The Andromeda Galaxy (IPA: /ænˈdrɒmɪdə/), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy approximately 2.5 million light-years (770 kiloparsecs) from Earth and the nearest major galaxy to the Milky Way.[4] 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 (or Phoenician) princess who was the wife of Perseus in Greek mythology.
The virial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at 1 trillion solar masses (2.0×1042 kilograms). The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy is more massive than the Milky Way by a margin of some 25% to 50%. This has been called into question by a 2018 study that cited a lower estimate on the mass of the Andromeda Galaxy,[12] combined with preliminary reports on a 2019 study estimating a higher mass of the Milky Way.[13][14] The Andromeda Galaxy has a diameter of about 220,000 ly (67 kpc), making it the largest member of the Local Group in terms of extension, if not mass.[citation needed]
The number of stars contained in the Andromeda Galaxy is estimated at one trillion (1×1012), or roughly twice the number estimated for the Milky Way.[15][needs update]
The Milky Way and Andromeda galaxies are expected to collide in around 4.5 billion years,[16][17] merging to form a giant elliptical galaxy[18] or a large lenticular galaxy.[19] With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects,[20] making it visible to the naked eye from Earth on moonless nights,[21] even when viewed from areas with moderate light pollution.
Observation history
Great Andromeda Nebula by Isaac Roberts, 1899.
Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi was the first to describe the Andromeda Galaxy. He referred to it in his Book of Fixed Stars as a "nebulous smear".[22]
Star charts of that period labeled it as the Little Cloud.[23] In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations.[24] Pierre Louis Maupertuis conjectured in 1745 that the blurry spot was an island universe.[25] In 1764, Charles Messier cataloged Andromeda as object M31 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the "great nebulae", and based on the color and magnitude of the nebula, he incorrectly guessed that it was no more than 2,000 times the distance of Sirius, or roughly 18,000 ly (5.5 kpc).[26] In 1850, William Parsons, 3rd Earl of Rosse made the first drawing of Andromeda's spiral structure.
In 1864 Sir William Huggins noted that the spectrum of Andromeda differed from that of a gaseous nebula.[27] The spectra of Andromeda displays a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. Andromeda's spectrum is very similar to the spectra of individual stars, and from this, it was deduced that Andromeda has a stellar nature. In 1885, a supernova (known as S Andromedae) was seen in Andromeda, the first and so far only one observed in that galaxy. At the time Andromeda was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly; "Nova 1885".[28]
In 1888, Isaac Roberts took one of the first photographs of Andromeda, which was still commonly thought to be a nebula within our galaxy. Roberts mistook Andromeda and similar spiral nebulae as solar systems being formed.[29][30]
In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of Andromeda with respect to our Solar System—the largest velocity yet measured, at 300 km/s (190 mi/s).[31]
Island universe
Location of the Andromeda Galaxy (M31) in the Andromeda constellation.
In 1917, Heber Curtis observed a nova within Andromeda. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred elsewhere in the sky. As a result, he was able to come up with a distance estimate of 500,000 ly (3.2×1010 AU). He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.[32]
Andromeda Galaxy above the Very Large Telescope.[33] The Triangulum Galaxy is visible on the top.
In 1920, the Great Debate between Harlow Shapley and Curtis took place concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim of the Great Andromeda Nebula being, in fact, an external galaxy, Curtis also noted the appearance of dark lanes within Andromeda which resembled the dust clouds in our own galaxy, as well as historical observations of Andromeda Galaxy's significant Doppler shift. In 1922 Ernst Öpik presented a method to estimate the distance of Andromeda using the measured velocities of its stars. His result placed the Andromeda Nebula far outside our galaxy at a distance of about 450 kpc (1,500 kly).[34] Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of Andromeda. These were made using the 2.5-metre (8 ft 2 in) Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our own galaxy, but an entirely separate galaxy located a significant distance from the Milky Way.[35]
In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their metallicity, naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge Type II. This nomenclature was subsequently adopted for stars within the Milky Way, and elsewhere. (The existence of two distinct populations had been noted earlier by Jan Oort.)[36] Baade also discovered that there were two types of Cepheid variable stars, which resulted in a doubling of the distance estimate to Andromeda, as well as the remainder of the universe.[37]
In 1950, radio emission from the Andromeda Galaxy was detected by Hanbury Brown and Cyril Hazard at Jodrell Bank Observatory.[38][39] The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group.[40] The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomy catalog. In 2009, the first planet may have been discovered in the Andromeda Galaxy. This was detected using a technique called microlensing, which is caused by the deflection of light by a massive object.[41]
Observations of linearly polarized radio emission with the Westerbork Synthesis Radio Telescope, the Effelsberg 100-m telescope, and the Very Large Array revealed ordered magnetic fields aligned along the "10-kpc ring" of gas and star formation.[42] The total magnetic field has a strength of about 0.5 nT, of which 0.3 nT are ordered.
