Milky Way
Milky Way | |
---|---|
Observation data (J2000 epoch) | |
Constellation | Sagittarius |
Right ascension | 17h 45m 40.03599s[1] |
Declination | −29° 00′ 28.1699″[1] |
Distance | 7.935–8.277 kpc (25,881–26,996 ly)[2][3][4][a] |
Characteristics | |
Type | Sb; Sbc; SB(rs)bc[5][6] |
Mass | 1.15×1012[7][8][9] M☉ |
Number of stars | 100–400 billion ((1–4)×1011)[12][13] |
Size | 26.8 ± 1.1 kpc (87,400 ± 3,600 ly) (diameter; D25 isophote)[10][b] |
Thickness of thin disk | 220–450 pc (718–1,470 ly)[14] |
Thickness of thick disk | 2.6 ± 0.5 kpc (8,500 ± 1,600 ly)[14] |
Angular momentum | ~1×1067 J s[15] |
Sun's Galactic rotation period | 212 Myr[16] |
Spiral pattern rotation period | 220–360 Myr[17] |
Bar pattern rotation period | 160–180 Myr[18] |
Speed relative to CMB rest frame | 552.2±5.5 km/s[19] |
Escape velocity at Sun's position | 550 km/s[20] |
Dark matter density at Sun's position | 0.0088+0.0024 −0.0018 M☉pc−3 (0.35+0.08 −0.07 GeV cm−3)[20] |
The Milky Way[c] is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos), meaning "milky circle".[26][27] 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.[28] Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis,[29] observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
The Milky Way is a
It is estimated to contain 100–400 billion stars
The Milky Way as a whole is moving at a velocity of approximately 600 km per second (372 miles per second) with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.[42]
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.[43][44] This story was once thought to have been based on an older Sumerian version in which Tiamat is instead slain by Enlil of Nippur,[45][46] but is now thought to be purely an invention of Babylonian propagandists with the intention to show Marduk as superior to the Sumerian deities.[46]
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 become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way. In another Greek story, the abandoned Heracles is given by Athena to Hera for feeding, but Heracles' forcefulness causes Hera to rip him from her breast in pain.[47][48][49]
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,[50][51] and Caer Arianrhod ("The Fortress of Arianrhod") being the constellation of Corona Borealis.[52][53]
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
The Milky Way, or "milk circle", was just one of 11 "circles" the Greeks identified in the sky, others being the
Appearance
The Milky Way is visible as a hazy band of white light, some 30° wide, arching the
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.[63] It should be visible if the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1.[64] This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but very prominent when viewed from rural areas when the Moon is below the horizon.[d] Maps of artificial night sky brightness show that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution.[65]
As viewed from Earth, the visible region of the Milky Way's galactic plane occupies an area of the sky that includes 30 constellations.[e] 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
Astronomical history
In
The
Proof of the Milky Way consisting of many stars came in 1610 when
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.[81]
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.[82][83]
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.[84] It was later realized that Kapteyn's data had been the first evidence of the rotation of our galaxy,[85] which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort.[citation needed]
In 1917,
The controversy was conclusively settled by
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.[91][92]
Data from Gaia has been described as "transformational". It has been estimated that Gaia has expanded the number of observations of stars from about 2 million stars as of the 1990s to 2 billion. It has expanded the measurable volume of space by a factor of 100 in radius and a factor of 1,000 in precision.[93]
A study in 2020 concluded that Gaia detected a wobbling motion of the galaxy, which might be caused by "
Sun's location and neighborhood
The
There are about 208 stars brighter than
The apex of the Sun's way, or the
It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a
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.[108]
Quadrants are described using
with the galactic longitude
Size and mass
Size
The Milky Way is one of the two largest galaxies in the Local Group (the other being the Andromeda Galaxy), although the size for its galactic disc and how much it defines the isophotal diameter is not well understood.[11] It is estimated that the significant bulk of stars in the galaxy lies within the 26 kiloparsecs (80,000 light-years) diameter, and that the number of stars beyond the outermost disc dramatically reduces to a very low number, with respect to an extrapolation of the exponential disk with the scale length of the inner disc.[113][11]
