Editing Andromeda Galaxy

{{short description|Barred spiral galaxy in the Local Group}}
{{Redirect|Galaxy M31|the smartphone|Samsung Galaxy M31|other uses|Andromeda (disambiguation){{!}}Andromeda}}
{{Good article}}
{{Use dmy dates|date=August 2024}}
{{Infobox galaxy
| name = Andromeda Galaxy
| image = M31-Andromede-16-09-2023-Hamois.jpg
| caption = A visible light image of the Andromeda Galaxy and two satellite galaxies. [[Messier 32]] is above the galactic nucleus and [[Messier 110]] is at the bottom.
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| pronounce = {{IPAc-en|æ|n|ˈ|d|r|ɒ|m|ᵻ|d|ə}}
| type = SA(s)b<ref name="ned"/>
| mass = {{val|1.5|0.2|e=12|u=}}<ref name="GalaxyMass"/>
| size = {{cvt|46.56|kpc|ly|sigfig=3|lk=on}}<br/>(diameter; [[Galaxy#Isophotal diameter|D<sub>25</sub> isophote]])<ref name="ned"/><ref name=RC3>{{cite book |last1=De Vaucouleurs |first1=Gerard |last2=De Vaucouleurs |first2=Antoinette |last3=Corwin |first3=Herold G. | last4=Buta |first4=Ronald J. |last5=Paturel |first5=Georges | last6=Fouque |first6=Pascal |title=Third Reference Catalogue of Bright Galaxies |year=1991 |bibcode=1991rc3..book.....D}}</ref>{{efn|name=AndromedaHalo|This is the diameter as measured through the D<sub>25</sub> standard. The halo extends up to a distance of {{convert|67.45|kpc|ly}}.<ref name="Chapman et al 2006"/>}}
| stars = ~1&nbsp;trillion (10<sup>12</sup>)<!-- exponential value to disambiguate "trillion" --><ref name="trillion-stars"/>
<!--| age = 5 to 9&nbsp;billion years<ref name="Moskvitch 2010"/>-->
| ra = {{RA|00|42|44.3}}<ref name="ned"/>
| dec = {{DEC|+41|16|9}}<ref name="ned"/>
| dist_ly = {{cvt|765|kpc|Mly|lk=on}}<ref name="adam"/>
| z = z = −0.001004 (minus sign indicates [[blueshift]])<ref name="ned"/>
| h_radial_v = −301 ± 1 km/s<ref name="Karachentsevetal2006"/>
| appmag_v = 3.44<ref name="SIMBAD-M31"/><ref name="GAXEL"/>
| size_v = {{Val|3.167|×|1|u=degree}}<ref name="ned"/>
| constellation name = [[Andromeda (constellation)|Andromeda]]
| absmag_v = −21.5{{efn|name=blue mag}}<ref name="Ribas2005"/>
| names = [[Messier object|M]]31, [[New General Catalogue|NGC]] 224, [[Uppsala General Catalogue|UGC]] 454, [[Principal Galaxies Catalogue|PGC]] 2557, [[Second Cambridge Catalogue of Radio Sources|2C]] 56 (Core),<ref name="ned"/> CGCG 535-17, [[Morphological Catalogue of Galaxies|MCG]] +07-02-016, [[IRAS]] 00400+4059, [[2MASX]] J00424433+4116074, GC 116, h 50, Bode 3, Flamsteed 58, Hevelius 32, Ha 3.3, IRC +40013
}}

The '''Andromeda Galaxy''' is a [[barred spiral galaxy]] and is the nearest major galaxy to the [[Milky Way]]. It was originally named the '''Andromeda Nebula''' and is cataloged as '''Messier 31''', '''M31''', and '''NGC 224'''. Andromeda has a [[Galaxy#Isophotal diameter|D<sub>25</sub> isophotal diameter]] of about {{convert|46.56|kpc|ly|sigfig=3|lk=on|abbr=off}}<ref name=RC3/> and is approximately {{convert|765|kpc|e6ly|1|abbr=in}} from Earth. 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 [[Andromeda (mythology)|the princess]] who was the wife of [[Perseus]] in [[Greek mythology]].<ref name="RC3"/>

The [[virial mass]] of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at {{convert|1|e12solar mass|kg|abbr=off|lk=on}}. 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%.<ref name="Kafle2018"/> However, this has been called into question by early-21st-century studies indicating a possibly lower mass for the Andromeda Galaxy<ref name="Kafle2018"/> and a higher mass for the Milky Way.<ref>{{cite journal |last1=López-Corredoira |first1=M. |last2=Prieto |first2=C. Allende |last3=Garzón |first3=F. |last4=Wang |first4=H. |last5=Liu |first5=C. |last6=Deng |first6=L. |date=1 April 2018 |title=Disk stars in the Milky Way detected beyond 25 kpc from its center |url=https://www.aanda.org/articles/aa/abs/2018/04/aa32880-18/aa32880-18.html |journal=Astronomy & Astrophysics |volume=612 |pages=L8 |arxiv=1804.03064 |bibcode=2018A&A...612L...8L |doi=10.1051/0004-6361/201832880 |via=aanda.org}}</ref><ref name="As-Now"/> The Andromeda Galaxy has a [[diameter]] of about {{cvt|46.56|kpc|ly|sigfig=3}}, making it the largest member of the [[Local Group]] of galaxies in terms of extension.<ref name="As-Now"/>

The Milky Way and Andromeda galaxies are [[Andromeda–Milky Way collision|expected to collide with each other]] in around 4–5&nbsp;billion years,<ref name="Schiavi"/> merging to potentially form a giant [[elliptical galaxy]]<ref name="milky-way-collide"/> or a large [[lenticular galaxy]].<ref name="Ueda2014"/>

With an [[apparent magnitude]] of 3.4, the Andromeda Galaxy is among the brightest of the [[Messier object]]s,<ref name="Frommert & Kronberg 2007"/> and is visible to the [[naked eye]] from Earth on moonless nights,<ref name="Messie"/> even when viewed from areas with moderate [[light pollution]].<ref name="RC3"/>

== Observation history ==
[[File:Auv0175.png|thumb|Oldest surviving depiction of the Andromeda (dots at the tip of the mouth of the lower), by [[Al-Sufi]] in ''[[The Book of Fixed Stars]]'' (from around 964 CE) in a manuscript from 1009–1010 CE<ref name="w718">{{cite web | title=Andromeda Galaxy al-Sufi | website=[[Ian Ridpath]] | url=http://www.ianridpath.com/startales/andromeda-alsufi.html | access-date=2024-11-22}}</ref><ref name="j307">{{cite web | title=The earliest image of another galaxy | website=Ivan Debono | date=2015-09-16 | url=http://www.idebono.eu/2015/09/16/the-earliest-image-of-another-galaxy/ | access-date=2024-11-22}}</ref>]]