General
The estimated distance of the Andromeda Galaxy from our own was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid variable star. In the 1990s, measurements of both standard red giants as well as red clump stars from the Hipparcos satellite measurements were used to calibrate the Cepheid distances.[43][44]
Formation and history
The Andromeda Galaxy as seen by NASA's Wide-field Infrared Survey Explorer.
The Andromeda Galaxy was formed roughly 10 billion years ago from the collision and subsequent merger of smaller protogalaxies.[45]
This violent collision formed most of the galaxy's (metal-rich) galactic halo and extended disk. During this epoch, its rate of star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years. Andromeda and the Triangulum Galaxy had a very close passage 2–4 billion years ago. This event produced high rates of star formation across the Andromeda Galaxy's disk—even some globular clusters—and disturbed M33's outer disk.
Over the past 2 billion years, star formation throughout Andromeda's disk is thought to have decreased to the point of near-inactivity. There have been interactions with satellite galaxies like M32, M110, or others that have already been absorbed by Andromeda Galaxy. These interactions have formed structures like Andromeda's Giant Stellar Stream. A galactic merger roughly 100 million years ago is believed to be responsible for a counter-rotating disk of gas found in the center of Andromeda as well as the presence there of a relatively young (100 million years old) stellar population.[45]
Distance estimate
At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy. In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value and a metallicity correction of −0.2 mag dex−1 in (O/H), an estimate of 2.57 ± 0.06 million light-years (1.625×1011 ± 3.8×109 astronomical units) was derived. A 2004 Cepheid variable method estimated the distance to be 2.51 ± 0.13 million light-years (770 ± 40 kpc).[2][3] In 2005, an eclipsing binary star was discovered in the Andromeda Galaxy. The binary[c] is two hot blue stars of types O and B. By studying the eclipses of the stars, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure their absolute magnitude. When the visual and absolute magnitudes are known, the distance to the star can be calculated. The stars lie at a distance of 2.52×106 ± 0.14×106 ly (1.594×1011 ± 8.9×109 AU) and the whole Andromeda Galaxy at about 2.5×106 ly (1.6×1011 AU).[4] This new value is in excellent agreement with the previous, independent Cepheid-based distance value. The TRGB method was also used in 2005 giving a distance of 2.56×106 ± 0.08×106 ly (1.619×1011 ± 5.1×109 AU).[5] Averaged together, these distance estimates give a value of 2.54×106 ± 0.11×106 ly (1.606×1011 ± 7.0×109 AU).[a] And, from this, the diameter of Andromeda at the widest point is estimated to be 220 ± 3 kly (67,450 ± 920 pc).[original research?] Applying trigonometry (angular diameter), this is equivalent to an apparent 4.96° angle in the sky.
Mass estimates
The Andromeda Galaxy pictured in ultraviolet light by GALEX (2003).
Illustration showing both the size of each galaxy and the distance between the two galaxies, to scale.
Giant halo around Andromeda Galaxy.[46]
Until 2018, mass estimates for the Andromeda Galaxy's halo (including dark matter) gave a value of approximately 1.5×1012 M☉,[15] compared to 8×1011 M☉ for the Milky Way. This contradicted earlier measurements that seemed to indicate that the Andromeda Galaxy and Milky Way are almost equal in mass. In 2018, the equality of mass was re-established by radio results as approximately 8×1011 M☉[47] [48] [49] [50] In 2006, Andromeda Galaxy's spheroid was determined to have a higher stellar density than that of the Milky Way,[51] and its galactic stellar disk was estimated at about twice the diameter of that of the Milky Way.[10] The total mass of Andromeda Galaxy is estimated to be between 8×1011 M☉[47] and 1.1×1012 M☉.[52][53] The stellar mass of M31 is 10-15×1010 M☉, with 30% of that mass in the central bulge, 56% in the disk, and the remaining 14% in the stellar halo.[54] The radio results (similar mass to Milky Way galaxy) should be taken as likeliest as of 2018, although clearly this matter is still under active investigation by a number of research groups worldwide.