There are several methods being used in astronomy in defining the size of a galaxy, and each of them can yield different results with respect to one another. The most commonly employed method is the
This is significantly smaller than the Andromeda Galaxy's isophotal diameter, and slightly below the mean isophotal sizes of the galaxies being at 28.3 kpc (92,000 ly).[10] The paper concludes that the Milky Way and Andromeda Galaxy were not overly large spiral galaxies and as well as one of the largest known (if the former not being the largest) as previously widely believed, but rather average ordinary spiral galaxies.[117] To compare the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter (24.3 mm (0.955 in)), the Milky Way would be approximately at least the greatest north–south line of the contiguous United States.[118] An even older study from 1978 gave a lower diameter for Milky Way about 23 kpc (75,000 ly).[10]
A 2015 paper discovered that there is a ring-like filament of stars called Triangulum–Andromeda Ring (TriAnd Ring) rippling above and below the relatively flat galactic plane, which alongside Monoceros Ring were both suggested to be primarily the result of disk oscillations and wrapping around the Milky Way, at a diameter of at least 50 kpc (160,000 ly),[119] which may be part of the Milky Way's outer disk itself, hence making the stellar disk larger by increasing to this size.[120] A more recent 2018 paper later somewhat ruled out this hypothesis, and supported a conclusion that the Monoceros Ring, A13 and TriAnd Ring were stellar overdensities rather kicked out from the main stellar disk, with the velocity dispersion of the RR Lyrae stars found to be higher and consistent with halo membership.[121]
Another 2018 study revealed the very probable presence of disk stars at 26–31.5 kpc (84,800–103,000 ly) from the Galactic Center or perhaps even farther, significantly beyond approximately 13–20 kpc (40,000–70,000 ly), in which it was once believed to be the abrupt drop-off of the stellar density of the disk, meaning that few or no stars were expected to be above this limit, save for stars that belong to the old population of the galactic halo.[11][122][123]
A 2020 study predicted the edge of the Milky Way's dark matter halo being around 292 ± 61 kpc (952,000 ± 199,000 ly), which translates to a diameter of 584 ± 122 kpc (1.905 ± 0.3979 Mly).[30][31] The Milky Way's stellar disk is also estimated to be approximately up to 1.35 kpc (4,000 ly) thick.[124][125]
Mass
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),[7][8][9] 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.[126][127][128] 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.[129]
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.[130] In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kiloparsecs is 7×1011 M☉.[131] In a 2014 study, the mass of the entire Milky Way is estimated to be 8.5×1011 M☉,[132] but this is only half the mass of the Andromeda Galaxy.[132] A recent 2019 mass estimate for the Milky Way is 1.29×1012 M☉.[133]
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 1–1.5×1012 M☉.[134][135][136] 2013 and 2014 studies indicate a range in mass, as large as 4.5×1012 M☉[137] and as small as 8×1011 M☉.[138] By comparison, the total mass of all the stars in the Milky Way is estimated to be between 4.6×1010 M☉[139] and 6.43×1010 M☉.[134]
In addition to the stars, there is also interstellar gas, comprising 90%
In March 2019, astronomers reported that the virial mass of the Milky Way Galaxy is 1.54 trillion solar masses within a radius of about 39.5 kpc (130,000 ly), over twice as much as was determined in earlier studies, suggesting that about 90% of the mass of the galaxy is dark matter.[7][8]
In September 2023, astronomers reported that the virial mass of the Milky Way Galaxy is only 2.06 1011 solar masses, only a 10th of the mass of previous studies. The mass was determined from data of the Gaia spacecraft.[142]
Contents
The Milky Way contains between 100 and 400 billion stars[12][13] and at least that many planets.[143] 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.[144] The Milky Way may contain ten billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes.[f][147][148] 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,[149] 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.[150][151]
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
Both
In November 2013, astronomers reported, based on
Structure
