle to the main disc.
Diagram of the galaxies in the
Local Group relative to the Milky Way
The Milky Way and the
Andromeda Galaxy are a
binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the
Local Group, surrounded by a Local Void, itself being part of the
Local Sheet and in turn the
Virgo Supercluster. Surrounding the Virgo Supercluster are a number of voids, devoid of many galaxies, the Microscopium Void to the "north", the Sculptor Void to the "left", the
Boötes Void to the "right" and the Canes-Major Void to the "south". These voids change shape over time, creating filamentous structures of galaxies. The Virgo Supercluster, for instance, is being drawn towards the
Great Attractor, which in turn forms part of a greater structure, called
with further confirmation in 2012 researchers reported that most satellite galaxies of the Milky Way lie in a very large disk and orbit in the same direction. This came as a surprise: according to standard cosmology, the satellite galaxies should form in dark matter halos, and they should be widely distributed and moving in random directions. This discrepancy is still not explained.
In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they orbit the Milky Way, causing vibrations when they pass through its edges. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, in a computer model, the movement of these two galaxies creates a dark matter wake that amplifies their influence on the larger Milky Way.
Current measurements suggest the Andromeda Galaxy is approaching the Milky Way at 100 to 140 km/s (220,000 to 310,000 mph). In 4.3 billion years, there may be an
Andromeda–Milky Way collision, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, the chance of individual
stars colliding with each other is extremely low, but instead the two galaxies will merge to form a single
elliptical galaxy or perhaps a large
disk galaxy over the course of about six billion years.
One such frame of reference is the
Hubble flow, the apparent motions of galaxy clusters due to the
expansion of space. Individual galaxies, including the Milky Way, have
peculiar velocities relative to the average flow. Thus, to compare the Milky Way to the Hubble flow, one must consider a volume large enough so that the expansion of the Universe dominates over local, random motions. A large enough volume means that the mean motion of galaxies within this volume is equal to the Hubble flow. Astronomers believe the Milky Way is moving at approximately 630 km/s (1,400,000 mph) with respect to this local co-moving frame of reference. The Milky Way is moving in the general direction of the
Great Attractor and other
galaxy clusters, including the
Shapley Supercluster, behind it. The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda Galaxy) is part of a
supercluster called the
Local Supercluster, centered near the
Virgo Cluster: although they are moving away from each other at 967 km/s (2,160,000 mph) as part of the Hubble flow, this velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.
Another reference frame is provided by the
cosmic microwave background (CMB), in which the CMB temperature is least distorted by Doppler shift (zero dipole moment). The Milky Way is moving at 552 ± 6 km/s (1,235,000 ± 13,000 mph) with respect to this frame, toward 10.5 right ascension, −24° declination (
J2000 epoch, near the center of
Hydra). This motion is observed by satellites such as the
Cosmic Background Explorer (COBE) and the
Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get
blue-shifted in the direction of the motion and
red-shifted in the opposite direction.
^This is the diameter measured using the D25 standard. It has been recently suggested that there is a presence of disk stars beyond this diameter, although it is not clear how much of this influences the surface brightness profile.
^Some authors use the term Milky Way to refer exclusively to the band of light that the galaxy forms in the night sky, while the galaxy receives the full name Milky Way Galaxy. See for example Lausten et al., Pasachoff, Jones, van der Kruit, and Hodge et al.
^These estimates are very uncertain, as most non-star objects are difficult to detect; for example, black hole estimates range from ten million to one billion.
^Karachentsev et al. give a blue absolute magnitude of −20.8. Combined with a
color index of 0.55 estimated
here, an absolute visual magnitude of −21.35 (−20.8 − 0.55 = −21.35) is obtained. Note that determining the absolute magnitude of the Milky Way is very difficult, because Earth is inside it.
abLambert, W. G. (1964). "E. O. James: The worship of the Skygod: A comparative study in Semitic and Indo-European religion. (School of Oriental and African Studies, University of London. Jordan Lectures in Comparative Religion, vi.) viii, 175 pp. London: University of London, the Athlone Press, 1963. 25s". Bulletin of the School of Oriental and African Studies. London, England: University of London. 27 (1): 157–158.
^Pache, Corinne Ondine (2010). "Hercules". In Gargarin, Michael; Fantham, Elaine (eds.).