As of 2019, current calculations based on escape velocity and dynamical mass measurements put the Andromeda Galaxy at 0.8×1012 M☉,[55] which is only half of the Milky Way's newer mass, calculated in 2019 at 1.5×1012 M☉.[56][57][58]
In addition to stars, Andromeda Galaxy's interstellar medium contains at least 7.2×109 M☉[59] in the form of neutral hydrogen, at least 3.4×108 M☉ as molecular hydrogen (within its innermost 10 kiloparsecs), and 5.4×107 M☉ of dust.[60]
Andromeda Galaxy is surrounded by a massive halo of hot gas that is estimated to contain half the mass of the stars in the galaxy. The nearly invisible halo stretches about a million light-years from its host galaxy, halfway to our Milky Way galaxy. Simulations of galaxies indicate the halo formed at the same time as the Andromeda Galaxy. The halo is enriched in elements heavier than hydrogen and helium, formed from supernovae and its properties are those expected for a galaxy that lies in the "green valley" of the Galaxy color–magnitude diagram (see below). Supernovae erupt in Andromeda Galaxy's star-filled disk and eject these heavier elements into space. Over Andromeda Galaxy's lifetime, nearly half of the heavy elements made by its stars have been ejected far beyond the galaxy's 200,000-light-year-diameter stellar disk.[61][62][63][64]
Luminosity estimates
Compared to the Milky Way, the Andromeda Galaxy appears to have predominantly older stars with ages >7×109 years.[54][clarification needed] The estimated luminosity of Andromeda Galaxy, ~2.6×1010 L☉, is about 25% higher than that of our own galaxy.[65] However, the galaxy has a high inclination as seen from Earth and its interstellar dust absorbs an unknown amount of light, so it is difficult to estimate its actual brightness and other authors have given other values for the luminosity of the Andromeda Galaxy (some authors even propose it is the second-brightest galaxy within a radius of 10 mega-parsecs of the Milky Way, after the Sombrero Galaxy,[66] with an absolute magnitude of around -22.21[d] or close[67]).
An estimation done with the help of Spitzer Space Telescope published in 2010 suggests an absolute magnitude (in the blue) of −20.89 (that with a color index of +0.63 translates to an absolute visual magnitude of −21.52,[b] compared to −20.9 for the Milky Way), and a total luminosity in that wavelength of 3.64×1010 L☉.[68]
The rate of star formation in the Milky Way is much higher, with Andromeda Galaxy producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of novae in the Milky Way is also double that of Andromeda Galaxy.[69] This suggests that the latter once experienced a great star formation phase, but is now in a relative state of quiescence, whereas the Milky Way is experiencing more active star formation.[65] Should this continue, the luminosity of the Milky Way may eventually overtake that of Andromeda Galaxy.
According to recent studies, the Andromeda Galaxy lies in what in the Galaxy color–magnitude diagram is known as the "green valley," a region populated by galaxies like the Milky Way 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 to Andromeda Galaxy, star formation is expected to extinguish within about five billion years from the now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between Andromeda Galaxy and the Milky Way.[70]
Structure
The Andromeda Galaxy seen in infrared by the Spitzer Space Telescope, one of NASA's four Great Space Observatories.
Image of the Andromeda Galaxy taken by Spitzer in infrared, 24 micrometres (Credit:NASA/JPL–Caltech/Karl D. Gordon, University of Arizona).
File:A Swift Tour of M31.ogvPlay media
A Swift Tour of Andromeda Galaxy.
A Galaxy Evolution Explorer image of the Andromeda Galaxy. The bands of blue-white making up the galaxy's striking rings are neighborhoods that harbor hot, young, massive stars. Dark blue-grey lanes of cooler dust show up starkly against these bright rings, tracing the regions where star formation is currently taking place in dense cloudy cocoons. When observed in visible light, Andromeda Galaxy's rings look more like spiral arms. The ultraviolet view shows that these arms more closely resemble the ring-like structure previously observed in infrared wavelengths with NASA's Spitzer Space Telescope. Astronomers using the latter interpreted these rings as evidence that the galaxy was involved in a direct collision with its neighbor, M32, more than 200 million years ago.
Based on its appearance in visible light, the Andromeda Galaxy is classified as an SA(s)b galaxy in the de Vaucouleurs–Sandage extended classification system of spiral galaxies.[1] However, infrared data from the 2MASS survey and from the Spitzer Space Telescope showed that Andromeda is actually a barred spiral galaxy, like the Milky Way, with Andromeda's bar major axis oriented 55 degrees anti-clockwise from the disc major axis.[71]
In 2005, astronomers used the Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself.[10] This means that the spiral disk of stars in the Andromeda Galaxy is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years (67 kiloparsecs) in diameter. Previously, estimates of the Andromeda Galaxy's size ranged from 70,000 to 120,000 light-years (21 to 37 kpc) across.
