Thevirial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at 1 trillionsolar masses (2.0×1042kilograms). The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy was more massive than the Milky Way by a margin of some 25% to 50%.[12] However, this has been called into question by early-21st-century studies indicating a possibly lower mass for the Andromeda Galaxy[12] and a higher mass for the Milky Way.[13][14] The Andromeda Galaxy has adiameter of about 46.56 kpc (152,000 ly), making it the largest member of theLocal Group of galaxies in terms of extension.[14]
Oldest surviving depiction of the Andromeda (dots at the tip of the mouth of the lower fish), byAl-Sufi inThe Book of Fixed Stars (from around 964 CE) in a manuscript from 1009 to 1010 CE[20][21]
The Andromeda Galaxy is visible to the naked eye in dark skies.[22] It has been speculated that the Babylonian constellation of the Rainbow, Mul Dingir Tir-an-na, may have referred to M31.[23] Around the year 964CE, thePersian astronomerAbd al-Rahman al-Sufi described the Andromeda Galaxy in hisBook of Fixed Stars as a "nebulous smear" or "small cloud".[24] This was the first historical reference to the Andromeda Galaxy and the earliest known reference to a galaxy other than theMilky Way.[25]Star charts of that period labeled it as theLittle Cloud.[26] In 1612, the German astronomerSimon Marius gave an early description of the Andromeda Galaxy based on telescopic observations.[27]John Flamsteed cataloged it as33 Andromedae.[28]Pierre Louis Maupertuis conjectured in 1745 that the blurry spot was an island universe.[29]Charles Messier cataloged Andromeda as object M31 in 1764 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye. In 1785, the astronomerWilliam Herschel noted a faint reddish hue in the core region of Andromeda.[19] He believed Andromeda to be the nearest of all the "greatnebulae," and based on the color and magnitude of thenebula, he incorrectly guessed that it was no more than 2,000 times the distance ofSirius, or roughly 18,000 ly (5.5 kpc).[30]
In 1864,William Huggins noted that thespectrum of Andromeda differed from that of a gaseous nebula.[32] The spectrum of Andromeda displays acontinuum offrequencies, superimposed with darkabsorption 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, asupernova (known asS Andromedae) was seen in Andromeda, the only one ever observed in that galaxy.[33] At the time, it was called "Nova 1885"[34]—the difference between "novae" in the modern sense and supernovae was not yet known. Andromeda was considered to be a nearby object, and it was not realized that the "nova" was much brighter than ordinary novae.[citation needed]
The earliest known photograph of the Great Andromeda "Nebula" (withM110 to the upper right), byIsaac Roberts (29 December 1888)
In 1888,Isaac Roberts took one of the first photographs of Andromeda, which was still commonly thought to be a nebula within the Milky Way galaxy. Roberts mistook Andromeda and similar "spiral nebulae" asstar systems being formed.[35][36]
Location of the Andromeda Galaxy (M31) in the Andromeda constellation
As early as 1755, the German philosopherImmanuel Kant proposed the hypothesis that the Milky Way is only one of many galaxies in his bookUniversal Natural History and Theory of the Heavens. Arguing that a structure like the Milky Way would look like a circular nebula viewed from above and like anellipsoid if viewed from an angle, he concluded that the observed elliptical nebulae like Andromeda, which could not be explained otherwise at the time, were indeed galaxies similar to the Milky Way, not nebulae, as Andromeda was commonly believed to be.[38]
In 1917,Heber Curtis observed anova within Andromeda. After searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10magnitudes 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 (32 billion AU). Although this estimate is about fivefold lower than the best estimates now available, it was the first known estimate of the distance to Andromeda that was correct to within an order of magnitude (i.e., to within a factor of ten of the current estimates, which place the distance around 2.5 million light-years[2][39][6][40]). Curtis became a proponent of the so-called "island universes" hypothesis: thatspiral nebulae were actually independent galaxies.[41]
In 1920, theGreat Debate betweenHarlow Shapley and Curtis took place concerning the nature of the Milky Way, spiral nebulae, and the dimensions of theuniverse.[42] To support his claim that the Great Andromeda Nebula is, in fact, an external galaxy, Curtis also noted the appearance of dark lanes within Andromeda that resembled the dust clouds in the Milky Way galaxy, as well as historical observations of the Andromeda Galaxy's significantDoppler 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 the Milky Way at a distance of about 450 kpc (1,500 kly).[33]Edwin Hubble settled the debate in 1925 when he identified extragalacticCepheid variable stars for the first time on astronomical photos of Andromeda. These were made using the 100-inch (2.5 m)Hooker telescope, and they enabled the distance of the Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within the Milky Way galaxy, but an entirely separate galaxy located a significant distance from the Milky Way.[42]
