The Milky Way[lower-alpha 1] 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 Latinvia lactea, from the Greekγαλακτικός κύκλος (galaktikos kýklos), meaning "milky circle."[20][21][22] 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.[23] Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis,[24] observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
It is estimated to contain 100–400 billion stars[27][28] and at least that number of planets.[29][30] The Solar System is located at a radius of about 27,000 light-years from the Galactic Center,[3] 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.[31][32] 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%)[33][34] 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".[35] The rotational period is about 240 million years at the radius of the Sun.[17] The Milky Way as a whole is moving at a velocity of approximately 600km 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 Big Bang.[36] 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.[37]
In 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.[38][39] This story was once thought to have been based on an older Sumerian version in which Tiamat is instead slain by Enlil of Nippur,[40][41] but is now thought to be purely an invention of Babylonian propagandists with the intention to show Marduk as superior to the Sumerian deities.[41]
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.[42] The Ancient Greeks believed that the appearance of the Milky Way was formed when Athena tore Heracles away from her breast which sprayed milk everywhere.[43][44]
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,[45][46] and Caer Arianrhod ("The Fortress of Arianrhod") being the constellation of Corona Borealis.[47][48]
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.[20][49][50][51]
A time-lapse video capturing the Milky Way arching over ALMA
The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching the night sky.[53] 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.[54][55] 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.[57]
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.[58] It should be visible if the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1.[59] 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.[lower-alpha 2] 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.[60]
As viewed from Earth, the visible region of the Milky Way's galactic plane occupies an area of the sky that includes 30 constellations.[lower-alpha 3] 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 anticenter 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.[citation needed]
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.[citation needed]
The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the Solar System was assumed near center
In Meteorologica, 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).[61] 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).[62][63] 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.[64]
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".[65] 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.[63] According to Jamil Ragep, the Persian astronomer Naṣīr al-Dīn al-Ṭūsī (1201–1274) in his Tadhkira writes: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color."[66]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.[67]
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.[68][69] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright,[70] 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.[71] 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.[72][73][74]
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.[75]
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.[76][77]
Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy
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.[78] It was later realized that Kapteyn's data had been the first evidence of the rotation of our galaxy,[79] which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort.[citation needed]
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.[80][81] 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.[82]
The controversy was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory 2.5m (100in) 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.[83][84]
Astrography
Map of the Milky Way Galaxy with the constellations that cross the galactic plane in each direction and the known prominent components annotated including main arms, spurs, bar, nucleus/bulge, notable nebulae and globular clusters.An all-sky view of stars in the Milky Way and neighbouring galaxies, based on the first year of observations from Gaia satellite, from July 2014 to September 2015. The map shows the density of stars in each portion of the sky. Brighter regions indicate denser concentrations of stars. Darker regions across the Galactic Plane correspond to dense clouds of interstellar gas and dust that absorb starlight.
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.[85][86] 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".[87]
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 Gould Belt).[88]
Artistic close-up of the Orion Arm with the main features of the Radcliffe Wave and Split linear structures, and with the Solar System surrounded by the closest large scale celestial features at the surface of the Local Bubble at a distance of 400-500 light years.
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).[88] Based upon studies of stellar orbits around SgrA* by Gillessen et al. (2016), the Sun lies at an estimated distance of 27.14±0.46kly (8.32±0.14kpc)[3] from the Galactic Center. Boehle et al. (2016) found a smaller value of 25.64±0.46kly (7.86±0.14kpc), also using a star orbit analysis.[2] The Sun is currently 5–30 parsecs (16–98ly) above, or north of, the central plane of the Galactic disk.[89] The distance between the local arm and the next arm out, the Perseus Arm, is about 2,000 parsecs (6,500ly).[90] The Sun, and thus the Solar System, is located in the Milky Way's galactic habitable zone.[91][92]
There are about 208stars brighter than absolute magnitude8.5 within a sphere with a radius of 15 parsecs (49ly) from the Sun, giving a density of one star per 69cubic parsecs, or one star per 2,360cubic light-years (from List of nearest bright stars). On the other hand, there are 64known stars (of any magnitude, not counting 4brown dwarfs) within 5 parsecs (16ly) of the Sun, giving a density of about one star per 8.2cubic parsecs, or one per 284cubic 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 500stars brighter than apparent magnitude4 but 15.5million stars brighter than apparent magnitude 14.[93]
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 60sky 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 2.7times per orbit.[94] 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.[95] A reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation.[96]
It takes the Solar System about 240million years to complete one orbit of the Milky Way (a galactic year),[17] so the Sun is thought to have completed 18–20orbits 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 220km/s (490,000mph) or 0.073% of the speed of light. The Sun moves through the heliosphere at 84,000km/h (52,000mph). At this speed, it takes around 1,400years for the Solar System to travel a distance of 1light-year, or 8days to travel 1AU (astronomical unit).[97] The Solar System is headed in the direction of the zodiacal constellation Scorpius, which follows the ecliptic.[98]
Diagram of the Sun's location in the Milky Way, the angles represent longitudes in the galactic coordinate system.