The Andromeda Galaxy is visible to the naked eye in dark skies.<ref name="firstName"/> Around the year 964 [[Common Era|CE]], the [[Greater Iran|Persian]] astronomer [[Abd al-Rahman al-Sufi]] described the Andromeda Galaxy in his ''[[Book of Fixed Stars]]'' as a "nebulous smear" or "small cloud".<ref name="Hafez"/> [[Star chart]]s of that period labeled it as the ''Little Cloud''.<ref name="NSOG"/> In 1612, the German astronomer [[Simon Marius]] gave an early description of the Andromeda Galaxy based on telescopic observations.<ref name="Aati"/> [[Pierre Louis Maupertuis]] conjectured in 1745 that the blurry spot was an island universe.<ref name="Kant"/> [[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 astronomer [[William Herschel]] noted a faint reddish hue in the core region of Andromeda.<ref name="Messie"/> He believed Andromeda to be the nearest of all the "great [[nebula]]e," 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 {{cvt|18000|ly|kpc|lk=on}}.<ref name="Herschel 1785"/>

In 1850, [[William Parsons, 3rd Earl of Rosse]], made a drawing of Andromeda's [[Spiral galaxy#Structure|spiral structure]].<!-- in doubt: Andromeda galaxy is _not_ listed here: http://articles.adsabs.harvard.edu/pdf/1991JHA....22..257D --><ref>{{Cite journal |last=Payne-Gaposchkin |first=Cecilia H. |year=1953 |title=Why do Galaxies have a Spiral Form? |journal=Scientific American |volume=189 |issue=3 |pages=89–99 |bibcode=1953SciAm.189c..89P |doi=10.1038/scientificamerican0953-89 |issn=0036-8733 |jstor=24944338 }}</ref>{{better source needed|date=January 2024}}

In 1864, [[William Huggins]] noted that the [[spectrum]] of Andromeda differed from that of a gaseous nebula.<ref name="Huggins & Miller 1864"/> The spectrum of Andromeda displays a [[wikt:continuum|continuum]] of [[frequency|frequencies]], superimposed with dark [[spectral line|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 [[SN 1885A|S Andromedae]]) was seen in Andromeda, the first and so far only one observed in that galaxy.<ref name="Öpik 1922"/> At the time, it was called "Nova 1885"<ref name="Backhouse 1888"/>—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|date=October 2023}}

[[File:Andromeda_Nebula_-_Isaac_Roberts,_29_December_1888 (cropped).jpg|thumb|The earliest known photograph of the Great Andromeda "Nebula" (with [[Messier 110|M110]] to the upper right), by [[Isaac 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 our galaxy. Roberts mistook Andromeda and similar "spiral nebulae" as [[protoplanetary disc|star systems being formed]].<ref name="NebPix"/><ref name="sciPho"/>

In 1912, [[Vesto Slipher]] used [[spectroscopy]] to measure the [[radial velocity]] of Andromeda with respect to the [[Solar System]]—the largest velocity yet measured, at {{cvt|300|km/s}}.<ref name="Slipher 1913"/>

=== "Island universes" hypothesis ===
[[File:Andromeda_constellation_map.svg|thumb|Location of the Andromeda Galaxy (M31) in the Andromeda constellation]]
As early as 1755, the German philosopher [[Immanuel Kant]] proposed the hypothesis that the Milky Way is only one of many galaxies in his book ''[[Universal 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 an [[Elliptical galaxy|ellipsoid]] 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.<ref>{{cite web |title=Seite:Allgemeine Naturgeschichte und Theorie des Himmels.djvu/41 – Wikisource |url=https://de.wikisource.org/wiki/Seite:Allgemeine_Naturgeschichte_und_Theorie_des_Himmels.djvu/41?useskin=vector |website=de.wikisource.org |language=de}}</ref>

In 1917, [[Heber Doust Curtis|Heber Curtis]] observed a [[nova]] within Andromeda. After searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 [[Magnitude (astronomy)|magnitudes]] fainter than those that occurred elsewhere in the sky. As a result, he was able to come up with a distance estimate of {{convert|500000|ly|e9AU|abbr=unit}}. 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&nbsp;million light-years<ref name="Karachentsevetal2006"/><ref name="karachentsevetal2004"/><ref name="Ribas2005"/><ref name="McConnachieetal2005"/>). Curtis became a proponent of the so-called "island universes" hypothesis: that [[Spiral galaxy#Spiral nebula|spiral nebulae]] were actually independent galaxies.<ref name="Curtis 1988"/>

In 1920, the [[Great Debate (astronomy)|Great Debate]] between [[Harlow Shapley]] and Curtis took place concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the [[universe]].<ref name="Hubble 1929"/> 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 our own galaxy, as well as historical observations of the 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 {{cvt|450|kpc}}.<ref name="Öpik 1922"/> [[Edwin Hubble]] settled the debate in 1925 when he identified extragalactic [[Cepheid variable|Cepheid variable star]]s for the first time on astronomical photos of Andromeda. These were made using the {{convert|100|in|m|adj=on}} [[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 our own galaxy, but an entirely separate galaxy located a significant distance from the Milky Way.<ref name="Hubble 1929"/>

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-A star|Type I]] and the older, red stars in the bulge Type II.<ref name="Baade 1944"/> 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]].)<ref name="Baade 1944"/> 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.<ref name="Gribbin 2001"/>

In 1950, radio [[Emission spectrum|emissions]] from the Andromeda Galaxy were detected by [[Robert Hanbury Brown]] and [[Cyril Hazard]] at the [[Jodrell Bank Observatory]].<ref name="Brown & Hazard 1950"/><ref name="Brown & Hazard 1951"/> The first [[radio astronomy|radio maps]] of the galaxy were made in the 1950s by [[John E. Baldwin|John Baldwin]] and collaborators at the [[Cavendish Astrophysics Group|Cambridge Radio Astronomy Group]].<ref name="van der Kruit & Allen 1976"/> The core of the Andromeda Galaxy is called 2C 56 in the [[Second Cambridge Catalogue of Radio Sources|2C]] radio astronomy catalog.

In 1959 rapid rotation of the semi-stellar nucleus of M31 was discovered by [[Andre Lallemand]], M. Duschene and Merle Walker<ref>PASP 1960, p.72</ref> at the [[Lick 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 10<sup>7</sup> solar masses. The second example of this phenomenon was found in 1961 in the nucleus of M32 by M.F Walker<ref>1962 Astrophysical Journal, 136, p.692</ref> at the [[Lick 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 10<sup>7</sup> solar masses. Such rotation is now considered to be evidence of the existence of supermassive black holes in the nuclei of these galaxies.

=== 21st century ===
[[File:Largest Mosaic of Andromeda by Hubble jan 16 2025-extra details.jpg | thumb | 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.<ref>{{cite web |title=Hubble's panoramic view of the Andromeda Galaxy (annotated) |url=https://esahubble.org/images/heic2501b/ |website=www.esahubble.org |access-date=17 February 2025 |language=en}}{{source-attribution}}</ref>]]
In 2009, an occurrence of [[microlensing]]—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.<ref name="Ingrosso 2009"/>

In 2020, observations of linearly polarized radio emission with the [[Westerbork Synthesis Radio Telescope]], the [[Effelsberg 100-m Radio Telescope]], and the [[Very Large Array]] revealed ordered magnetic fields aligned along the "10-kpc ring" of gas and star formation.<ref name="Beck 2020"/>

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 our Sun, but still a fraction of the galaxy’s total estimated stellar population.