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 theirmetallicity, naming the young, high-velocity stars in the diskType I and the older, red stars in the bulge Type II.[43] This nomenclature was subsequently adopted for stars within the Milky Way and elsewhere. (The existence of two distinct populations had been noted earlier byJan Oort.)[43] Baade also discovered that there were two types of Cepheid variable stars, which resulted in doubling the distance estimate to Andromeda, as well as the remainder of the universe.[44]
In 1959 rapid rotation of the semi-stellar nucleus of M31 was discovered byAndre Lallemand, M. Duschene and Merle Walker[48] at theLick Observatory, using the 120-inch telescope, coudé Spectrograph, and Lallemand electronographic camera. They estimated the mass of the nucleus to be about 1.3 x 107 solar masses. The second example of this phenomenon was found in 1961 in the nucleus of M32 by M.F Walker[49] at theLick Observatory, using the same equipment as used for the discovery of the nucleus of M31. He estimated the nuclear mass to be between 0.8 and 1 x 107 solar masses. Such rotation is now considered to be evidence of the existence of supermassive black holes in the nuclei of these galaxies.
Largest Mosaic of Andromeda by Hubble with details: (a) Clusters of bright blue stars embedded within the galaxy, background galaxies seen much farther away, and photo-bombing by a couple bright foreground stars that are actually inside our Milky Way; (b) NGC 206 the most conspicuous star cloud in Andromeda; (c) A young cluster of blue newborn stars; (d) The satellite galaxy M32, that may be the residual core of a galaxy that once collided with Andromeda; (e) Dark dust lanes across myriad stars.[50]
In 2009, an occurrence ofmicrolensing—a phenomenon caused by the deflection of light by a massive object—may have led to the first discovery of a planet in the Andromeda Galaxy.[51]
In 2023, amateur astronomers Marcel Drechsler, Xavier Strottner and Yann Sainty announced the discovery of a huge, oxygen-rich emission nebula just south of M31, near the bright star 35 And. This nebula, now classified as SDSO-1, is exceedingly faint, requiring dozens of hours of exposure time minimum to detect, and appears to only emit inoxygen-III.[53] Deep studies of the surrounding regions showed no signs of similarly bright oxygen nebulae near M31, nor any sign of connecting hydrogen filaments to SDSO-1, suggesting a high oxygen-hydrogen ratio. Current research suggests SDSO-1 is extragalactic in nature, specifically caused by interaction between the Milky Way's and M31's circumgalactic halos, although more research is needed to fully understand this object.[54] A later study using spectroscopy found the nebula to be in the Milky Way.[55] One study found the nebula to be abow shock of a ghostplanetary nebula around the binaryEG Andromedae.[56]
In 2025, NASA published a huge mosaic made by the Hubble Space Telescope, assembled from approximately 600 separate overlapping fields of view taken over 10 years of Hubble observation. Hubble resolves an estimated 200 million stars that are hotter than the Sun, but still a fraction of the galaxy's total estimated stellar population.
The estimated distance of the Andromeda Galaxy from our own was doubled in 1953 when it was discovered that there is a second, dimmer type ofCepheid variable star. In the 1990s, measurements of both standardred giants as well asred clump stars from theHipparcos satellite measurements were used to calibrate the Cepheid distances.[57][58]
Processed image of the Andromeda Galaxy, with enhancement ofH-alpha to highlight its star-forming regions
A majormerger occurred 2 to 3 billion years ago at the Andromeda location, involving two galaxies with a mass ratio of approximately 4.[59][60]
The discovery of a recent merger in the Andromeda galaxy was first based on interpreting its anomalous age-velocity dispersion relation,[61] as well as the fact that 2 billion years ago, star formation throughout Andromeda's disk was much more active than today.[62]
Modeling[59] of this violent collision shows that it has formed most of the galaxy's (metal-rich)galactic halo, including the Giant Stream,[63] and also the extended thick disk, the young age thin disk, and the static 10 kpc ring. During this epoch, its rate ofstar formation would have beenvery high, to the point of becoming aluminous infrared galaxy for roughly 100 million years. Modeling also recovers the bulge profile, the large bar, and the overall halo density profile.