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.[99]
Quadrants are described using ordinals–for example, "1st galactic quadrant",[100] "second galactic quadrant",[101] or "third quadrant of the Milky Way".[102] 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:
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 structure of the Milky Way is thought to be similar to this galaxy (UGC 12158 imaged by Hubble)
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 (52–61kpc) in diameter and, on average, approximately 1,000ly (0.3kpc) thick.[14][15] To compare the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter (24.3mm (0.955in)), the Milky Way would be approximately the size of the contiguous United States.[104] 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 (46–55kpc),[105] which may be part of the Milky Way itself.[106]
A schematic profile of the Milky Way. 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),[33][34] 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.[107][108][109] Measurements using the Very Long Baseline Array in 2009 found velocities as large as 254km/s (570,000mph) for stars at the outer edge of the Milky Way.[110] 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 160,000ly (49kpc) of its center.[111] In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kiloparsecs is 7×1011M☉.[112] According to a study published in 2014, the mass of the entire Milky Way is estimated to be 8.5×1011M☉,[113] but this is only half the mass of the Andromeda Galaxy.[113] A recent mass estimate for the Milky Way is 1.29×1012M☉.[114]
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☉.[6][7][115] Recent studies indicate a range in mass, as large as 4.5×1012M☉[116] and as small as 8×1011M☉.[117] By comparison, the total mass of all the stars in the Milky Way is estimated to be between 4.6×1010M☉[118] and 6.43×1010M☉.[6] In addition to the stars, there is also interstellar gas, comprising 90% hydrogen and 10% helium by mass,[119] with two thirds of the hydrogen found in the atomic form and the remaining one-third as molecular hydrogen.[120] The mass of the Milky Way's interstellar gas is equal to between 10%[120] and 15%[119] of the total mass of its stars. Interstellar dust accounts for an additional 1% of the total mass of the gas.[119]
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.[33][34]
Contents
360-degree panorama view of the Milky Way (an assembled mosaic of photographs) by ESO, the galactic centre is in the middle of the view, with galactic north up360-degree rendering of the Milky Way using Gaia EDR3 data showing interstellar gas, dust backlit by stars (main patches labeled in black; white labels are main bright patches of stars). Left hemisphere is facing the galactic center, right hemisphere is facing the galactic anticenter.
The Milky Way contains between 100 and 400billion stars[121][122] and at least that many planets.[123] 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 300ly (90pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars.[124] The Milky Way may contain ten billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes.[lower-alpha 4][127][128] 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,[129] 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.[130][131]
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.[132] 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 180,000ly (55kpc).[133] 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.[134][135][lower-alpha 5]
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,[29][136] and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.[137][138] 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.[30] A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sizedexoplanets reside in the Milky Way.[139] 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.[140][141][142] 11 billion of these estimated planets may be orbiting Sun-like stars.[143] The nearest exoplanet may be 4.2 light-years away, orbiting the red dwarfProxima Centauri, according to a 2016 study.[144] Such Earth-sized planets may be more numerous than gas giants.[29] Besides exoplanets, "exocomets", comets beyond the Solar System, have also been detected and may be common in the Milky Way.[145] More recently, in November 2020, over 300 million habitable exoplanets are estimated to exist in the Milky Way Galaxy.[146]
Structure
Overview of different elements of the overall structure of the Milky Way.