== General ==
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 of [[Cepheid variable|Cepheid variable star]]. In the 1990s, measurements of both standard [[red giant]]s as well as [[red clump]] stars from the ''[[Hipparcos]]'' satellite measurements were used to calibrate the Cepheid distances.<ref name="Holland 1998"/><ref name="Stanek & Garnavich 1998"/>

=== Formation and history ===

[[File:Andromeda Galaxy 560mm FL.jpg|thumb|Processed image of the Andromeda Galaxy, with enhancement of [[H-alpha]] to highlight its star-forming regions]]
A major [[Galaxy merger|merger]] occurred 2 to 3&nbsp;billion years ago at the Andromeda location, involving two galaxies with a mass ratio of approximately 4.<ref name="Hammer et al. 2018">{{cite journal |last1=Hammer |first1=F |last2=Yang |first2=Y B |last3=Wang |first3=J L |last4=Ibata |first4=R |last5=Flores |first5=H |last6=Puech |first6=M |title=A 2–3 billion year old major merger paradigm for the Andromeda galaxy and its outskirts |journal=Monthly Notices of the Royal Astronomical Society |date=1 April 2018 |volume=475 |issue=2 |pages=2754–2767 |doi=10.1093/mnras/stx3343|doi-access=free |arxiv=1801.04279 }}</ref><ref name="D'Souza and Bell">{{cite journal |last1=D’Souza |first1=Richard |last2=Bell |first2=Eric F. |title=The Andromeda galaxy's most important merger about 2 billion years ago as M32's likely progenitor |journal=Nature Astronomy |date=September 2018 |volume=2 |issue=9 |pages=737–743 |doi=10.1038/s41550-018-0533-x |arxiv=1807.08819 |bibcode=2018NatAs...2..737D |s2cid=256713163 |url=https://www.nature.com/articles/s41550-018-0533-x |language=en |issn=2397-3366}}</ref>

The discovery of a recent merger in the Andromeda galaxy was first based on interpreting its anomalous age-velocity dispersion relation,<ref name="Dorman et al. 2014">{{Cite journal |last1=Dorman |first1=Claire E. |last2=Guhathakurta |first2=Puragra |last3=Seth |first3=Anil C. |last4=Weisz |first4=Daniel R. |last5=Bell |first5=Eric F. |last6=Dalcanton |first6=Julianne J. |last7=Gilbert |first7=Karoline M. |last8=Hamren |first8=Katherine M. |last9=Lewis |first9=Alexia R. |last10=Skillman |first10=Evan D. |last11=Toloba |first11=Elisa |last12=Williams |first12=Benjamin F. |date=9 April 2015 |title=A clear age-velocity dispersion correlation in Andromeda's stellar disk |journal=The Astrophysical Journal |volume=803 |issue=1 |pages=24 |arxiv=1502.03820 |bibcode=2015ApJ...803...24D |doi=10.1088/0004-637X/803/1/24 |s2cid=119223754}}</ref> as well as the fact that 2&nbsp;billion years ago, star formation throughout Andromeda's disk was much more active than today.<ref name="Williams et al. 2015">{{Cite journal |last1=Williams |first1=Benjamin F. |last2=Dalcanton |first2=Julianne J. |last3=Dolphin |first3=Andrew E. |last4=Weisz |first4=Daniel R. |last5=Lewis |first5=Alexia R. |last6=Lang |first6=Dustin |last7=Bell |first7=Eric F. |last8=Boyer |first8=Martha |last9=Fouesneau |first9=Morgan |last10=Gilbert |first10=Karoline M. |last11=Monachesi |first11=Antonela |last12=Skillman |first12=Evan |date=5 June 2015 |title=A Global Star-forming Episode in M31 2-4 Gyr Ago |journal=The Astrophysical Journal |volume=806 |issue=1 |pages=48 |arxiv=1504.02120 |bibcode=2015ApJ...806...48W |doi=10.1088/0004-637X/806/1/48 |s2cid=118435748}}</ref>

Modeling<ref name="Hammer et al. 2018"/> of this violent collision shows that it has formed most of the galaxy's (metal-rich) [[galactic halo]], including the Giant Stream,<ref name="Ibata et al. 2001">{{cite journal |last1=Ibata |first1=Rodrigo |last2=Irwin |first2=Michael |last3=Lewis |first3=Geraint |last4=Ferguson |first4=Annette M. N. |last5=Tanvir |first5=Nial |title=A giant stream of metal-rich stars in the halo of the galaxy M31 |journal=Nature |date=July 2001 |volume=412 |issue=6842 |pages=49–52 |doi=10.1038/35083506|pmid=11452300 |arxiv=astro-ph/0107090 |bibcode=2001Natur.412...49I |s2cid=4413139 }}</ref> and also the extended thick disk, the young age thin disk, and the static 10 kpc ring. During this epoch, its rate of [[star formation]] would have been [[Starburst galaxy|very high]], to the point of becoming a [[luminous infrared galaxy]] for roughly 100 million years. Modeling also recovers the bulge profile, the large bar, and the overall halo density profile.

Andromeda and the [[Triangulum Galaxy]] (M33) might have had a very close passage 2–4&nbsp;billion years ago, but it seems unlikely from the last measurements from the Hubble Space Telescope.<ref name="Patel et al. 2017">{{cite journal |last1=Patel |first1=Ekta |last2=Besla |first2=Gurtina |last3=Sohn |first3=Sangmo Tony |title=Orbits of massive satellite galaxies – I. A close look at the Large Magellanic Cloud and a new orbital history for M33 |journal=Monthly Notices of the Royal Astronomical Society |date=1 February 2017 |volume=464 |issue=4 |pages=3825–3849 |doi=10.1093/mnras/stw2616|doi-access=free |hdl=10150/623269 |hdl-access=free }}</ref>

=== Distance estimate ===
[[File:Milky Way and Andromeda in space, to scale.jpg|thumb|left|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 infrared [[surface brightness fluctuation]]s (I-SBF) and adjusting for the new period-luminosity value and a metallicity correction of −0.2 mag dex<sup>−1</sup> in (O/H), an estimate of {{convert|2.57|+/-|0.06|e6ly|e9AU|lk=on|abbr=unit}} was derived. A 2004 Cepheid variable method estimated the distance to be 2.51 ± 0.13&nbsp;million light-years (770 ± 40 kpc).<ref name="Karachentsevetal2006"/><ref name="karachentsevetal2004"/>

In 2005, an eclipsing [[binary star]] was discovered in the Andromeda Galaxy. The binary{{efn|name=M31VJ}} is made up of two hot blue stars of [[stellar classification|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 [[apparent magnitude|visual]] and absolute magnitudes are known, the distance to the star can be calculated. The stars lie at a distance of {{convert|2.52|+/-|0.14|e6ly|e9AU|abbr=unit}} and the whole Andromeda Galaxy at about {{convert|2.5|e6ly|e9AU|abbr=unit}}.<ref name="Ribas2005"/> This new value is in excellent agreement with the previous, independent Cepheid-based distance value. The [[tip of the red-giant branch|TRGB]] method was also used in 2005 giving a distance of {{convert|2.56|+/-|0.08|e6ly|e9AU|abbr=unit}}.<ref name="McConnachieetal2005"/> Averaged together, these distance estimates give a value of {{convert|2.54|+/-|0.11|e6ly|e9AU|abbr=unit}}.{{efn|name=avg dist}}