Ancient Greece and Rome. Vol. 1: Academy-Bible. Oxford, England: Oxford University Press. p. 400.
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^Aristotle with W. D. Ross, ed., The Works of Aristotle ... (Oxford, England: Clarendon Press, 1931), vol. III, Meteorologica, E. W. Webster, trans., Book 1, Part 8,
pp. 39–40Archived April 11, 2016, at the
Wayback Machine : "(2) Anaxagoras, Democritus, and their schools say that the milky way is the light of certain stars ... shaded by the earth from the sun's rays."
^Ragep, Jamil (1993). Nasir al-Din al-Tusi's Memoir on Astronomy (al-Tadhkira fi 'ilm al-hay' a). New York: Springer-Verlag. p. 129.
^Livingston, John W. (1971). "Ibn Qayyim al-Jawziyyah: A Fourteenth Century Defense against Astrological Divination and Alchemical Transmutation". Journal of the American Oriental Society. 91 (1): 96–103 .
^O'Connor, J. J.; Robertson, E. F. (November 2002).
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^Thomas Wright, An Original Theory or New Hypothesis of the Universe … (London, England: H. Chapelle, 1750).
On page 57Archived November 20, 2016, at the
Wayback Machine, Wright stated that despite their mutual gravitational attraction, the stars in the constellations don't collide because they are in orbit, so centrifugal force keeps them separated: " … centrifugal force, which not only preserves them in their orbits, but prevents them from rushing all together, by the common universal law of gravity, … "
On page 48Archived November 20, 2016, at the
Wayback Machine, Wright stated that the form of the Milky Way is a ring: " … the stars are not infinitely dispersed and distributed in a promiscuous manner throughout all the mundane space, without order or design, … this phænomenon [is] no other than a certain effect arising from the observer's situation, … To a spectator placed in an indefinite space, … it [i.e. the Milky Way (Via Lactea)] [is] a vast ring of stars … "
On page 65Archived November 20, 2016, at the
Wayback Machine, 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; ... "
On page 73Archived November 20, 2016, at the
Wayback Machine, Wright called the Milky Way the Vortex Magnus (the great whirlpool) and estimated its diameter to be 8.64×1012 miles (13.9×1012 km).
On page 33Archived November 20, 2016, at the
Wayback Machine, Wright speculated that there are a vast number of inhabited planets in the galaxy: " … ; therefore we may justly suppose, that so many radiant bodies [i.e. stars] were not created barely to enlighten an infinite void, but to … display an infinite shapeless universe, crowded with myriads of glorious worlds, all variously revolving round them; and … with an inconceivable variety of beings and states, animate … "
Allgemeine Naturgeschichte und Theorie des Himmels …Archived November 20, 2016, at the
Wayback Machine [Universal Natural History and Theory of Heaven … ], (Koenigsberg and Leipzig, (Germany): Johann Friederich Petersen, 1755).
On pages 2–3, Kant acknowledged his debt to Thomas Wright: "Dem Herrn Wright von Durham, einen Engeländer, war es vorbehalten, einen glücklichen Schritt zu einer Bemerkung zu thun, welche von ihm selber zu keiner gar zu tüchtigen Absicht gebraucht zu seyn scheinet, und deren nützliche Anwendung er nicht genugsam beobachtet hat. Er betrachtete die Fixsterne nicht als ein ungeordnetes und ohne Absicht zerstreutes Gewimmel, sondern er fand eine systematische Verfassung im Ganzen, und eine allgemeine Beziehung dieser Gestirne gegen einen Hauptplan der Raume, die sie einnehmen." (To Mr. Wright of Durham, an Englishman, it was reserved to take a happy step towards an observation, which seemed, to him and to no one else, to be needed for a clever idea, the exploitation of which he hasn't studied sufficiently. He regarded the fixed stars not as a disorganized swarm that was scattered without a design; rather, he found a systematic shape in the whole, and a general relation between these stars and the principal plane of the space that they occupy.)