The galaxy is inclined an estimated 77° relative to Earth (where an angle of 90° would be viewed directly from the side). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk.[72] A possible cause of such a warp could be gravitational interaction with the satellite galaxies near the Andromeda Galaxy. The Galaxy M33 could be responsible for some warp in Andromeda's arms, though more precise distances and radial velocities are required.
Spectroscopic studies have provided detailed measurements of the rotational velocity of the Andromeda Galaxy as a function of radial distance from the core. The rotational velocity has a maximum value of 225 km/s (140 mi/s) at 1,300 ly (82,000,000 AU) from the core, and it has its minimum possibly as low as 50 km/s (31 mi/s) at 7,000 ly (440,000,000 AU) from the core. Further out, rotational velocity rises out to a radius of 33,000 ly (2.1×109 AU), where it reaches a peak of 250 km/s (160 mi/s). The velocities slowly decline beyond that distance, dropping to around 200 km/s (120 mi/s) at 80,000 ly (5.1×109 AU). These velocity measurements imply a concentrated mass of about 6×109 M☉ in the nucleus. The total mass of the galaxy increases linearly out to 45,000 ly (2.8×109 AU), then more slowly beyond that radius.[73]
The spiral arms of the Andromeda Galaxy are outlined by a series of HII regions, first studied in great detail by Walter Baade and described by him as resembling "beads on a string". His studies show two spiral arms that appear to be tightly wound, although they are more widely spaced than in our galaxy.[74] His descriptions of the spiral structure, as each arm crosses the major axis of the Andromeda Galaxy, are as follows[75]§pp1062[76]§pp92:
Baade's spiral arms of M31 Arms (N=cross M31's major axis at north, S=cross M31's major axis at south) Distance from center (arcminutes) (N*/S*) Distance from center (kpc) (N*/S*) Notes
N1/S1 3.4/1.7 0.7/0.4 Dust arms with no OB associations of HII regions.
N2/S2 8.0/10.0 1.7/2.1 Dust arms with some OB associations.
N3/S3 25/30 5.3/6.3 As per N2/S2, but with some HII regions too.
N4/S4 50/47 11/9.9 Large numbers of OB associations, HII regions, and little dust.
N5/S5 70/66 15/14 As per N4/S4 but much fainter.
N6/S6 91/95 19/20 Loose OB associations. No dust visible.
N7/S7 110/116 23/24 As per N6/S6 but fainter and inconspicuous.
Since the Andromeda Galaxy is seen close to edge-on, it is difficult to study its spiral structure. Rectified images of the galaxy seem to show a fairly normal spiral galaxy, exhibiting two continuous trailing arms that are separated from each other by a minimum of about 13,000 ly (820,000,000 AU) and that can be followed outward from a distance of roughly 1,600 ly (100,000,000 AU) from the core. Alternative spiral structures have been proposed such as a single spiral arm[77] or a flocculent[78] pattern of long, filamentary, and thick spiral arms.[1][79]
The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellites M32 and M110.[80] This can be seen by the displacement of the neutral hydrogen clouds from the stars.[81]
In 1998, images from the European Space Agency's Infrared Space Observatory demonstrated that the overall form of the Andromeda Galaxy may be transitioning into a ring galaxy. The gas and dust within the galaxy is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 ly (9.8 kpc) from the core,[82] nicknamed by some astronomers the ring of fire.[83] This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust, and most of the star formation that is taking place in the Andromeda Galaxy is concentrated there.[84]
Later studies with the help of the Spitzer Space Telescope showed how Andromeda Galaxy's spiral structure in the infrared appears to be composed of two spiral arms that emerge from a central bar and continue beyond the large ring mentioned above. Those arms, however, are not continuous and have a segmented structure.[80]
Close examination of the inner region of the Andromeda Galaxy with the same telescope also showed a smaller dust ring that is believed to have been caused by the interaction with M32 more than 200 million years ago. Simulations show that the smaller galaxy passed through the disk of the Andromeda Galaxy along the latter's polar axis. This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda.[85] It is the co-existence of the long-known large ring-like feature in the gas of Messier 31, together with this newly discovered inner ring-like structure, offset from the barycenter, that suggested a nearly head-on collision with the satellite M32, a milder version of the Cartwheel encounter.[86]
Studies of the extended halo of the Andromeda Galaxy show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally "metal-poor", and increasingly so with greater distance.[51] This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 100–200 low-mass galaxies during the past 12 billion years.[87] The stars in the extended halos of the Andromeda Galaxy and the Milky Way may extend nearly one third the distance separating the two galaxies.