=== Mass estimates ===
[[File:Hubble Finds Giant Halo Around the Andromeda Galaxy.jpg|thumb|right|Giant halo around Andromeda Galaxy<ref name="Hubble-halo"/>]]

Until 2018, mass estimates for the Andromeda Galaxy's halo (including [[dark matter]]) gave a value of approximately {{Solar mass|1.5{{e|12}}|link=y}},<ref name="Jorge Peñarrubia2014"/> compared to {{Solar mass|8{{e|11}}}} 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 {{Solar mass|8{{e|11}}}}.<ref name="Kafle et al. 2018"/><ref name="Need4Speed-1"/><ref name="cosmic"/><ref name="Space.com"/> In 2006, the Andromeda Galaxy's [[Spiral galaxy#Galactic spheroid|spheroid]] was determined to have a higher stellar density than that of the Milky Way,<ref name="Kalirai et al 2006"/> and its galactic stellar disk was estimated at twice the diameter of that of the Milky Way.<ref name="Chapman et al 2006"/> The total mass of the Andromeda Galaxy is estimated to be between {{Solar mass|8{{e|11}}}}<ref name="Kafle et al. 2018"/> and {{Solar mass|1.1{{e|12}}}}.<ref name="Barmby 2006"/><ref name="Barmby2007"/> The stellar mass of M31 is {{Solar mass|10–15{{e|10}}}}, with 30% of that mass in the central [[Bulge (astronomy)|bulge]], 56% in the [[Disc (galaxy)|disk]], and the remaining 14% in the [[Spiral galaxy#Galactic spheroid|stellar halo]].<ref name="Tamm2012"/> 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 {{Solar mass|0.8{{e|12}}}},<ref name="Need4Speed"/> which is only half of the Milky Way's newer mass, calculated in 2019 at {{Solar mass|1.5{{e|12}}}}.<ref name="daily"/><ref name="SA-20190308"/><ref name="ARX-20190208"/>

In addition to stars, the Andromeda Galaxy's [[interstellar medium]] contains at least {{Solar mass|7.2{{e|9}}}}<ref name="BraunThilkerWalterbosCorbelli2009"/> in the form of [[neutral hydrogen]], at least {{Solar mass|3.4{{e|8}}}} as molecular [[hydrogen]] (within its innermost 10 kiloparsecs), and {{Solar mass|5.4{{e|7}}}} of [[interstellar dust|dust]].<ref name="Andromedandust"/>

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 from [[supernova]]e, and its properties are those expected for a galaxy that lies in the "green valley" of the [[Galaxy color–magnitude diagram|Galaxy color-magnitude diagram]] (see [[#Luminosity estimates|below]]). 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.<ref name="Hubble-halo-a"/><ref name="Massive"/><ref name="Evidence"/><ref name="HaloFind"/>

=== Luminosity estimates ===
The estimated [[luminosity]] of the Andromeda Galaxy, {{Solar luminosity|~2.6{{e|10}}|link=y}}, is about 25% higher than that of our own galaxy.<ref name="vdb"/><ref name="Moskvitch 2010"/> However, the galaxy has a high [[Orbital inclination|inclination]] as seen from Earth, and its [[Cosmic dust|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 [[megaparsec]]s of the Milky Way, after the [[Sombrero Galaxy]],<ref name="Karachentsev2004"/> with an absolute magnitude of around −22.21{{efn|name=bright m31}} or close<ref name="McCall2014"/>).

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,{{efn|name=blue mag}} compared to −20.9 for the Milky Way), and a total luminosity in that [[wavelength]] of {{Solar luminosity|3.64{{e|10}}}}.<ref name="Tempel2010"/>

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 of [[nova]]e in the Milky Way is also double that of the Andromeda Galaxy.<ref name="Liller & Mayer 1987"/> 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.<ref name="vdb"/> 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 the [[galaxy 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.<ref name="MWAY"/>

== Structure ==
{{Wide image|Andromeda Galaxy M31 - Heic1502a Full resolution.jpg|600px|''[[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.}}
[[File:A Swift Tour of M31.ogv|thumb|A narrated tour of the Andromeda Galaxy, made by NASA's [[Swift satellite]] team]]
Based on its appearance in visible light, the Andromeda Galaxy is classified as an SA(s)b galaxy in the [[Galaxy morphological classification#De Vaucouleurs system|de Vaucouleurs–Sandage extended classification system]] of spiral galaxies.<ref name="ned"/> However, infrared data from the [[2MASS]] survey and 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.<ref name="Beaton 2006"/>

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 D<sub>25</sub> standard, the [[isophote]] where the photometric brightness of a galaxy in the B-band (445&nbsp;nm wavelength of light, in the blue part of the [[visible spectrum]]) reaches 25 mag/arcsec<sup>2</sup>.<ref name="GalaxySize">{{Cite web|url=https://ned.ipac.caltech.edu/level5/PROPERTIES/dog.html|title=Dimensions of Galaxies|website=ned.ipac.caltech.edu}}</ref> The Third Reference Catalogue of Bright Galaxies (RC3) used this standard for Andromeda in 1991, yielding an isophotal diameter of {{convert|46.56|kpc|ly|sigfig=3|abbr=off}} at a distance of 2.5&nbsp;million light-years.<ref name="RC3"/> An earlier estimate from 1981 gave a diameter for Andromeda at {{convert|54|kpc|ly|sigfig=3|abbr=off}}.<ref name="AtlasofAndromeda">{{Cite web|url=https://ned.ipac.caltech.edu/level5/ANDROMEDA_Atlas/frames.html|title=Atlas of the Andromeda Galaxy|website=ned.ipac.caltech.edu}}</ref>

A study in 2005 by the [[Keck telescopes]] shows the existence of a tenuous sprinkle of stars, or [[galactic halo]], extending outward from the galaxy.<ref name="Chapman et al 2006"/> 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&nbsp;km/s.<ref name="Chapman et al 2006"/> This diffuse halo extends outwards away from Andromeda's main disc with the diameter of {{convert|67.45|kpc|ly|sigfig=3|abbr=off}}.<ref name="Chapman et al 2006"/>

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.<ref name="UC Santa Cruz 2001"/> A possible cause of such a warp could be gravitational interaction with the satellite galaxies near the Andromeda Galaxy. The Galaxy [[Triangulum 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 [[Galaxy rotation curve|rotational velocity of the Andromeda Galaxy]] as a function of radial distance from the core. The rotational velocity has a maximum value of {{cvt|225|km/s}} at {{convert|1300|ly|e6AU|lk=on|abbr=unit}} from the core, and it has its minimum possibly as low as {{cvt|50|km/s}} at {{convert|7000|ly|e6AU|abbr=unit}} from the core. Further out, rotational velocity rises out to a radius of {{convert|33000|ly|e9AU|abbr=unit}}, where it reaches a peak of {{cvt|250|km/s}}. The velocities slowly decline beyond that distance, dropping to around {{cvt|200|km/s}} at {{convert|80000|ly|e9AU|abbr=unit}}. These velocity measurements imply a concentrated mass of about {{Solar mass|6{{e|9}}|link=y}} in the [[Galaxy nucleus|nucleus]]. The total mass of the galaxy increases [[linearity|linearly]] out to {{convert|45000|ly|e9AU|abbr=unit}}, then more slowly beyond that radius.<ref name="Rubin & Ford 1970"/>