pages xxxiii–xxxvi of the Preface (Vorrede):Archived November 20, 2016, at the
Wayback Machine"Ich betrachtete die Art neblichter Sterne, deren Herr von Maupertuis in der Abhandlung von der Figur der Gestirne gedenket, und die die Figur von mehr oder weniger offenen Ellipsen vorstellen, und versicherte mich leicht, daß sie nichts anders als eine Häufung vieler Fixsterne seyn können. Die jederzeit abgemessene Rundung dieser Figuren belehrte mich, daß hier ein unbegreiflich zahlreiches Sternenheer, und zwar um einen gemeinschaftlichen Mittelpunkt, müste geordnet seyn, weil sonst ihre freye Stellungen gegen einander, wohl irreguläre Gestalten, aber nicht abgemessene Figuren vorstellen würden. Ich sahe auch ein: daß sie in dem System, darinn sie sich vereinigt befinden, vornemlich auf eine Fläche beschränkt seyn müßten, weil sie nicht zirkelrunde, sondern elliptische Figuren abbilden, und daß sie wegen ihres blassen Lichts unbegreiflich weit von uns abstehen." (I considered the type of nebulous stars, which Mr. de Maupertuis considered in his treatise on the shape of stars, and which present the figures of more or less open ellipses, and I readily assured myself, that they could be nothing else than a cluster of fixed stars. That these figures always measured round informed me that here an inconceivably numerous host of stars, [which were clustered] around a common center, must be orderly, because otherwise their free positions among each other would probably present irregular forms, not measurable figures. I also realized: that in the system in which they find themselves bound, they must be restricted primarily to a plane, because they display not circular, but elliptical figures, and that on account of their faint light, they are located inconceivably far from us.)
^Evans, J. C. (November 24, 1998).
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^The term Weltinsel (island universe) appears nowhere in Kant's book of 1755. The term first appeared in 1850, in the third volume of von Humboldt's Kosmos: Alexander von Humboldt, Kosmos, vol. 3 (Stuttgart & Tübingen, (Germany): J.G. Cotta, 1850), pp. 187, 189.
From p. 187:Archived November 20, 2016, at the
Wayback Machine"Thomas Wright von Durham, Kant, Lambert und zuerst auch William Herschel waren geneigt die Gestalt der Milchstraße und die scheinbare Anhäufung der Sterne in derselben als eine Folge der abgeplatteten Gestalt und ungleichen Dimensionen der Weltinsel (Sternschict) zu betrachten, in welche unser Sonnensystem eingeschlossen ist." (Thomas Wright of Durham, Kant, Lambert and first of all also William Herschel were inclined to regard the shape of the Milky Way and the apparent clustering of stars in it as a consequence of the oblate shape and unequal dimensions of the world island (star stratum), in which our solar system is included.)
In the English translation – Alexander von Humboldt with
E.C. Otté, trans., Cosmos ... (New York City: Harper & Brothers, 1897), vols. 3–5. see
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^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 Milky Way appears immediately after the article's last page. See:
Rosse revealed the spiral structure of
Whirlpool Galaxy (M51) at the 1845 meeting of the British Association for the Advancement of Science. Rosse's illustration of M51 was reproduced in J.P. Nichol's book of 1846.
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Robinson, T. R. (1845).
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Rosse, The Earl of (1850).
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Kapteyn, Jacobus Cornelius (1906).
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^Licquia, Timothy; Newman, J. (2013). "Improved Constraints on the Total Stellar Mass, Color, and Luminosity of the Milky Way". American Astronomical Society, AAS Meeting #221, #254.11. 221: 254.11.
^Blandford, R. D. (August 8–12, 1998). Origin and Evolution of Massive Black Holes in Galactic Nuclei. Galaxy Dynamics, proceedings of a conference held at Rutgers University, ASP Conference Series. Vol. 182. Rutgers University (published August 1999).
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2008ASPC..387..375B. See also Bryner, Jeanna (June 3, 2008).
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^van Woerden, H.; et al. (1957). "Expansion d'une structure spirale dans le noyau du Système Galactique, et position de la radiosource Sagittarius A". Comptes Rendus de l'Académie des Sciences (in French). 244: 1691–1695.
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^Pawlowski, M.; Famaey, B.; Jerjen, H.; Merritt, D.; Kroupa, P.; Dabringhausen, J.; Lueghausen, F.; Forbes, D.; Hensler, G.; Hammer, F.; Puech, M.; Fouquet, S.; Flores, H.; Yang, Y. (August 2014). "Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies". Monthly Notices of the Royal Astronomical Society. 423 (3): 2362–2380.