The Milky Way consists of a bar-shaped core region surrounded by a warped disk of gas, dust and stars.[166][167] 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.[5] Astronomers first began to conjecture that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1960s.[168][169][170] 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.[171]
Galactic Center
The Sun is 25,000–28,000 ly (7.7–8.6 kpc) from the Galactic Center. This value is estimated using
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 Sagittarius A* harbors a massive, compact object.[180] This concentration of mass is best explained as a supermassive black hole[h][173][181] (SMBH) with an estimated mass of 4.1–4.5 million times the mass of the Sun.[181] The rate of accretion of the SMBH is consistent with an inactive galactic nucleus, being estimated at 1×10−5 M☉ per year.[182] Observations indicate that there are SMBHs located near the center of most normal galaxies.[183][184]
The nature of the Milky Way's bar is actively debated, with estimates for its half-length and orientation spanning from 1 to 5 kpc (3,000–16,000 ly) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.[175][176][185] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[186] However, RR Lyrae-type stars do not trace a prominent Galactic bar.[176][187][188] The bar may be surrounded by a ring called the "5 kpc ring" that contains a large fraction of the molecular hydrogen present in the Milky Way, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.[189] X-ray emission from the core is aligned with the massive stars surrounding the central bar[182] and the Galactic ridge.[190]
In June 2023, astronomers reported using a new cascade neutrino technique[191] to detect, for the first time, the release of neutrinos from the galactic plane of the Milky Way galaxy, creating the first neutrino view of the Milky Way.[192][193]
Gamma rays and x-rays
Since 1970, various gamma-ray detection missions have discovered 511-
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
Later, on January 5, 2015,
Spiral arms
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.[200] 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[201][202] and molecular clouds.[203]
The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's arms.[204] Perfect logarithmic spiral patterns only crudely describe features near the Sun,[202][205] because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.[176][205][206] The possible scenario of the Sun within a spur / Local arm[202] emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way.[205] Estimates of the pitch angle of the arms range from about 7° to 25°.[149][207] There are thought to be four spiral arms that all start near the Milky Way Galaxy's center.[208] These are named as follows, with the positions of the arms shown in the image below:
Color | Arm(s) |
---|---|
turquoise | Near 3 kpc and Perseus Arm |
blue | Outer arm (Along with extension discovered in 2004[209] )
|
green | Far 3 kpc and Scutum–Centaurus Arm |
red | Carina–Sagittarius Arm |
There are at least two smaller arms or spurs, including: | |
orange | Orion–Cygnus Arm (which contains the Sun and Solar System)
|
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.[207][210]
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.[204] 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.[211][212][213] 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.[213]
The
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
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,[218] and the two patterns would be connected by the Cygnus arm.[219]
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.[220] The structure of the Milky Way's disk is warped along an "S" curve.[221]
Halo
The Galactic disk is surrounded by a
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.[105] Open clusters are also located primarily in the disk.[225]
Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much farther than previously thought,
The Sloan Digital Sky Survey of the northern sky shows a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Milky Way. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years (9 kpc) away.[229]
Gaseous halo
In addition to the stellar halo, the Chandra X-ray Observatory, XMM-Newton, and Suzaku have provided evidence that there is also a gaseous halo containing a large amount of hot gas. This halo extends for hundreds of thousands 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.[230][231][232] The temperature of this halo gas is between 1 and 2.5 million K (1.8 and 4.5 million °F).[233]
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.[234] 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.[234]
Galactic rotation
The stars and gas in the Milky Way rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 ± 10 km/s (470,000 ± 22,000 mph).[237] 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.