Nucleus
Hubble image of the Andromeda Galaxy core showing possible double structure. NASA/ESA photo.
Artist's concept of the Andromeda Galaxy's core, showing a view across a disk of young, blue stars encircling a supermassive black hole. NASA/ESA photo.
The Andromeda Galaxy is known to harbor a dense and compact star cluster at its very center. In a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. In 1991, the Hubble Space Telescope was used to image Andromeda Galaxy's inner nucleus. The nucleus consists of two concentrations separated by 1.5 pc (4.9 ly). The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a black hole measured at 3–5 × 107 M☉ in 1993,[88] and at 1.1–2.3 × 108 M☉ in 2005.[89] The velocity dispersion of material around it is measured to be ≈ 160 km/s (99 mi/s).[90]
Chandra X-ray telescope image of the center of Andromeda Galaxy. A number of X-ray sources, likely X-ray binary stars, within the galaxy's central region appear as yellowish dots. The blue source at the center is at the position of the supermassive black hole.
It has been proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an eccentric orbit around the central black hole.[91] The eccentricity is such that stars linger at the orbital apocenter, creating a concentration of stars. P2 also contains a compact disk of hot, spectral-class A stars. The A stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.[92]
While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus is the remnant of a small galaxy "cannibalized" by Andromeda Galaxy,[93] this is no longer considered a viable explanation, largely because such a nucleus would have an exceedingly short lifetime due to tidal disruption by the central black hole. While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 does not suggest that there is a black hole at its center.[91]
Discrete sources
The Andromeda Galaxy in high-energy X-ray and ultraviolet light (released 5 January 2016).
Apparently, by late 1968, no X-rays had been detected from the Andromeda Galaxy.[94] A balloon flight on 20 October 1970, set an upper limit for detectable hard X-rays from the Andromeda Galaxy.[95] The Swift BAT all-sky survey successfully detected hard X-rays coming from a region centered 6 arcseconds away from the galaxy center. The emission above 25 keV was later found to be originating from a single source named 3XMM J004232.1+411314, and identified as a binary system where a compact object (a neutron star or a black hole) accretes matter from a star.[96]
Multiple X-ray sources have since been detected in the Andromeda Galaxy, using observations from the European Space Agency's (ESA) XMM-Newton orbiting observatory. Robin Barnard et al. hypothesized that these are candidate black holes or neutron stars, which are heating the incoming gas to millions of kelvins and emitting X-rays. Neutron stars and black holes can be distinguished mainly by measuring their masses.[97] An observation campaign of NuSTAR space mission identified 40 objects of this kind in the galaxy.[98] In 2012, a microquasar, a radio burst emanating from a smaller black hole was detected in the Andromeda Galaxy. The progenitor black hole is located near the galactic center and has about 10 M☉. It was discovered through data collected by the European Space Agency's XMM-Newton probe and was subsequently observed by NASA's Swift Gamma-Ray Burst Mission and Chandra X-Ray Observatory, the Very Large Array, and the Very Long Baseline Array. The microquasar was the first observed within the Andromeda Galaxy and the first outside of the Milky Way galaxy.[99]
Globular clusters
Star clusters in the Andromeda Galaxy.[100]
There are approximately 460 globular clusters associated with the Andromeda Galaxy.[101] The most massive of these clusters, identified as Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the Local Group of galaxies.[102] It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a dwarf galaxy that was consumed by Andromeda in the distant past.[103] The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half.[23] Another massive globular cluster, named 037-B327 and discovered in 2006 as is heavily reddened by the Andromeda Galaxy's interstellar dust, was thought to be more massive than G1 and the largest cluster of the Local Group;[104] however, other studies have shown it is actually similar in properties to G1.[105]
Unlike the globular clusters of the Milky Way, which show a relatively low age dispersion, Andromeda Galaxy's globular clusters have a much larger range of ages: from systems as old as the galaxy itself to much younger systems, with ages between a few hundred million years to five billion years.[106]
In 2005, astronomers discovered a completely new type of star cluster in the Andromeda Galaxy. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger—several hundred light-years across—and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.[107]
Satellites
Main article: Andromeda's satellite galaxies
Messier 32 is to the left of the center, Messier 110 is to the bottom-right of the center.