Andromeda and theTriangulum Galaxy (M33) might have had a very close passage 2–4 billion years ago, but it seems unlikely from the last measurements from the Hubble Space Telescope.[64]
Illustration showing both the size of each galaxy and the distance between the two galaxies, to scale
At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy. In 2003, using the infraredsurface 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 ly (162.5 ± 3.8 billionAU) was derived. A 2004 Cepheid variable method estimated the distance to be 2.51 ± 0.13 million light-years (770 ± 40 kpc).[2][39]
In 2005, an eclipsingbinary star was discovered in the Andromeda Galaxy. The binary[c] is made up of two hot blue stars oftypes 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 theirabsolute magnitude. When thevisual and absolute magnitudes are known, the distance to the star can be calculated. The stars lie at a distance of 2.52 ± 0.14 million ly (159.4 ± 8.9 billion AU) and the whole Andromeda Galaxy at about 2.5 million ly (160 billion AU).[6] This new value is in excellent agreement with the previous, independent Cepheid-based distance value. TheTRGB method was also used in 2005 giving a distance of 2.56 ± 0.08 million ly (161.9 ± 5.1 billion AU).[40] Averaged together, these distance estimates give a value of 2.54 ± 0.11 million ly (160.6 ± 7.0 billion AU).[d]
Until 2018, mass estimates for the Andromeda Galaxy's halo (includingdark matter) gave a value of approximately 1.5×1012M☉,[66] compared to 8×1011M☉ for the Milky Way. This contradicted even earlier measurements that seemed to indicate that the Andromeda Galaxy and Milky Way are almost equal in mass. In 2018, the earlier measurements for equality of mass were re-established by radio results as approximately 8×1011M☉.[67][68][69][70] In 2006, the Andromeda Galaxy'sspheroid was determined to have a higher stellar density than that of the Milky Way,[71] and its galactic stellar disk was estimated at twice the diameter of that of the Milky Way.[9] The total mass of the Andromeda Galaxy is estimated to be between 8×1011M☉[67] and 1.1×1012M☉.[72][73] The stellar mass of M31 is 10–15×1010M☉, with 30% of that mass in the centralbulge, 56% in thedisk, and the remaining 14% in thestellar halo.[74] The radio results (similar mass to the Milky Way Galaxy) should be taken as likeliest as of 2018, although clearly, this matter is still under active investigation by several research groups worldwide.
As of 2019, current calculations based on escape velocity and dynamical mass measurements put the Andromeda Galaxy at 0.8×1012M☉,[75] which is only half of the Milky Way's newer mass, calculated in 2019 at 1.5×1012M☉.[76][77][78]
In addition to stars, the Andromeda Galaxy'sinterstellar medium contains at least 7.2×109M☉[79] in the form ofneutral hydrogen, at least 3.4×108M☉ as molecularhydrogen (within its innermost 10 kiloparsecs), and 5.4×107M☉ ofdust.[80]
The 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 fromsupernovae, and its properties are those expected for a galaxy that lies in the "green valley" of theGalaxy color-magnitude diagram (seebelow). Supernovae erupt in the Andromeda Galaxy's star-filled disk and eject these heavier elements into space. Over the 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.[81][82][83][84]
The estimatedluminosity of the Andromeda Galaxy, ~2.6×1010L☉, is about 25% higher than that of our own galaxy.[85][86] However, the galaxy has a highinclination as seen from Earth, and itsinterstellar 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 10megaparsecs of the Milky Way, after theSombrero Galaxy,[87] with an absolute magnitude of around −22.21[e] or close[88]).
An estimation done with the help ofSpitzer Space Telescope published in 2010 suggests anabsolute magnitude (in the blue) of −20.89 (that with acolor index of +0.63 translates to an absolute visual magnitude of −21.52,[a] compared to −20.9 for the Milky Way), and a total luminosity in thatwavelength of 3.64×1010L☉.[89]
The rate of star formation in the Milky Way is much higher, with the Andromeda Galaxy producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate ofnovae in the Milky Way is also double that of the Andromeda Galaxy.[90] 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.[85] Should this continue, the luminosity of the Milky Way may eventually overtake that of the Andromeda Galaxy.