Artist's impression of how the Milky Way would look from different vantage points – from edge-on lines-of-sight, the peanut-shell-shaped structure, not to be confused with the galaxy's central bulge, is evident; viewed from above, the central narrow bar that is responsible for this structure appears clearly, as would many spiral arms and their associated dust clouds
The Milky Way consists of a bar-shaped core region surrounded by a warped disk of gas, dust and stars.[148][149] 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.[4] Astronomers first began to conjecture that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1960s.[150][151][152] 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.[153]
The Sun is 25,000–28,000ly (7.7–8.6kpc) 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.[154][2][3][155][156][157] In the inner few kiloparsecs (around 10,000light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge.[158] 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.[159] 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 0.5kpc, abounds.[160]
The Galactic Center is marked by an intense radio source named Sagittarius A* (pronounced Sagittarius A-star). The motion of material around the center indicates that SagittariusA* harbors a massive, compact object.[161] This concentration of mass is best explained as a supermassive black hole[lower-alpha 6][154][162] (SMBH) with an estimated mass of 4.1–4.5 million times the mass of the Sun.[162] The rate of accretion of the SMBH is consistent with an inactive galactic nucleus, being estimated at around 1×10−5M☉ per year.[163] Observations indicate that there are SMBHs located near the center of most normal galaxies.[164][165]
The nature of the Milky Way's bar is actively debated, with estimates for its half-length and orientation spanning from 1to 5kpc (3,000–16,000ly) and 10–50degrees relative to the line of sight from Earth to the Galactic Center.[156][157][166] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[167] However, RRLyrae-type stars do not trace a prominent Galactic bar.[157][168][169] The bar may be surrounded by a ring called the "5kpc 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.[170] X-ray emission from the core is aligned with the massive stars surrounding the central bar[163] and the Galactic ridge.[171]
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.[172][173][174] The observations were both made by NASA and 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.[173]
Illustration of the two gigantic X-ray/gamma-ray bubbles (blue-violet) of the Milky Way (center)
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 (7.7kpc) (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.[175][176] 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 640ly (200pc) of the Milky Way.[177]
Later, on January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from SagittariusA*. 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 SagittariusA*.[147]
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.[178] 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[179][180] and molecular clouds.[181]
The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's arms.[182] Perfect logarithmic spiral patterns only crudely describe features near the Sun,[180][183] because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.[157][183][184] The possible scenario of the Sun within a spur / Local arm[180] emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way.[183] Estimates of the pitch angle of the arms range from about 7° to 25°.[129][185] There are thought to be four spiral arms that all start near the Milky Way Galaxy's center.[186] These are named as follows, with the positions of the arms shown in the image below:
Observed (normal lines) and extrapolated (dotted lines) structure of the spiral arms of the Milky Way, viewed from north of the galaxy – the galaxy rotates clockwise in this view. The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations
Spitzer reveals what cannot be seen in visible light: cooler stars (blue), heated dust (reddish hue), and SgrA* 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.[182]
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.[185][188] 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.[182] 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.[189][190][191] 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.[191]
Clusters detected by WISE used to trace the Milky Way's spiral arms.
The Near 3 kpc Arm (also called the Expanding 3kpc Arm or simply the 3kpc Arm) was discovered in the 1950s by astronomer vanWoerden and collaborators through 21centimeter radio measurements of HI (atomic hydrogen).[192][193] It was found to be expanding away from the central bulge at more than 50km/s. It is located in the fourth galactic quadrant at a distance of about 5.2kpc from the Sun and 3.3kpc from the Galactic Center. The Far 3kpc Arm was discovered in 2008 by astronomer Tom Dame (Harvard–Smithsonian CfA). It is located in the first galactic quadrant at a distance of 3kpc (about 10,000ly) from the Galactic Center.[193][194]
A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.[195]
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,[196] and the two patterns would be connected by the Cygnus arm.[197]
The long filamentary molecular cloud dubbed "Nessie" probably forms a dense "spine" of the Scutum–Centarus 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.[198] The structure of the Milky Way's disk is warped along an "S" curve.[199]
Halo
The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30kpc) of the Galactic Center.[200] However, a few globular clusters have been found farther, such as PAL4 and AM1 at more than 200,000light-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.[201] The globular clusters can follow rosette orbits about the Milky Way, in contrast to the elliptical orbit of a planet around a star.[202]
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.[17]Open clusters are also located primarily in the disk.[203]
Discoveries in the early 21stcentury 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,[204] 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[187][205] and of a similar extension of the Scutum–Centaurus Arm.[206] 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.[citation needed]
The Sloan Digital Sky Survey of the northern sky shows a huge and diffuse structure (spread out across an area around 5,000times 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 (9kpc) away.[207]
Gaseous halo
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 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.[208][209][210] The temperature of this halo gas is between 1 and 2.5millionK (1.8 and 4.5million°F).[211]
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.[212] 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.[212]
Galactic rotation
Galaxy rotation curve for the Milky Way – vertical axis is speed of rotation about the galactic center; horizontal axis is distance from the galactic center in kpcs; the sun is marked with a yellow ball; the observed curve of speed of rotation is blue; the predicted curve based upon stellar mass and gas in the Milky Way is red; scatter in observations roughly indicated by gray bars, the difference is due to dark matter
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±10km/s (470,000±22,000mph).[215] 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.[citation needed]
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.[35] 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.[216]
The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang.[217] 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.[217] 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.[218][219]
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.[219] The Milky Way is currently accreting material from several small galaxies, including two of its largest 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.[220][221] Cosmological simulations indicate that, 11 billion years ago, it merged with a particularly large galaxy that has been labeled the Kraken.[222][223] 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.[224][225]
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.[226] 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.[227]
Age and cosmological history
Comparison of the night sky with the night sky of a hypothetical planet within the Milky Way 10 billion years ago, at an age of about 3.6 billion years and 5 billion years before the Sun formed.