The [[Spiral galaxy#Spiral arms|spiral arms]] of the Andromeda Galaxy are outlined by a series of [[H II region|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.<ref name="Arp 1964"/> His descriptions of the spiral structure, as each arm crosses the major axis of the Andromeda Galaxy, are as follows<ref name="Bergh1991"/><sup>§pp1062</sup><!--
--><ref name="Hodge1966"/><sup>§pp92</sup>:
{| class="wikitable"
|+ 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 ([[Minute and second of arc|arcminutes]]) (N*/S*)
! Distance from the center (kpc) (N*/S*)
! Notes
|-
| align="left" |N1/S1|| 3.4/1.7|| 0.7/0.4|| Dust arms with no [[Stellar kinematics#OB associations|OB associations]] of [[H II region|HII region]]s
|-
| align="left" |N2/S2||8.0/10.0||1.7/2.1|| Dust arms with some OB associations
|-
| align="left" |N3/S3||25/30||5.3/6.3|| As per N2/S2, but with some HII regions too
|-
| align="left" |N4/S4||50/47||11/9.9|| Large numbers of OB associations, HII regions, and little dust
|-
| align="left" |N5/S5||70/66||15/14|| As per N4/S4 but much fainter
|-
| align="left" |N6/S6||91/95||19/20|| Loose OB associations. No dust is visible.
|-
| align="left" |N7/S7||110/116||23/24|| As per N6/S6 but fainter and inconspicuous
|}[[File:Andromeda galaxy Ssc2005-20a1.jpg|thumb|300x300px|Image of the Andromeda Galaxy taken by Spitzer in infrared, 24 [[micrometre]]s (Credit: [[NASA]]/[[JPL]]–[[Caltech]]/Karl D. Gordon, [[University of Arizona]])]]
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 {{convert|13000|ly|e6AU|lk=on|abbr=unit}} and that can be followed outward from a distance of roughly {{convert|1600|ly|e6AU|abbr=unit}} from the core. Alternative spiral structures have been proposed such as a single spiral arm<ref name="Simien1978"/> or a [[flocculent spiral galaxy|flocculent]]<ref name="Haas2000"/> pattern of long, filamentary, and thick spiral arms.<ref name="ned"/><ref name="Walterbos1988"/>

The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellites [[Messier 32|M32]] and [[Messier 110|M110]].<ref name="Gordon2006"/> This can be seen by the displacement of the [[H I region|neutral hydrogen clouds]] from the stars.<ref name="Braun 1991"/>

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 are generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of {{cvt|32000|ly|kpc}} from the core,<ref name="ESA 1998-10-14"/> nicknamed by some astronomers the ''ring of fire''.<ref name="M31youngclusters"/> 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.<ref name="Andromedan10kpcring"/>

Later studies with the help of the [[Spitzer 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.<ref name="Gordon2006"/>

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.<ref name="Harvard-Smithsonian 2006-10-18"/>
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 Galaxy|Cartwheel encounter]].<ref name="Block2006"/>

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 "[[Metallicity|metal-poor]]", and increasingly so with greater distance.<ref name="Kalirai et al 2006"/> 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&nbsp;billion years.<ref name="Bullock & Johnston 2005"/> 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 ===
[[File:M31 nucleus (labels).jpg|thumb|[[Hubble Space Telescope]] image of the Andromeda Galaxy core showing P1, P2 and P3, with P3 containing M31*. [[NASA]]/[[European Space Agency|ESA]] photo]]
The Andromeda Galaxy is known to harbor a dense and compact star cluster at its very center, similar to the [[Milky Way]] galaxy. A large telescope creates a visual impression of a star embedded in the more diffuse surrounding bulge. In 1991, the [[Hubble Space Telescope]] was used to image the Andromeda Galaxy's inner nucleus. The nucleus consists of two concentrations separated by {{cvt|1.5|pc|lk=on}}. 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,<ref name="Bender et al 2005"/> containing many [[Ultraviolet|UV]]-bright [[A-type main-sequence star|A-stars]] and the [[supermassive black hole]], called [[M31*]].<ref name=":0">{{Cite journal |last1=Garcia |first1=Michael R. |last2=Hextall |first2=Richard |last3=Baganoff |first3=Frederick K. |last4=Galache |first4=Jose |last5=Melia |first5=Fulvio |last6=Murray |first6=Stephen S. |last7=Primini |first7=F. A. |last8=Sjouwerman |first8=Loránt O. |last9=Williams |first9=Ben |date=1 February 2010 |title=X-ray and Radio Variability of M31*, The Andromeda Galaxy Nuclear Supermassive Black Hole |url=https://ui.adsabs.harvard.edu/abs/2010ApJ...710..755G |journal=The Astrophysical Journal |volume=710 |issue=1 |pages=755–763 |arxiv=0907.4977 |bibcode=2010ApJ...710..755G |doi=10.1088/0004-637X/710/1/755 |hdl=1721.1/96091 |issn=0004-637X}}</ref><ref name=":1">{{Cite journal |last1=Yang |first1=Yang |last2=Li |first2=Zhiyuan |last3=Sjouwerman |first3=Loránt O. |last4=Yuan |first4=Feng |last5=Shen |first5=Zhi-Qiang |date=1 August 2017 |title=Very Large Array Multiband Monitoring Observations of M31* |journal=The Astrophysical Journal |volume=845 |issue=2 |pages=140 |arxiv=1707.08317 |bibcode=2017ApJ...845..140Y |doi=10.3847/1538-4357/aa8265 |doi-access=free |issn=0004-637X}}</ref> The black hole is classified as a low-luminosity [[Active galactic nucleus|AGN]] (LLAGN) and it was detected only in [[Radio astronomy|radio wavelengths]] and in [[X-ray astronomy|x-rays]].<ref name=":1"/> It was quiescent in 2004–2005, but it was highly variable in 2006–2007.<ref name=":0"/> An additional x-ray flare occurred in 2013.<ref>{{Cite journal |last1=DiKerby |first1=Stephen |last2=Zhang |first2=Shuo |last3=Irwin |first3=Jimmy |date=2025-03-01 |title=Fifteen Years of M31* X-Ray Variability and Flares |journal=The Astrophysical Journal |volume=981 |issue=1 |pages=50 |arxiv=2502.01365 |bibcode=2025ApJ...981...50D |doi=10.3847/1538-4357/adb1d5 |doi-access=free |issn=0004-637X}}</ref> The mass of M31* was measured at 3–5 × 10<sup>7</sup> {{Solar mass}} in 1993,<ref name="Lauer"/> and at 1.1–2.3 × 10<sup>8</sup> {{Solar mass}} in 2005.<ref name="Bender et al 2005"/> The [[velocity dispersion]] of material around it is measured to be ≈ {{cvt|160|km/s|round=10|lk=on}}.<ref name="Gebhardt et al 2000"/>