According to recent studies, the Andromeda Galaxy lies in what is known in thegalaxy color–magnitude diagram 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 the Andromeda Galaxy, star formation is expected to extinguish within about five billion years, even accounting for the expected, short-term increase in the rate of star formation due to the collision between the Andromeda Galaxy and the Milky Way.[91]
Zooming In on the Andromeda Galaxy – A panorama of foreground stars and the Andromeda Galaxy's nucleus. The image is the largest ever taken by the Hubble Space Telescope.
A narrated tour of the Andromeda Galaxy, made by NASA'sSwift satellite team
There are various methods used in astronomy in defining the size of a galaxy, and each method can yield different results concerning one another. The most commonly employed is the D25 standard, theisophote where the photometric brightness of a galaxy in the B-band (445 nm wavelength of light, in the blue part of thevisible spectrum) reaches 25 mag/arcsec2.[93] The Third Reference Catalogue of Bright Galaxies (RC3) used this standard for Andromeda in 1991, yielding an isophotal diameter of 46.56 kiloparsecs (152,000 light-years) at a distance of 2.5 million light-years.[8] An earlier estimate from 1981 gave a diameter for Andromeda at 54 kiloparsecs (176,000 light-years).[94]
A study in 2005 by theKeck telescopes shows the existence of a tenuous sprinkle of stars, orgalactic halo, extending outward from the galaxy.[9] The stars in this halo behave differently from the ones in Andromeda's main galactic disc, where they show rather disorganized orbital motions as opposed to the stars in the main disc having more orderly orbits and uniform velocities of 200 km/s.[9] This diffuse halo extends outwards away from Andromeda's main disc with the diameter of 67.45 kiloparsecs (220,000 light-years).[9]
The galaxy is inclined an estimated 77° relative to Earth (where an angle of 90° would be edge-on). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk.[95] A possible cause of such a warp could be gravitational interaction with the satellite galaxies near the Andromeda Galaxy. The GalaxyM33 could be responsible for some warp in Andromeda's arms, though more precise distances and radial velocities are required.[original research?]
Spectroscopic studies have provided detailed measurements of therotational 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 million AU) from the core, and it has its minimum possibly as low as 50 km/s (31 mi/s) at 7,000 ly (440 million AU) from the core. Further out, rotational velocity rises out to a radius of 33,000 ly (2.1 billion 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 billion AU). These velocity measurements imply a concentrated mass of about 6×109M☉ in thenucleus. The total mass of the galaxy increaseslinearly out to 45,000 ly (2.8 billion AU), then more slowly beyond that radius.[96]
Thespiral arms of the Andromeda Galaxy are outlined by a series ofHII regions, first studied in great detail byWalter 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.[97] His descriptions of the spiral structure, as each arm crosses the major axis of the Andromeda Galaxy, are as follows[98]§pp1062[99]§pp92:
Baade's spiral arms of M31
Arms (N=cross M31's major axis at north, S=cross M31's major axis at south)
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 million AU) and that can be followed outward from a distance of roughly 1,600 ly (100 million AU) from the core. Alternative spiral structures have been proposed such as a single spiral arm[100] or aflocculent[101] pattern of long, filamentary, and thick spiral arms.[1][102]
The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellitesM32 andM110.[103] This can be seen by the displacement of theneutral hydrogen clouds from the stars.[104]
In 1998, images from theEuropean Space Agency'sInfrared Space Observatory demonstrated that the overall form of the Andromeda Galaxy may be transitioning into aring galaxy. The gas and dust within the galaxy are 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,[105] nicknamed by some astronomers thering of fire.[106] 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.[107]
Later studies with the help of theSpitzer Space Telescope showed how the 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.[103]
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.[108]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 thebarycenter, that suggested a nearly head-on collision with the satellite M32, a milder version of theCartwheel encounter.[109]
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.[71] 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.[110] 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.
