Editing Andromeda Galaxy (section)

== 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.