The Milky Way is the galaxy that includes our 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 Greek γαλακτικός κύκλος (galaktikos 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 Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
The Milky Way is a barred spiral galaxy with an estimated visible diameter of 100,000–200,000 light-years, but only about 1000 light years thick at the spiral arms (more at the bulge). Recent simulations suggest that a dark matter area, also containing some visible stars, may extend up to a diameter of almost 2 million light-years. The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.
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 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. 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 240 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 with respect to extra-galactic 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 Big Bang. On 12 May 2022, astronomers announced the image, for the first time, of Sagittarius A, the supermassive black hole at the center of the Milky Way galaxy.
Etymology and mythology
In the Babylonian epic poem Enūma Eliš, the Milky Way is created from the severed tail of the primeval salt water dragoness Tiamat, 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 Enlil of 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.
In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on 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 Athena to rip him from her breast in pain.
Llys Dôn (literally "The Court of Dôn") is the traditional Welsh name for the constellation Cassiopeia. At least three of Dôn's children also have astronomical associations: Caer Gwydion ("The fortress of Gwydion") is the traditional Welsh name for the Milky Way, and Caer Arianrhod ("The Fortress of Arianrhod") being the constellation of Corona Borealis.
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 (galaxías kýklos), meaning "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.
The Milky Way, or "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.
Appearance
The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching the night sky. In night sky observing, although all the individual naked-eye stars in the entire sky are part of the Milky Way Galaxy, the term "Milky Way" is limited to this band of light. The light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Brighter regions around the band appear as soft visual patches known as star clouds. The most conspicuous of these is the Large Sagittarius Star Cloud, a portion of the central bulge of the galaxy.[56] Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars. The area of sky that the Milky Way obscures is called the Zone of Avoidance.
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 arc-second 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. 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.
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 Galactic Center lies in the direction of Sagittarius, where the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass around to the galactic anti-center in Auriga. The band then continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two roughly equal hemispheres.
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° (B1950) near 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.
Astronomical history
In Meteorologica, Aristotle (384–322 BC) states that the Greek philosophers Anaxagoras (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). The Neoplatonist philosopher 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 Islamic world.
The Persian astronomer Al-Biruni (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars". 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 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" and that these stars are larger than planets.
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. 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 "extra-galactic 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.
In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.
In 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.
In 1917, 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 Doppler shift.
The controversy was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory 2.5 m (100 in) Hooker telescope. With the light-gathering power of this new telescope, he was able to produce astronomical photographs that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some Cepheid variables that he could use as a benchmark to estimate the distance to the nebulae. He found that the Andromeda Nebula is 275,000 parsecs from the Sun, far too distant to be part of the Milky Way.
Astrography
The 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. 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 principle 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".
Sun's location and neighborhood
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 life-form 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.
Galactic quadrants
A galactic quadrant, or quadrant of the Milky Way, refers to one of four circular sectors in the division of the Milky Way. In astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the origin of the mapping system.
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:
Galactic
quadrant
Galactic
longitude
(ℓ)
Reference
1st 0° ≤ ℓ ≤ 90°
2nd 90° ≤ ℓ ≤ 180°
3rd 180° ≤ ℓ ≤ 270°
4th
270° ≤ ℓ ≤ 360°
(360° ≅ 0°)
with the galactic longitude (ℓ) increasing in the counter-clockwise direction (positive rotation) as viewed from north of the galactic center (a view-point several hundred thousand light-years distant from Earth in the direction of the constellation Coma Berenices); if viewed from south of the galactic center (a view-point similarly distant in the constellation Sculptor), ℓ would increase in the clockwise direction (negative rotation).
Size and mass
The Milky Way is the second-largest galaxy in the Local Group (after the Andromeda Galaxy), with its stellar disk approximately 170,000–200,000 light-years (52–61 kpc) in diameter and, on average, approximately 1,000 ly (0.3 kpc) thick. 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 the size of the contiguous United States. There is a ring-like filament of stars rippling above and below the relatively flat galactic plane, wrapping around the Milky Way at a diameter of 150,000–180,000 light-years (46–55 kpc), which may be part of the Milky Way itself.
Abbreviations: GNP/GSP: Galactic North and South Poles
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 5.8×1011 solar 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 7×1011 M☉ 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 7×1011 M☉. According to a study published in 2014, the mass of the entire Milky Way is estimated to be 8.5×1011 M☉, but this is only half the mass of the Andromeda Galaxy. A recent mass estimate for the Milky Way is 1.29×1012 M☉.
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 kilo-parsecs (kpc) from the Galactic Center. Mathematical models of the Milky Way suggest that the mass of dark matter is 1–1.5×1012 M☉. Recent studies indicate a range in mass, as large as 4.5×1012 M☉ and as small as 8×1011 M☉. By comparison, the total mass of all the stars in the Milky Way is estimated to be between 4.6×1010 M☉ and 6.43×1010 M☉. 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 mass of the Milky Way galaxy is 1.5 trillion solar masses within a radius of about 129,000 light-years, over twice as much as was determined in earlier studies, and suggesting that about 90% of the mass of the galaxy is dark matter.
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. 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.
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, 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-sized exoplanets 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 dwarf Proxima 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, "exocomets", 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.
Structure
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.
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