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 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[229] and 13.8 ± 4 billion years for BD +17° 3248.[230] 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 12.6billion years, and a 95% confidence upper limit of 16billion years.[231]
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.[232][233][234]
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.[235] 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.[235] Another star, HD 140283, is 14.5 ± 0.7 billion years old.[36][236]
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.[237]
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.[238] 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.[239]
The satellite galaxies surrounding the Milky way are not randomly distributed but seem to be the result of a break-up of some larger system producing a ring structure 500,000 light-years in diameter and 50,000 light-years wide.[240] 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.[241]
Intergalactic neighbourhood
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[242] 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 Bootes 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,[243] which in turn forms part of a greater structure, called Laniakea.[244]
In 2014 researchers reported that most satellite galaxies of the Milky Way lie in a very large disk and orbit in the same direction.[248] 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 fully explained.[249]
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.[250]
Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140km/s (220,000 to 310,000mph). In 3 to 4 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[251] over the course of about a billion years.[252]
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 630km/s (1,400,000mph) with respect to this local co-moving frame of reference.[253][254] The Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters, including the Shapley supercluster, behind it.[255] 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 967km/s (2,160,000mph) as part of the Hubble flow, this velocity is less than would be expected given the 16.8million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.[256]
Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at 552±6km/s (1,235,000±13,000mph)[19] with respect to the photons of the CMB, toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.[19]
↑ 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.[125][126]
↑ 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.
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, that clustering and merging allows galaxies to accumulate mass, determining both their shape and structure.
The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with diameter of about 220,000 ly approximately 2.5 million light-years from Earth and the nearest large galaxy to the Milky Way. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the Ethiopian princess who was the wife of Perseus in Greek mythology.
The rotation curve of a disc galaxy is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived by applying gravity theory to the matter observed in a galaxy. Theories involving dark matter are the main postulated solutions to account for the variance.
The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598. The Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Andromeda Galaxy and the Milky Way. It is one of the most distant permanent objects that can be viewed with the naked eye.
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.
The Galactic Center is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact radio source which is almost exactly at the galactic rotational center. The Galactic Center is approximately 8 kiloparsecs (26,000 ly) away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest, visually close to the Butterfly Cluster (M6) or the star Shaula, south to the Pipe Nebula.
In astronomy, a galactic bulge is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.
The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy, is an elliptical loop-shaped satellite galaxy of the Milky Way. It contains four globular clusters in its main body, with the brightest of them – NGC 6715 (M54) – being known well before the discovery of the galaxy itself in 1994. Sgr dSph is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth, travelling in a polar orbit at a distance of about 50,000 light-years from the core of the Milky Way. In its looping, spiraling path, it has passed through the plane of the Milky Way several times in the past. In 2018 the Gaia project of the European Space Agency showed that Sgr dSph had caused perturbations in a set of stars near the Milky Way's core, causing unexpected rippling movements of the stars triggered when it moved past the Milky Way between 300 and 900 million years ago.
Sagittarius A*, abbreviated Sgr A* is the supermassive black hole at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, visually close to the Butterfly Cluster (M6) and Lambda Scorpii.