Like the Milky Way, the Andromeda Galaxy has satellite galaxies, consisting of over 20 known dwarf galaxies. The best known and most readily observed satellite galaxies are M32 and M110. Based on current evidence, it appears that M32 underwent a close encounter with the Andromeda Galaxy in the past. M32 may once have been a larger galaxy that had its stellar disk removed by M31, and underwent a sharp increase of star formation in the core region, which lasted until the relatively recent past.[108]
M110 also appears to be interacting with the Andromeda Galaxy, and astronomers have found in the halo of the latter a stream of metal-rich stars that appear to have been stripped from these satellite galaxies.[109] M110 does contain a dusty lane, which may indicate recent or ongoing star formation.[110] M32 has a young stellar population as well.[111]
In 2006, it was discovered that nine of the satellite galaxies lie in a plane that intersects the core of the Andromeda Galaxy; they are not randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.[112]
PA-99-N2 event and possible exoplanet in galaxy
Main article: PA-99-N2
PA-99-N2 was a microlensing event detected in the Andromeda Galaxy in 1999. One of the explanations for this is the gravitational lensing of a red giant by a star with a mass between 0.02 and 3.6 times that of the Sun, which suggested that the star is likely orbited by a planet. This possible exoplanet would have a mass 6.34 times that of Jupiter. If finally confirmed, it would be the first ever found extragalactic planet. However, anomalies in the event were later found.[113]
Collision with the Milky Way
Main article: Andromeda–Milky Way collision
The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second (68 miles per second).[114] It has been measured approaching relative to the Sun at around 300 km/s (190 mi/s)[1] as the Sun orbits around the center of the galaxy at a speed of approximately 225 km/s (140 mi/s). This makes the Andromeda Galaxy one of about 100 observable blueshifted galaxies.[115] Andromeda Galaxy's tangential or sideways velocity with respect to the Milky Way is relatively much smaller than the approaching velocity and therefore it is expected to collide directly with the Milky Way in about 4 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy[116] or perhaps even a large disc galaxy.[19] Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision is currently unknown. Before the galaxies merge, there is a small chance that the Solar System could be ejected from the Milky Way or join the Andromeda Galaxy.[117]
Amateur observing
The Andromeda Galaxy is the most distant object and the only spiral galaxy outside our Milky Way able to be seen with the naked eye.[118][119][120] The galaxy is commonly located in the sky in reference to the constellations Cassiopeia and Pegasus. Andromeda is best seen during autumn nights in the Northern Hemisphere when it passes high overhead, reaching its highest point around midnight in October, and two hours later each successive month. In early evening, it rises in the east in September and sets in the west in February.[121] From the Southern Hemisphere the Andromeda Galaxy is visible between October and December, best viewed from as far north as possible. Binoculars can reveal some larger structures of the galaxy and its two brightest satellite galaxies, M32 and M110.[122] An amateur telescope can reveal Andromeda's disk, some of its brightest globular clusters, dark dust lanes and the large star cloud NGC 206.[123][124]
See also
Astronomy portal
Andromeda Nebula in fiction
Galaxies in fiction
List of galaxies
Messier object
New General Catalogue
NGC 206 – the brightest star cloud in the Andromeda Galaxy
Notes
average(787 ± 18, 770 ± 40, 772 ± 44, 783 ± 25) = ((787 + 770 + 772 + 783) / 4) ± (182 + 402 + 442 + 252)0.5 / 2 = 778 ± 33.
Blue absolute magnitude of −20.89 – Color index of 0.63 = −21.52
J00443799+4129236 is at celestial coordinates R.A. 00h 44m 37.99s, Dec. +41° 29′ 23.6″.
Blue absolute magnitude of −21.58 (see reference) – Color index of 0.63 = absolute visual magnitude of −22.21
References
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Bekki, Kenji; Couch, Warrick J.; Drinkwater, Michael J.; et al. (2001). "A New Formation Model for M32: A Threshed Early-type Spiral?". Astrophysical Journal Letters. 557 (1): L39–L42. arXiv:astro-ph/0107117. Bibcode:2001ApJ...557L..39B. doi:10.1086/323075. S2CID 18707442.
Ibata, Rodrigo A.; Irwin, Michael J.; Lewis, Geraint F.; et al. (2001). "A giant stream of metal-rich stars in the halo of the galaxy M31". Nature. 412 (6842): 49–52. arXiv:astro-ph/0107090. Bibcode:2001Natur.412...49I. doi:10.1038/35083506. PMID 11452300. S2CID 4413139.
