Andromeda Galaxy

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 D₂₅ isophotal diameter of about and is approximately 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 the princess who was the wife of Perseus in Greek mythology.
The virial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at. 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%. However, this has been called into question by early-21st-century studies indicating a possibly lower mass for the Andromeda Galaxy and a higher mass for the Milky Way. The Andromeda Galaxy has a diameter of about, making it the largest member of the Local Group of galaxies in terms of extension.
The Milky Way and Andromeda galaxies have about a 50% chance of colliding with each other in the next 10 billion years, merging to potentially form a giant elliptical galaxy or a large lenticular galaxy.
With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects, and is visible to the naked eye from Earth on moonless nights, even when viewed from areas with moderate light pollution.

Observation history

The Andromeda Galaxy is visible to the naked eye in dark skies. It has been speculated that the Babylonian constellation of the Rainbow, Mul Dingir Tir-an-na, may have referred to M31. Around the year 964 CE, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy in his Book of Fixed Stars as a "nebulous smear" or "small cloud". This was the first historical reference to the Andromeda Galaxy and the earliest known reference to a galaxy other than the Milky Way. Star charts of that period labeled it as the Little Cloud. In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations. John Flamsteed cataloged it as 33 Andromedae. Pierre Louis Maupertuis conjectured in 1745 that the blurry spot was an island universe. 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. He believed Andromeda to be the nearest of all the "great nebulae," and based on the color and magnitude of the nebula, he incorrectly guessed that it was no more than 2,000 times the distance of Sirius, or roughly.
In 1850, William Parsons, 3rd Earl of Rosse, made a drawing of Andromeda's spiral structure.
In 1864, William Huggins noted that the spectrum of Andromeda differed from that of a gaseous nebula. The spectrum of Andromeda displays a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. Andromeda's spectrum is very similar to the spectra of individual stars, and from this, it was deduced that Andromeda has a stellar nature. In 1885, a supernova was seen in Andromeda, the only one ever observed in that galaxy. At the time, it was called "Nova 1885"—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.
File:Andromeda Nebula - Isaac Roberts, 29 December 1888.jpg|thumb|The earliest known photograph of the Great Andromeda "Nebula", by Isaac Roberts
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" as star systems being formed.
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.

"Island universes" hypothesis

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 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.
In 1917, 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 magnitudes fainter than those that occurred elsewhere in the sky. As a result, he was able to come up with a distance estimate of. 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. Curtis became a proponent of the so-called "island universes" hypothesis: that spiral nebulae were actually independent galaxies.
In 1920, the Great Debate between Harlow Shapley and Curtis took place concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim 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 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 the Milky Way at a distance of about. Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of Andromeda. These were made using the 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.
In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their metallicity, naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge Type II. This nomenclature was subsequently adopted for stars within the Milky Way and elsewhere. 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.
In 1950, radio emissions from the Andromeda Galaxy were detected by Robert Hanbury Brown and Cyril Hazard at the Jodrell Bank Observatory. The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group. The core of the Andromeda Galaxy is called 2C 56 in the 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 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 10⁷ solar masses. The second example of this phenomenon was found in 1961 in the nucleus of M32 by M.F Walker 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 10⁷ 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

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.
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.
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 in oxygen-III. 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. A later study using spectroscopy found the nebula to be in the Milky Way. One study found the nebula to be a bow shock of a ghost planetary nebula around the binary EG Andromedae.
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.