According to modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matter halo may contain multiple virialized clumps of dark matter bound together by gravity, known as subhalos. Modern cosmological models, such as ΛCDM, propose that dark matter halos and subhalos may contain galaxies. The dark matter halo of a galaxy envelops the galactic disc and extends well beyond the edge of the visible galaxy. Thought to consist of dark matter, halos have not been observed directly. Their existence is inferred through observations of their effects on the motions of stars and gas in galaxies and gravitational lensing. Dark matter halos play a key role in current models of galaxy formation and evolution. Theories that attempt to explain the nature of dark matter halos with varying degrees of success include cold dark matter (CDM), warm dark matter, and massive compact halo objects (MACHOs).
The Milky Way has several smaller galaxies gravitationally bound to it, as part of the Milky Way subgroup, which is part of the local galaxy cluster, the Local Group.
In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space.
Sagittarius A or Sgr A is a complex radio source at the center of the Milky Way which contains a supermassive black hole. It is located in the constellation Sagittarius, and is hidden from view at optical wavelengths by large clouds of cosmic dust in the spiral arms of the Milky Way.
Coma Berenices or Com is a dwarf spheroidal galaxy situated in the Coma Berenices constellation and discovered in 2006 in data obtained by the Sloan Digital Sky Survey. The galaxy is located at the distance of about 44 kpc from the Sun and moves away from the Sun with the velocity of about 98 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an elliptical shape with the half-light radius of about 70 pc.
Boötes II or Boo II is a dwarf spheroidal galaxy situated in the constellation Boötes and discovered in 2007 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 42 kpc from the Sun and moves towards the Sun with the speed of 120 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately round shape with the half-light radius of about 51 pc.
The thick disk is one of the structural components of about 2/3 of all disk galaxies, including the Milky Way. It was discovered first in external edge-on galaxies. Soon after, it was proposed as a unique galactic structure in the Milky Way, different from the thin disk and the halo in the 1983 article by Gilmore & Reid. It is supposed to dominate the stellar number density between 1 and 5 kiloparsecs above the galactic plane and, in the solar neighborhood, is composed almost exclusively of older stars. Its stellar chemistry and stellar kinematics are also said to set it apart from the thin disk. Compared to the thin disk, thick disk stars typically have significantly lower levels of metals—that is, the abundance of elements other than hydrogen and helium.
In astronomy, the Sagittarius Stream is a long, complex structure made of stars that wrap around the Milky Way galaxy in an orbit that nearly crosses the galactic poles. It consists of tidally stripped stars from the Sagittarius Dwarf Elliptical Galaxy, resulting from the process of merging with the Milky Way over a period of billions of years.
Galaxy X is a postulated dark satellite dwarf galaxy of the Milky Way Galaxy. If it exists, it would be composed mostly of dark matter and interstellar gas with few stars. Its proposed location is some 90 kpc (290 kly) from the Sun, behind the disk of the Milky Way, and some 12 kpc (39 kly) in extent. Galactic coordinates would be (l= -27.4°,b=-1.08°).
NGC 5053 is the New General Catalogue designation for a globular cluster in the northern constellation of Coma Berenices. It was discovered by German-British astronomer William Herschel on March 14, 1784 and cataloged as VI-7. In his abbreviated notation, he described it as, "an extremely faint cluster of extremely small stars with resolvable nebula 8 or 10′ diameter, verified by a power of 240, beyond doubt". Danish-Irish astronomer John Louis Emil Dreyer reported in 1888 that the cluster appeared, "very faint, pretty large, irregular round shape, growing very gradually brighter at the middle".
The Gaia Sausage or Gaia Enceladus is the remains of a dwarf galaxy that merged with the Milky Way about 8–11 billion years ago. At least eight globular clusters were added to the Milky Way along with 50 billion solar masses of stars, gas and dark matter.
↑ Karachentsev, Igor. "Double Galaxies §7.1". ned.ipac.caltech.edu. Izdatel'stvo Nauka. Archived from the original on March 4, 2016. Retrieved April 5, 2015.
↑ Shapley, H.; Curtis, H. D. (1921). "The Scale of the Universe". Bulletin of the National Research Council. 2 (11): 171–217. Bibcode:1921BuNRC...2..171S.