It has been proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an [[orbital eccentricity|eccentric orbit]] around the central black hole.<ref name="Tremaine 1995"/> The eccentricity is such that stars linger at the orbital [[apsis|apocenter]], 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.<ref>{{Cite journal |last1=Akiba |first1=Tatsuya |last2=Madigan |first2=Ann-Marie |date=1 November 2021 |title=On the Formation of an Eccentric Nuclear Disk following the Gravitational Recoil Kick of a Supermassive Black Hole |journal=The Astrophysical Journal Letters |volume=921 |issue=1 |pages=L12 |doi=10.3847/2041-8213/ac30d9 |arxiv=2110.10163 |bibcode=2021ApJ...921L..12A |s2cid=239049969 |issn=2041-8205 |doi-access=free }}</ref> P2 also contains a compact disk of hot, [[stellar classification|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.<ref name="hubblesite 1993-07-20"/>

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,<ref name="Schewe & Stein 1993"/> this is no longer considered a viable explanation, largely because such a nucleus would have an exceedingly short lifetime due to [[tidal force|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.<ref name="Tremaine 1995"/>

== Discrete sources ==
[[File:PIA20061 - Andromeda in High-Energy X-rays, Figure 1.jpg|thumb|400px|The Andromeda Galaxy in high-energy X-ray and ultraviolet light (released 5 January 2016)]]
Apparently, by late 1968, no [[X-ray]]s had been detected from the Andromeda Galaxy.<ref name="Fujimoto et al 1969"/> A balloon flight on 20 October 1970 set an upper limit for detectable hard X-rays from the Andromeda Galaxy.<ref name="Peterson 1973"/> The [[Neil Gehrels Swift Observatory|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.<ref name="Marelli et al. 2017"/>

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 star]]s, which are heating the incoming gas to millions of [[kelvin]]s and emitting X-rays. Neutron stars and black holes can be distinguished mainly by measuring their masses.<ref name="Barnard Kolb & Osborne 2005"/> An observation campaign of [[NuSTAR]] space mission identified 40 objects of this kind in the galaxy.<ref name="And-Scand"/>
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 {{Solar mass}}. It was discovered through data collected by the [[European Space Agency]]'s [[XMM-Newton]] probe and was subsequently observed by [[NASA]]'s [[Neil Gehrels Swift Observatory|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.<ref name="prostak-2012"/>

== Globular clusters ==
[[File:Star cluster in the Andromeda galaxy.jpg|thumb|Star clusters in the Andromeda Galaxy<ref name="And-Clustr"/>|400x400px]]
There are approximately 460 [[globular cluster]]s associated with the Andromeda Galaxy.<ref name="Barmby & Huchra 2001"/> 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.<ref name="hubblesite 1996-04-24"/> It contains several million stars and is about twice as luminous as [[Omega 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 a [[dwarf galaxy]] that was consumed by Andromeda in the distant past.<ref name="Meylan et al 2001"/> The cluster with the greatest apparent brightness is G76 which is located in the southwest arm's eastern half.<ref name="NSOG"/>
Another massive globular cluster, named 037-B327 (also known as Bol 37) and discovered in 2006 as is heavily reddened by the Andromeda Galaxy's [[Cosmic dust|interstellar dust]], was thought to be more massive than Mayall II and the largest cluster of the Local Group;<ref name="SuperStr"/> however, other studies have shown it is actually similar in properties to Mayall II.<ref name="Cohen06"/>

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.<ref name="Formations"/>

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.<ref name="Huxor et al 2005"/>

The most massive globular cluster in the Andromeda Galaxy, B023-G078, likely has a central intermediate black hole of almost 100,000 solar masses.<ref name="central black hole">{{cite journal | last1=Pechetti | first1=Renuka | last2=Seth | first2=Anil | last3=Kamann | first3=Sebastian | last4=Caldwell | first4=Nelson | last5=Strader | first5= Jay |title=Detection of a 100,000 M ⊙ black hole in M31's Most Massive Globular Cluster: A Tidally Stripped Nucleus |journal=The Astrophysical Journal |date=January 2022 |volume=924 |issue=2 |pages=13 |doi=10.3847/1538-4357/ac339f | arxiv=2111.08720 | bibcode=2022ApJ...924...48P | s2cid=245876938 | doi-access=free }}</ref>

=== PA-99-N2 event and possible exoplanet in galaxy ===
{{Main|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.<ref name="Better needed"/>

== Nearby and satellite galaxies ==
{{Main|Andromeda's satellite galaxies}}
[[File:M31, the Andromeda Galaxy, Killarney Provincial Park Observatory.jpg|thumb|The Andromeda Galaxy with [[Satellite galaxy|satellite galaxies]] [[Messier 32|M32]] (center left above the [[galactic nucleus]]) and [[Messier 110|M110]] (center right below the galaxy)]]
Like the Milky Way, the Andromeda Galaxy has smaller [[satellite galaxy|satellite galaxies]], consisting of over 20 known [[dwarf galaxy|dwarf galaxies]]. The Andromeda Galaxy's dwarf galaxy population is very similar to the Milky Way's, but the galaxies are much more numerous.<ref>{{cite journal|doi=10.1093/mnras/stab1754|title=Solo dwarfs IV: Comparing and contrasting satellite and isolated dwarf galaxies in the Local Group|year=2021|last1=Higgs|first1=C. R.|last2=McConnachie|first2=A. W.|journal=Monthly Notices of the Royal Astronomical Society|volume=506|issue=2|pages=2766–2779|doi-access=free |arxiv=2106.12649}}</ref> The best-known and most readily observed satellite galaxies are [[Elliptical Galaxy M32|M32]] and [[Elliptical Galaxy M110|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.<ref name="Bekki et al 2001"/>

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.<ref name="Ibata et al 2001"/> M110 does contain a dusty lane, which may indicate recent or ongoing star formation.<ref name="Young 2000"/> M32 has a young stellar population as well.<ref name="Rudenko"/>

The [[Triangulum 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.<ref>{{cite web |url=http://www.messier.seds.org/m/m033.html |title=Messier 33 |access-date=22 July 2024 |website=SEDS Messier Catalog}}</ref>

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.<ref name="Koch & Grebel 2006"/>

== Collision with the Milky Way ==
{{Main|Andromeda–Milky Way collision}}
[[File:Collision paths of our Milky Way galaxy and the Andromeda galaxy.jpg|thumb|300x300px|Illustration of the collision path between the Milky Way and Andromeda Galaxy]]
The Andromeda Galaxy is approaching the Milky Way at about {{convert|110|km|abbr=off}} per second.<ref name="nature"/> It has been measured approaching relative to the Sun at around {{cvt|300|km/s}}<ref name="ned"/> as the Sun orbits around the center of the galaxy at a speed of approximately {{cvt|225|km/s}}. This makes the Andromeda Galaxy one of about 100 observable [[blueshift]]ed galaxies.<ref name="Answers"/> Andromeda Galaxy's tangential or sideways velocity concerning 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 2.5–4&nbsp;billion years. A likely outcome of the collision is that the [[Galaxy merger|galaxies will merge]] to form a giant [[elliptical galaxy]]<ref name="Cox & Loeb 2008"/> or possibly large [[disc galaxy]].<ref name="Ueda2014"/> Such events are frequent among the galaxies in [[galaxy group]]s. The fate of 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.<ref name="Cain 2007"/>