Young, Lisa M. (2000). "Properties of the Molecular Clouds in NGC 205". Astronomical Journal. 120 (5): 2460–2470. arXiv:astro-ph/0007169. Bibcode:2000AJ....120.2460Y. doi:10.1086/316806. S2CID 18728927.
Rudenko, Pavlo; Worthey, Guy; Mateo, Mario (2009). "Intermediate age clusters in the field containing M31 and M32 stars". The Astronomical Journal. 138 (6): 1985–1989. Bibcode:2009AJ....138.1985R. doi:10.1088/0004-6256/138/6/1985.
Koch, Andreas; Grebel, Eva K. (March 2006). "The Anisotropic Distribution of M31 Satellite Galaxies: A Polar Great Plane of Early-type Companions". Astronomical Journal. 131 (3): 1405–1415. arXiv:astro-ph/0509258. Bibcode:2006AJ....131.1405K. doi:10.1086/499534. S2CID 3075266.
"The Anomaly in the Candidate Microlensing Event PA-99-N2".
Cowen, Ron (2012). "Andromeda on collision course with the Milky Way". Nature. doi:10.1038/nature.2012.10765. S2CID 124815138. Retrieved 6 October 2014.
"Apart from Andromeda, are any other galaxies moving towards us? - Space Facts – Astronomy, the Solar System & Outer Space - All About Space Magazine". Retrieved 3 April 2016.
Cox, Thomas J.; Loeb, Abraham (2008). "The collision between the Milky Way and Andromeda". Monthly Notices of the Royal Astronomical Society. 386 (1): 461–474. arXiv:0705.1170. Bibcode:2008MNRAS.386..461C. doi:10.1111/j.1365-2966.2008.13048.x. S2CID 14964036.
Cain, Fraser (2007). "When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?". Universe Today. Archived from the original on 17 May 2007. Retrieved 16 May 2007.
https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question15.html
"Tonight, Find the Andromeda Galaxy". 18 September 2019.
"How to See the Farthest Thing You Can See - Sky & Telescope". 9 September 2015.
http://www.physics.ucla.edu/~huffman/m31.html
"Watch Andromeda Blossom in Binoculars - Sky & Telescope". 16 September 2015.
"Observing M31, the Andromeda Galaxy".
"Globular Clusters in the Andromeda Galaxy".
External links
Wikimedia Commons has media related to Andromeda Galaxy.
The Andromeda Galaxy on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images
StarDate: M31 Fact Sheet
Messier 31, SEDS Messier pages
Astronomy Picture of the Day
A Giant Globular Cluster in M31 1998 October 17.
M31: The Andromeda Galaxy 2004 July 18.
Andromeda Island Universe 2005 December 22.
Andromeda Island Universe 2010 January 9.
WISE Infrared Andromeda 2010 February 19
M31 and its central Nuclear Spiral
Amateur photography – M31
Globular Clusters in M31 at The Curdridge Observatory
First direct distance to Andromeda − Astronomy magazine article
Andromeda Galaxy at SolStation.com
Andromeda Galaxy at The Encyclopedia of Astrobiology, Astronomy, & Spaceflight
M31, the Andromeda Galaxy at NightSkyInfo.com
Than, Ker (23 January 2006). "Strange Setup: Andromeda's Satellite Galaxies All Lined Up". Space.com.
Hubble Finds Mysterious Disk of Blue Stars Around Black Hole Hubble observations (20 September 2005) put the mass of the Andromeda core black hole at 140 million solar masses
M31 (Apparent) Novae Page (IAU)
Multi-wavelength composite
Andromeda Project (crowd-source)
Gray, Meghan; Szymanek, Nik; Merrifield, Michael. "M31 – Andromeda Galaxy". Deep Sky Videos. Brady Haran.
Andromeda Galaxy (M31) at Constellation Guide
APOD – 2013 August 1 (M31's angular size compared with full Moon)
Hubble's High-Definition Panoramic View of the Andromeda Galaxy
Creative Commons Astrophotography M31 Andromeda image download & processing guide
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Andromeda Galaxy
Location
Andromeda Galaxy → Andromeda subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Observable universe → Universe
Each → may be read as "within" or "part of".