Hubble Space Telescope image of the Andromeda Galaxy core showing P1, P2 and P3, with P3 containing M31*.NASA/ESA photo
The Andromeda Galaxy is known to harbor a dense and compact star cluster at its very center, similar to theMilky Way galaxy. A large telescope creates a visual impression of a star embedded in the more diffuse surrounding bulge. In 1991, theHubble Space Telescope was used to image the 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 an embedded star cluster, called P3,[111] containing manyUV-brightA-stars and thesupermassive black hole, calledM31*.[112][113] The black hole is classified as a low-luminosityAGN (LLAGN) and it was detected only inradio wavelengths and inx-rays.[113] It was quiescent in 2004–2005, but it was highly variable in 2006–2007.[112] An additional x-ray flare occurred in 2013.[114] The mass of M31* was measured at 3–5 × 107M☉ in 1993,[115] and at 1.1–2.3 × 108M☉ in 2005.[111] Thevelocity dispersion of material around it is measured to be ≈ 160 km/s (100 mi/s).[116]
It has been proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in aneccentric orbit around the central black hole.[117] The eccentricity is such that stars linger at the orbitalapocenter, creating a concentration of stars. It has been postulated that such an eccentric disk could have been formed from the result of a previous black hole merger, where the release of gravitational waves could have "kicked" the stars into their current eccentric distribution.[118] 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.[119]
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 the Andromeda Galaxy,[120] this is no longer considered a viable explanation, largely because such a nucleus would have an exceedingly short lifetime due totidal 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.[117]
The Andromeda Galaxy in high-energy X-ray and ultraviolet light (released 5 January 2016)
Apparently, by late 1968, noX-rays had been detected from the Andromeda Galaxy.[121] A balloon flight on 20 October 1970 set an upper limit for detectable hard X-rays from the Andromeda Galaxy.[122] TheSwift 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 named3XMM J004232.1+411314, and identified as a binary system where a compact object (aneutron star or a black hole) accretes matter from a star.[123]
Multiple X-ray sources have since been detected in the Andromeda Galaxy, using observations from theEuropean Space Agency's (ESA)XMM-Newton orbiting observatory. Robin Barnardet al. hypothesized that these are candidate black holes orneutron stars, which are heating the incoming gas to millions ofkelvins and emitting X-rays. Neutron stars and black holes can be distinguished mainly by measuring their masses.[124] An observation campaign ofNuSTAR space mission identified 40 objects of this kind in the galaxy.[125]In 2012, amicroquasar, 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 10M☉. It was discovered through data collected by theEuropean Space Agency'sXMM-Newton probe and was subsequently observed byNASA'sSwift Gamma-Ray Burst Mission andChandra X-Ray Observatory, theVery Large Array, and theVery Long Baseline Array. The microquasar was the first observed within the Andromeda Galaxy and the first outside of the Milky Way Galaxy.[126]
There are approximately 460globular clusters associated with the Andromeda Galaxy.[128] The most massive of these clusters, identified asMayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in theLocal Group of galaxies.[129] It contains several million stars and is about twice as luminous asOmega Centauri, the brightest known globular cluster in the Milky Way.Mayall II (also known as Globular One or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider Mayall II to be the remnant core of adwarf galaxy that was consumed by Andromeda in the distant past.[130] The cluster with the greatest apparent brightness is G76 which is located in the southwest arm's eastern half.[26]Another massive globular cluster, named 037-B327 (also known as Bol 37) and discovered in 2006 as is heavily reddened by the Andromeda Galaxy'sinterstellar dust, was thought to be more massive than Mayall II and the largest cluster of the Local Group;[131] however, other studies have shown it is actually similar in properties to Mayall II.[132]
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.[133]
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.[134]
The most massive globular cluster in the Andromeda Galaxy, B023-G078, likely has a central intermediate black hole of almost 100,000 solar masses.[135]
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 theSun, 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 foundextragalactic planet. However, anomalies in the event were later found.[136]
Like the Milky Way, the Andromeda Galaxy has smallersatellite galaxies, consisting of over 20 knowndwarf galaxies. The Andromeda Galaxy's dwarf galaxy population is very similar to the Milky Way's, but the galaxies are much more numerous.[137] The best-known and most readily observed satellite galaxies areM32 andM110. 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 ofstar formation in the core region, which lasted until the relatively recent past.[138]