== Amateur observation==
[[File:Moon over Andromeda (rotated).jpg|thumb|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.<ref>{{cite web |last1=Plait |first1=Phil |title=Yes, That Picture of the Moon and the Andromeda Galaxy Is About Right |url=https://slate.com/technology/2014/01/moon-and-andromeda-relative-size-in-the-sky.html |website=Slate Magazine |access-date=29 July 2022 |language=en |date=1 January 2014}}</ref><ref>{{cite web |title=Andromeda and the Moon collage |url=https://noirlab.edu/public/images/noao-andromn/ |website=www.noirlab.edu |access-date=29 July 2022 |language=en}}</ref>]]

Under most viewing conditions, the Andromeda Galaxy is one of the most distant objects that can be seen with the [[naked eye]], due to its sheer size. ([[Triangulum Galaxy|M33]] and, for observers with exceptionally good vision, [[Messier 81|M81]] can be seen under [[Bortle scale|very dark]] skies.)<ref name="StarChild"/><ref name="S&T-obs"/><ref>{{Cite web|last=Garner|first=Rob|date=20 February 2019|title=Messier 33 (The Triangulum Galaxy)|url=https://www.nasa.gov/feature/goddard/2019/messier-33-the-triangulum-galaxy|access-date=6 August 2021|website=NASA}}</ref><ref>{{cite book |last=Harrington |first=Philip S. |date=2010 |title=Cosmic Challenge: The Ultimate Observing List for Amateurs |publisher=Cambridge University Press |pages=28–29 |isbn=9781139493680 |quote=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.}}</ref>
The constellation of Andromeda, in which the galaxy is located, is usually found with the aid of the constellations [[Cassiopeia (constellation)|Cassiopeia]] or [[Pegasus (constellation)|Pegasus]], which are usually easier to recognize at first glance.
Andromeda is best seen during autumn nights in the [[Northern Hemisphere]] when it passes high overhead, reaching its [[zenith|highest 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.<ref name=physics/>
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 galaxy|satellite galaxies]], [[Messier 32|M32]] and [[Messier 110|M110]].<ref name="S&T"/> An [[Amateur astronomy|amateur telescope]] can reveal Andromeda's disk, some of its brightest globular clusters, dark dust lanes, and the large star cloud [[NGC 206]].<ref name=Backyard/><ref name="A-Mall"/>

==See also==
* [[List of Messier objects]]
* [[List of galaxies]]
* ''[[New General Catalogue]]''

== Notes ==
{{notelist
| notes =
{{efn
| name = avg dist
| 1 = average(787 ± 18, 770 ± 40, 772 ± 44, 783 ± 25) = ((787 + 770 + 772 + 783) / 4) ± (18<sup>2</sup> + 40<sup>2</sup> + 44<sup>2</sup> + 25<sup>2</sup>)<sup>0.5</sup> / 2 = 778 ± 33.
}}

{{efn
| name = M31VJ
| 1 = J00443799+4129236 is at [[Celestial coordinate system|celestial coordinates]] [[Right Ascension|R.A.]] {{RA|00|44|37.99}}, [[Declination|Dec.]] {{DEC|+41|29|23.6}}.
}}

{{efn
| name = blue mag
| 1 = Blue absolute magnitude of −20.89 – [[Color index]] of 0.63 = −21.52
}}

{{efn
| name = bright m31
| 1 = Blue absolute magnitude of −21.58 (see reference) – [[Color index]] of 0.63 = absolute visual magnitude of −22.21
}}

}}
{{clear}}

== References ==
{{reflist|refs=
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<ref name="GalaxyMass">"The median values of the Milky Way and Andromeda masses are M<sub>G</sub> = {{val|0.8|0.4|0.3|e=12|u=solar mass}} and M<sub>A</sub> = {{val|1.5|0.5|0.4|e=12|u=solar mass}} at a 68% level" {{cite journal | first1=Jorge |last1=Peñarrubia | first2=Yin-Zhe |last2=Ma | first3=Matthew G. |last3=Walker | first4=Alan W. |last4=McConnachie | title=A dynamical model of the local cosmic expansion | journal=Monthly Notices of the Royal Astronomical Society | volume=433 | issue=3 | date=29 July 2014 | pages=2204–2222 | bibcode= 2014MNRAS.443.2204P | doi=10.1093/mnras/stu879|doi-access=free |arxiv = 1405.0306 |s2cid=119295582}}, but compare "[we estimate] the virial mass and radius of the galaxy to be {{cvt|0.8|+/-|0.1|e12solar mass|kg|lk=on}}" {{cite journal | first1 = Prajwal R. |last1=Kafle | first2 = Sanjib |last2=Sharma | first3 = Geraint F. |last3=Lewis | display-authors=3 | first4 = Aaron S. G. |last4=Robotham | first5 = Simon P. | last5=Driver | title = The Need for Speed: Escape velocity and dynamical mass measurements of the Andromeda Galaxy | journal = Monthly Notices of the Royal Astronomical Society | volume = 475 | issue = 3 | date = 1 February 2018 | pages=4043–4054 | bibcode = 2018MNRAS.475.4043K | issn=0035-8711 | doi = 10.1093/mnras/sty082|doi-access=free |arxiv = 1801.03949 |s2cid=54039546}}</ref>
<ref name="As-Now">{{Cite web |date=11 March 2019 |title=Milky Way tips the scales at 1.5 trillion solar masses |url=https://astronomynow.com/2019/03/11/milky-way-tips-the-scales-at-1-5-trillion-solar-masses/ |access-date=2024-09-11 |website=Astronomy Now |language=en-US}}</ref>
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<ref name="Messie">{{cite web |url=https://messierobjects101.com/messier-object-m31-m32-m110-andromeda-galaxy/ |title=M 31, M 32 & M 110 |date=15 October 2016}}</ref>
<ref name="Hafez">{{Cite thesis | last= Hafez | first= Ihsan | date= 2010 | title= Abd al-Rahman al-Sufi and his book of the fixed stars: a journey of re-discovery. | url= http://eprints.jcu.edu.au/28854/ | type= PhD Thesis | publisher= James Cook University | access-date= 23 June 2016 | bibcode= 2010PhDT.......295H}}</ref>
<ref name="Kant">{{Cite book |last=Kant |first=Immanuel |author-link=Immanuel Kant |url=https://archive.org/details/universalnatural0000kant |title=Universal natural history and theory of the heavens |date=1969 |publisher=University of Michigan Press}}</ref>
<ref name="NebPix">{{Cite journal |last=Roberts |first=I. |year=1888 |title=Photographs of the Nebulae M 31, h 44, and h 51 Andromedae, and M 27 Vulpeculae |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=49 |issue=2 |pages=65–66 |bibcode=1888MNRAS..49...65R |doi=10.1093/mnras/49.2.65 |issn=0035-8711 |doi-access=free}}</ref>
<ref name="firstName">{{Cite web|url=https://www.space.com/15590-andromeda-galaxy-m31.html|title=The Andromeda Galaxy (M31): Location, Characteristics & Images|website=[[Space.com]]|date=10 January 2018 }}</ref>
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<ref name="nake">{{cite news |title=Two naked-eye galaxies above the VLT |url=http://www.eso.org/public/images/potw1342a/ |access-date=22 October 2013 |newspaper=ESO Picture of the Week}}</ref>
-->
<ref name="Hubble-halo">{{cite web|title=Hubble Finds Giant Halo Around the Andromeda Galaxy|url=http://www.spacetelescope.org/images/opo1515a/|access-date=14 June 2015}}</ref>
<ref name="Need4Speed-1">{{Cite journal |arxiv = 1801.03949|doi = 10.1093/mnras/sty082|title = The need for speed: Escape velocity and dynamical mass measurements of the Andromeda galaxy|journal = Monthly Notices of the Royal Astronomical Society|volume = 475|issue = 3|pages = 4043–4054|year = 2018|last1 = Kafle|first1 = Prajwal R.|last2 = Sharma|first2 = Sanjib|last3 = Lewis|first3 = Geraint F.|last4 = Robotham|first4 = Aaron S G.|last5 = Driver|first5 = Simon P.| doi-access=free |bibcode = 2018MNRAS.475.4043K|s2cid = 54039546}}</ref>
<ref name="cosmic">{{cite web|url=https://www.icrar.org/cosmic-collision/|title=Milky Way ties with neighbour in galactic arms race|date=15 February 2018}}</ref>
<ref name="Space.com">{{cite web|url=https://www.space.com/39751-andromeda-galaxy-not-bigger-than-milky-way.html|title=The Andromeda Galaxy Is Not Bigger Than the Milky Way After All|first1=Samantha Mathewson 2018-02-20T19:05:26Z|last1=Science|last2=Astronomy|website=Space.com|date=20 February 2018 }}</ref>
<ref name="Need4Speed">{{cite journal|title=The Need for Speed: Escape velocity and dynamical mass measurements of the Andromeda galaxy|year=2018|doi=10.1093/mnras/sty082|arxiv=1801.03949|last1=Kafle|first1=Prajwal R.|last2=Sharma|first2=Sanjib|last3=Lewis|first3=Geraint F.|last4=Robotham|first4=Aaron S G.|last5=Driver|first5=Simon P.|journal=Monthly Notices of the Royal Astronomical Society|volume=475|issue=3|pages=4043–4054|doi-access=free |bibcode=2018MNRAS.475.4043K|s2cid=54039546}}</ref>
<ref name="daily">{{Cite web|url=https://scitechdaily.com/hubble-gaia-reveal-weight-of-the-milky-way-1-5-trillion-solar-masses/|title=Hubble & Gaia Reveal Weight of the Milky Way: 1.5 Trillion Solar Masses|first1=Bethany|last1=Downer|first2=ESA/Hubble|last2=Telescope|date=10 March 2019}}</ref>
<ref name="Hubble-halo-a">{{Cite web|title = HubbleSite – NewsCenter – Hubble Finds Giant Halo Around the Andromeda Galaxy (05/07/2015) – The Full Story|url = http://hubblesite.org/newscenter/archive/releases/2015/15/full/|website = hubblesite.org|access-date = 7 May 2015}}</ref>
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<ref name="StarChild">{{cite web|url=https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question15.html|title=Can you see other galaxies without a telescope?|website=starchild.gsfc.nasa.gov}}</ref>
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}}