Andromeda Galaxy
Satellite galaxies
M32 M110 NGC 185 NGC 147 Andromeda I Andromeda II Andromeda III Andromeda IV* Andromeda V Andromeda VI Andromeda VII Andromeda VIII Andromeda IX Andromeda X Andromeda XI Andromeda XII Andromeda XIII Andromeda XIV Andromeda XV Andromeda XVI Andromeda XVII Andromeda XVIII Andromeda XIX Andromeda XX Andromeda XXI Andromeda XXII Andromeda XXIII Andromeda XXIV Andromeda XXV Andromeda XXVI Andromeda XXVII Andromeda XXVIII Andromeda XXIX Triangulum subgroup*
*It is uncertain whether Triangulum is a companion galaxy of the Andromeda Galaxy
Catalogued stars
AE Andromedae M31-RV
Other
Andromeda–Milky Way collision S Andromedae Mayall II M32p NGC 206 3XMM J004232.1+411314
Category Category
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Messier objects
List
M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20 M21 M22 M23 M24 M25 M26 M27 M28 M29 M30 M31 M32 M33 M34 M35 M36 M37 M38 M39 M40 M41 M42 M43 M44 M45 M46 M47 M48 M49 M50 M51 M52 M53 M54 M55 M56 M57 M58 M59 M60 M61 M62 M63 M64 M65 M66 M67 M68 M69 M70 M71 M72 M73 M74 M75 M76 M77 M78 M79 M80 M81 M82 M83 M84 M85 M86 M87 M88 M89 M90 M91 M92 M93 M94 M95 M96 M97 M98 M99 M100 M101 M102 M103 Added
M104 M105 M106 M107 M108 M109 M110
Charles Messier.jpg
See also
Caldwell catalogue Catalogue of Nebulae and Clusters of Stars Herschel 400 Catalogue Index Catalogue New General Catalogue Revised New General Catalogue
Wikipedia book Book Category Category Commons page Commons Portal Portal
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New General Catalogue 1 to 499
1 2 3 5 6 7 8 9 10 12 13 14 15 16 17 18 19 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499
Astronomical catalog List of NGC objects
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Constellation of Andromeda
Adhil Andromeda in Chinese astronomy List of Andromeda's satellite galaxies List of stars in Andromeda
Stars
Bayer
α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ σ τ υ φ χ ψ ω
Flamsteed
2 3 4 5 6 7 8 9 10 11 12 13 14 15 18 22 23 26 28 32 36 39 41 44 45 47 49 51 55 56 58 59 60 62 63 64 65 66
Variable
R T U W Z RT RU RV RX SU SW TU TW WZ XZ AB AC AD AE AR BM BX CC CN DS DX EG ET EU FF GP GR GY HN KK KX KZ LL LP LQ OP OS OU PX QR QV QX V385 V455 V529 V807
HD
3 166 895 1185 1606 2421 2767 2942 3322 3346 4778 5608 5789/5788 6114 7853 8673 10307 13931 14622 15082 16004 16028 16175 217811 220105 221246 221776 222109 222155 222399 223229 224365 224801 225218
Other
3XMM J004232.1+411314 6 Persei ADS 48 Groombridge 34 HAT-P-6 HAT-P-16 HAT-P-19 HAT-P-28 HR 178 HR 8768 M31-RV NGC 44 NGC 82 NGC 84 NGC 91 NGC 162 NGC 464 PA-99-N2 Ross 248 WASP-1 WISE 0146+4234 WISE J0005+3737 WISE J004945.61+215120.0
Exoplanets
14 Andromedae b Upsilon Andromedae b c d e Gliese 15 Ab HD 13931 b HD 16175 b HAT-P-6b HAT-P-32b WASP-1b WASP-33b
Star clusters
Mayall II NGC 206 NGC 272 NGC 752 NGC 956 NGC 7686
Nebulae
NGC 7662
Galaxies
NGC
5 13 19 21 27 39 43 48 49 51 67 68 69 70 71 72 74 76 79 80 81 83 85 86 90 93 94 96 97 108 109 112 140 149 160 169 181 183 184 214 218 226 228 229 233 243 252 258 260 262 280 304 317 389 393 404 425 431 477 478 512 513 523 528 531 536 668 679 687 700 703 704 705 708 709 710 712 714 717 732 753 759 891 1000 7640 7836
Numbered
I II III IV V VIII IX X XI XVIII XIX XXI
Other
3C 66A 3C 66B Andromeda Galaxy Donatiello I Messier 32 Messier 110
Galaxy clusters
Abell 262 NGC 68 group
Astronomical events
GRB 101225A SN 1885A
Hellenica World - Scientific Library
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