Formation
History
The Milky Way began as one or several small overdensities in the mass distribution in the
Since the first stars began to form, the Milky Way has grown through both
Cosmological simulations indicate that, 11 billion years ago, it merged with a particularly large galaxy that has been labeled the Kraken.[246][247] 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.[248][249]
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.[250] 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.[251]
Age and cosmological history
Globular clusters are among the oldest objects in the Milky Way, which thus set a lower limit on the age of the Milky Way. The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived
Once a white dwarf is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as 12.7 ± 0.7 billion years. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.[255]
In November 2018, astronomers reported the discovery of one of the oldest stars in the universe. About 13.5 billion-years-old,
Several individual stars have been found in the Milky Way's halo with measured ages very close to the 13.80-billion-year
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.[261]
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.[262] 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.[263]
The satellite galaxies surrounding the Milky Way are not randomly distributed but seem to be the result of a breakup of some larger system producing a ring structure 500,000 light-years in diameter and 50,000 light-years wide.[264] Close encounters between galaxies, like that expected in 4 billion years with the Andromeda Galaxy, can rip 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.[265]
Intergalactic neighbourhood
The Milky Way and the
Two smaller galaxies and a number of
The smallest dwarf galaxies of the Milky Way are only 500 light-years in diameter. These include
In 2005[273] with further confirmation in 2012[274] researchers reported that most satellite galaxies of the Milky Way lie in a very large disk and orbit in the same direction. This came as a surprise: according to standard cosmology, the satellite galaxies should form in dark matter halos, and they should be widely distributed and moving in random directions. This discrepancy is still not explained.[275]
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.[276]
Current measurements suggest the Andromeda Galaxy is approaching the Milky Way at 100 to 140 km/s (220,000 to 310,000 mph). In 4.3 billion years, there may be an
Velocity
Although
One such frame of reference is the
The Milky Way is moving in the general direction of the
Another reference frame is provided by the
See also
Notes
- ^ The distance towards its center (Sagittarius A*).
- ^ This is the diameter measured using the D25 standard. It has been recently suggested that there is a presence of disk stars beyond this diameter, although it is not clear how much of this influences the surface brightness profile.[11]
- ^ Some authors use the term Milky Way to refer exclusively to the band of light that the galaxy forms in the night sky, while the galaxy receives the full name Milky Way Galaxy. See for example Lausten et al.,[21] Pasachoff,[22] Jones,[23] van der Kruit,[24] and Hodge et al.[25]
- Bortle Dark-Sky Scale.
- ^ The bright center of the galaxy is located in the constellation Sagittarius. From Sagittarius, the hazy band of white light appears to pass westward through the constellations of Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion and Gemini, Taurus, to the galactic anticenter in Auriga. From there, it passes through Perseus, Andromeda, Cassiopeia, Cepheus and Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius.
- ^ 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.[145][146]
- ^ 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.
- ^ For a photo see: "Sagittarius A*: Milky Way monster stars in cosmic reality show". Chandra X-ray Observatory. Center for Astrophysics | Harvard & Smithsonian. January 6, 2003. Archived from the original on March 17, 2008. Retrieved May 20, 2012.
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Further reading
- Dambeck, Thorsten (March 2008). "Gaia's Mission to the Milky Way". Sky & Telescope: 36–39.
- Chiappini, Cristina (November–December 2001). "The Formation and Evolution of the Milky Way" (PDF). .
- McTier, Moiya (August 16, 2022). The Milky Way. Grand Central Publishing. ISBN 978-1-5387-5415-3.
- scientific revolutions", Scientific American, vol. 329, no. 4 (November 2023), pp. 86–87.
External links
- Milky Way – IRAS (infrared) survey – wikisky.org
- Milky Way – H-Alpha survey – wikisky.org
- Multiwavelength Milky Way – Images and VRML models (NASA)
- Milky Way – Panorama (9 billion pixels).
- Milky Way – SEDS Messier website
- Milky Way – Infrared Images
- Milky Way – Mosaic of galactic plane (March 19, 2021)
- The clickable Milky Way