== External links ==
{{Commons}}
* {{WikiSky|name=The Andromeda Galaxy}}
* [https://web.archive.org/web/20110722021459/http://blackholes.stardate.org/directory/factsheet.php?p=M31 StarDate: M31 Fact Sheet]
* [http://www.messier.seds.org/m/m031.html Messier 31, SEDS Messier pages]
* [[Astronomy Picture of the Day]]
** [https://apod.nasa.gov/apod/ap981017.html A Giant Globular Cluster in M31] 1998 October 17.
** [https://apod.nasa.gov/apod/ap040718.html M31: The Andromeda Galaxy] 2004 July 18.
** [https://apod.nasa.gov/apod/ap051222.html Andromeda Island Universe] 2005 December 22.
** [https://apod.nasa.gov/apod/ap100109.html Andromeda Island Universe] 2010 January 9.
** [https://apod.nasa.gov/apod/ap100219.html WISE Infrared Andromeda] 2010 February 19.
** [https://apod.nasa.gov/apod/ap130801.html M31's angular size compared with full Moon] 2013 August 1.
* [http://www.beskeen.com/gallery/galaxy/m31/m31.shtml M31 and its central Nuclear Spiral]
* [http://www.starpointing.com/ccd/m31.html Amateur photography&nbsp;– M31]
* [http://astro.neutral.org/imagehtml/20050903_m31.html Globular Clusters in M31] at The Curdridge Observatory
* [https://web.archive.org/web/20181116120359/http://www.astronomy.com/news/2005/11/first-direct-distance-to-andromeda First direct distance to Andromeda] − [[Astronomy (magazine)|Astronomy magazine]] article
* [http://www.solstation.com/x-objects/andromeda.htm Andromeda Galaxy] at SolStation.com
* [https://www.daviddarling.info/encyclopedia/A/Andromeda.html Andromeda Galaxy at The Encyclopedia of Astrobiology, Astronomy, & Spaceflight]
* [http://www.nightskyinfo.com/archive/m31_galaxy M31, the Andromeda Galaxy] at NightSkyInfo.com
* {{cite news |last=Than |first=Ker |title=Strange Setup: Andromeda's Satellite Galaxies All Lined Up |url=http://space.com/scienceastronomy/060123_andromeda_plane.html |work=Space.com |date=23 January 2006}}
* [https://web.archive.org/web/20110719204305/http://cbat.eps.harvard.edu/CBAT_M31.html M31 (Apparent) Novae Page] (IAU)
* [http://herschel.esac.esa.int/Images/2011/M31_COMPO_A.jpg Multi-wavelength composite]
* [https://www.zooniverse.org/projects/zooniverse/andromeda-project Andromeda Project] (crowd-source)
* {{cite web |title=M31 – Andromeda Galaxy|url=http://www.deepskyvideos.com/videos/messier/M31_andromeda.html |work=Deep Sky Videos |publisher=[[Brady Haran]] |last1=Gray |first1=Meghan| last2=Szymanek |first2=Nik |author2-link=Nik Szymanek |last3=Merrifield |first3=Michael}}
* [https://hubblesite.org/contents/news-releases/2015/news-2015-02.html Hubble's High-Definition Panoramic View of the Andromeda Galaxy]
* [https://hubblesite.org/contents/media/images/2022/027/01G4NM6207PRA9YES60C5RKKA5 Infrared-Radio Image of the Andromeda Galaxy (M31)]
* [https://rockchucksummit.com/m31-andromeda-galaxy/ Creative Commons Astrophotography M31 Andromeda image download & processing guide]

{{Andromeda galaxy}}
{{Messier objects}}
{{NGC objects:1-499}}
{{Stars of Andromeda}}
{{Portal bar|Astronomy|Stars|Outer space}}
{{Authority control}}
{{Sky|00|42|44.3|+|41|16|9|2540000}}

{{DEFAULTSORT:Andromeda Galaxy}}
[[Category:Andromeda Galaxy| ]]
[[Category:Andromeda (constellation)]]
[[Category:Andromeda Subgroup]]
[[Category:Articles containing video clips]]
[[Category:Astronomical objects known since antiquity]]
[[Category:Barred spiral galaxies]]
[[Category:IRAS catalogue objects|00400+4059]]
[[Category:Local Group]]
[[Category:MCG objects|+07-02-016]]
[[Category:Messier objects]]
[[Category:NGC objects]]
[[Category:Principal Galaxies Catalogue objects|002557]]
[[Category:UGC objects|00454]]