↑ James, E. O. (1963). The Worship of the Skygod: A Comparative Study in Semitic and Indo-European Religion. Jordan Lectures in Comparative religion. London, England: University of London. pp.24, 27f.
1 2 Lambert, W. G. (1964). "Bulletin of the School of Oriental and African Studies". 27 (1). London, England: University of London: 157–158.{{cite journal}}: Cite journal requires |journal= (help)
↑ Simpson, John; Weiner, Edmund, eds. (March 30, 1989). The Oxford English Dictionary (2nded.). Oxford University Press. ISBN978-0-19-861186-8. See the entries for "Milky Way" and "galaxy".
↑ 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."
↑ (Aristotle with Ross, 1931), p. 41–43: Archived April 11, 2016, at the Wayback Machine "For it is natural to suppose that, if the motion of a single star excites a flame, that of all the stars should have a similar result, and especially in that region in which the stars are biggest and most numerous and nearest to one another. ... all this matter is continually and ceaselessly collecting there ... in that part of the terrestrial world which is continuous with the motions of the heavens."
↑ 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 [99]. doi:10.2307/600445. JSTOR600445.
↑ Galileo Galilei, Sidereus Nuncius (Venice, (Italy): Thomas Baglioni, 1610), pages 15 and 16.Archived March 16, 2016, at the Wayback Machine English translation: Galileo Galilei with Edward Stafford Carlos, trans., The Sidereal Messenger (London: Rivingtons, 1880), pages 42 and 43.Archived December 2, 2012, at the Wayback Machine
↑ O'Connor, J. J.; Robertson, E. F. (November 2002). "Galileo Galilei". University of St. Andrews. Archived from the original on May 30, 2012. Retrieved January 8, 2007.
↑ 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 … "
↑ Immanuel Kant, 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.)
↑ Kant (1755), 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). "Our Galaxy". George Mason University. Archived from the original on June 30, 2012. Retrieved January 4, 2007.
↑ 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 p. 147Archived November 6, 2018, at the Wayback Machine .
↑ 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.
Rosse, Earl of (1846). "On the nebula 25 Herschel, or 61 [should read: 51] of Messier's catalogue". Report of the Fifteenth Meeting of the British Association for the Advancement of Science; Held at Cambridge in June 1845 § Notices and Abstracts of Miscellaneous Communications to the Sections. Report of the ... Meeting of the British Association for the Advancement of Science (1833): 4.
Robinson, T.R. (1845). "On Lord Rosse's telescope". Proceedings of the Royal Irish Academy. 3 (50): 114–133. Rosse's early observations of nebulae and galaxies are discussed on pp. 127–130.
Rosse, The Earl of (1850). "Observations on the nebulae". Philosophical Transactions of the Royal Society of London. 140: 499–514. doi:10.1098/rstl.1850.0026. Rosse's illustrations of nebulae and galaxies appear on the plates that immediately precede the article.
Kapteyn, Jacobus Cornelius (1906). "Statistical methods in stellar astronomy". In Rogers, Howard J. (ed.). Congress of Arts and Science, Universal Exposition, St. Louis, 1904. Vol.4. Boston and New York: Houghton, Mifflin and Co. pp.396–425. From pp. 419–420: "It follows that the one set of the stars must have a systematic motion relative to the other. … these two main directions of motion must be in reality diametrically opposite."
Kapteyn, J.C. (1905). "Star streaming". Report of the Seventy-fifth Meeting of the British Association for the Advancement of Science, South Africa. Report of the ... Meeting of the British Association for the Advancement of Science (1833): 257–265.
Schwarzschild, K. (1907). "Ueber die Eigenbewegungen der Fixsterne"[On the proper motions of the fixed stars]. Nachrichten von der Königlichen Gesellschaft der Wissenschaften zu Göttingen (Reports of the Royal Society of Science at Göttingen) (in German). 5: 614–632. Bibcode:1907NWGot...5..614S.
↑ Mullen, Leslie (May 18, 2001). "Galactic Habitable Zones". NAI Features Archive. Nasa Astrobiology Institute. Archived from the original on April 9, 2013. Retrieved May 9, 2013.
↑ Blaauw, A.; etal. (1960), "The new I. A. U. system of galactic coordinates (1958 revision)", Monthly Notices of the Royal Astronomical Society, 121 (2): 123–131, Bibcode:1960MNRAS.121..123B, doi:10.1093/mnras/121.2.123
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