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.[139] M110 does contain a dusty lane, which may indicate recent or ongoing star formation.[140] M32 has a young stellar population as well.[141]
TheTriangulum Galaxy is a non-dwarf galaxy that lies 750,000 light-years from Andromeda. It is currently unknown whether it is a satellite of Andromeda.[142]
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.[143]
Illustration of the collision path between the Milky Way and Andromeda Galaxy
The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres (68 miles) per second.[144] 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 observableblueshifted galaxies.[145] Andromeda Galaxy's tangential or sideways velocity concerning the Milky Way is uncertain, but estimated to be smaller than the approaching velocity. After the sideways velocity was first measured, Andromeda was predicted to collide directly with the Milky Way in about 4 billion years.[146] However, later calculations, including a higher sideways velocity measurement from theGaia (spacecraft) and the effect of otherLocal Group galaxies found a much lower probability of a merger.[15]
A likely outcome of the collision would be that thegalaxies will merge to form a giantelliptical galaxy[147] or possibly largedisc galaxy.[17] Such events are frequent among the galaxies ingalaxy groups. The fate of Earth and theSolar 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.[148]
Superimposing picture showing sizes of the Moon and the Andromeda Galaxy as observed from Earth. Note that this photographic image shows much more of the galaxy's darker outer regions than an amateur observer is able to see.[149][150]
Under most viewing conditions, the Andromeda Galaxy is one of the most distant objects that can be seen with thenaked eye, due to its sheer size. (M33 and, for observers with exceptionally good vision,M81 can be seen undervery dark skies.)[151][152][153][154]The constellation of Andromeda, in which the galaxy is located, is usually found with the aid of the constellationsCassiopeia orPegasus, which are usually easier to recognize at first glance.Andromeda is best seen during autumn nights in theNorthern Hemisphere when it passes high overhead, reaching itshighest point around midnight in October, and two hours earlier each successive month. In the early evening, it rises in the east in September and sets in the west in February.[155]From theSouthern 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 brightestsatellite galaxies,M32 andM110.[156] Anamateur telescope can reveal Andromeda's disk, some of its brightest globular clusters, dark dust lanes, and the large star cloudNGC 206.[157][158]
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^White, Gavin (2008).Babylonian Star-Lore: An Illustrated Guide to the Star-lore and Constellations of Ancient Babylonia. London: Solaria Publications. p. 165.
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^Davidson, Norman (1985).Astronomy and the imagination: a new approach to man's experience of the stars. Routledge Kegan & Paul. p. 203.ISBN978-0-7102-0371-7.
^Simien, François; Pellet, André; Monnet, Guy; et al. (1978). "The spiral structure of M31 – A morphological approach".Astronomy and Astrophysics.67 (1):73–79.Bibcode:1978A&A....67...73S.
^Haas, Martin (2000). "Cold dust in M31 as mapped by ISO".The Interstellar Medium in M31 and M33. Proceedings 232. WE-Heraeus Seminar:69–72.Bibcode:2000immm.proc...69H.
^Walterbos, René A. M.; Kennicutt, Robert C. Jr. (1988). "An optical study of stars and dust in the Andromeda galaxy".Astronomy and Astrophysics.198:61–86.Bibcode:1988A&A...198...61W.
^Pagani, Laurent; Lequeux, James; Cesarsky, Diego; et al. (1999). "Mid-infrared and far-ultraviolet observations of the star-forming ring of M 31".Astronomy & Astrophysics.351:447–458.arXiv:astro-ph/9909347.Bibcode:1999A&A...351..447P.
^Barnard, Robin; Kolb, Ulrich C.; Osborne, Julian P. (2005). "Timing the bright X-ray population of the core of M31 with XMM-Newton".arXiv:astro-ph/0508284.
^Burstein, David; Li, Yong; Freeman, Kenneth C.; et al. (2004). "Globular Cluster and Galaxy Formation: M31, the Milky Way, and Implications for Globular Cluster Systems of Spiral Galaxies".Astrophysical Journal.614 (1):158–166.arXiv:astro-ph/0406564.Bibcode:2004ApJ...614..158B.doi:10.1086/423334.S2CID56003193.
^Harrington, Philip S. (2010).Cosmic Challenge: The Ultimate Observing List for Amateurs. Cambridge University Press. pp. 28–29.ISBN9781139493680.But can [M81's] diffuse 7.9-magnitude glow actually be glimpsed without any optical aid? The answer is yes, but with a few important qualifications. Not only must the observing site be extraordinarily dark and completely absent of any atmospheric interferences, either natural or artificial, but the observer must have exceptionally keen vision.
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