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The Andromeda Galaxy (IPA: /ænˈdrɒmɪdə/), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with diameter of about 46.56 kiloparsecs (152,000 light-years)^[8] approximately 2.5 million light-years (770 kiloparsecs) from Earth and the nearest large galaxy to the Milky Way.^[6] 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 1 trillion solar masses (2.0×10⁴² kilograms). The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy is more massive than the Milky Way by a margin of some 25% to 50%. This has been called into question by a 2018 study that cited a lower estimate on the mass of the Andromeda Galaxy,^[12] combined with preliminary reports on a 2019 study estimating a higher mass of the Milky Way.^[13][14] The Andromeda Galaxy has a diameter of about 46.56 kpc (152,000 ly), making it the largest member of the Local Group in terms of extension.

The Milky Way and Andromeda galaxies are expected to collide in around 4–5 billion years,^[15] merging to form a giant elliptical galaxy^[16] or a large lenticular galaxy.^[17] With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects,^[18] and is visible to the naked eye from Earth on moonless nights,^[19] even when viewed from areas with moderate light pollution.

Observation history[edit]

Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi was the first to formally describe the Andromeda Galaxy. He referred to it in his Book of Fixed Stars as a “nebulous smear” or “small cloud”.^[20][21]

Star charts of that period labeled it as the Little Cloud.^[22] In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations.^[23] Pierre Louis Maupertuis conjectured in 1745 that the blurry spot was an island universe.^[24] In 1764, Charles Messier cataloged Andromeda as object M31 and incorrectly credited Marius as the discoverer despite its being visible to the naked eye. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the “great nebulae“, and based on the color and magnitude of the nebula, he incorrectly guessed that it was no more than 2,000 times the distance of Sirius, or roughly 18,000 ly (5.5 kpc).^[25] In 1850, William Parsons, 3rd Earl of Rosse made the first drawing of Andromeda’s spiral structure.

In 1864 Sir William Huggins noted that the spectrum of Andromeda differed from that of a gaseous nebula.^[26] The spectra of Andromeda displays a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. Andromeda’s spectrum is very similar to the spectra of individual stars, and from this, it was deduced that Andromeda has a stellar nature. In 1885, a supernova (known as S Andromedae) was seen in Andromeda, the first and so far only one observed in that galaxy. At the time it was called “Nova 1885”^[27] – 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.

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 star systems being formed.^[28][29]

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 300 km/s (190 mi/s).^[30]

Island universe[edit]

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 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.

In 1917, Heber Curtis observed a nova within Andromeda. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred elsewhere in the sky. As a result, he was able to come up with a distance estimate of 500,000 ly (3.2×10¹⁰ AU). He became a proponent of the so-called “island universes” hypothesis, which held that spiral nebulae were actually independent galaxies.^[31]

In 1920, the Great Debate between Harlow Shapley and Curtis took place concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim of the Great Andromeda Nebula being, in fact, an external galaxy, Curtis also noted the appearance of dark lanes within Andromeda which resembled the dust clouds in our own galaxy, as well as historical observations of Andromeda Galaxy’s significant Doppler shift. In 1922 Ernst Öpik presented a method to estimate the distance of Andromeda using the measured velocities of its stars. His result placed the Andromeda Nebula far outside our galaxy at a distance of about 450 kpc (1,500 kly).^[33] 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 100-inch (2.5 m) Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our own galaxy, but an entirely separate galaxy located a significant distance from the Milky Way.^[34]

In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their metallicity, naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge Type II. This nomenclature was subsequently adopted for stars within the Milky Way, and elsewhere. (The existence of two distinct populations had been noted earlier by Jan Oort.)^[35] Baade also discovered that there were two types of Cepheid variable stars, which resulted in a doubling of the distance estimate to Andromeda, as well as the remainder of the universe.^[36]

In 1950, radio emission from the Andromeda Galaxy was detected by Hanbury Brown and Cyril Hazard at Jodrell Bank Observatory.^[37][38] The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group.^[39] The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomy catalog. In 2009, the first planet may have been discovered in the Andromeda Galaxy. This was detected using a technique called microlensing, which is caused by the deflection of light by a massive object.^[40]

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.^[41] The total magnetic field has a strength of about 0.5 nT, of which 0.3 nT are ordered.

General[edit]

The estimated distance of the Andromeda Galaxy from our own was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid variable star. In the 1990s, measurements of both standard red giants as well as red clump stars from the Hipparcos satellite measurements were used to calibrate the Cepheid distances.^[42][43]

Formation and history[edit]

The Andromeda Galaxy was formed roughly 10 billion years ago from the collision and subsequent merger of smaller protogalaxies.^[44]

This violent collision formed most of the galaxy’s (metal-rich) galactic halo and extended disk. During this epoch, its rate of star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years. Andromeda and the Triangulum Galaxy (M33) had a very close passage 2–4 billion years ago. This event produced high rates of star formation across the Andromeda Galaxy’s disk—even some globular clusters—and disturbed M33’s outer disk.

Over the past 2 billion years, star formation throughout Andromeda’s disk is thought to have decreased to the point of near-inactivity. There have been interactions with satellite galaxies such as M32, M110, or others that have already been absorbed by the Andromeda Galaxy. These interactions have formed structures like Andromeda’s Giant Stellar Stream. A galactic merger roughly 100 million years ago is believed to be responsible for a counter-rotating disk of gas found in the center of Andromeda as well as the presence there of a relatively young (100 million years old) stellar population.^[44]

Distance estimate[edit]

At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy. In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value and a metallicity correction of −0.2 mag dex⁻¹ in (O/H), an estimate of 2.57 ± 0.06 million light-years (1.625×10¹¹ ± 3.8×10⁹ astronomical units) was derived. A 2004 Cepheid variable method estimated the distance to be 2.51 ± 0.13 million light-years (770 ± 40 kpc).^[2][45] In 2005, an eclipsing binary star was discovered in the Andromeda Galaxy. The binary^[c] is two hot blue stars of types O and B. By studying the eclipses of the stars, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure their absolute magnitude. When the visual and absolute magnitudes are known, the distance to the star can be calculated. The stars lie at a distance of 2.52×10⁶ ± 0.14×10⁶ ly (1.594×10¹¹ ± 8.9×10⁹ AU) and the whole Andromeda Galaxy at about 2.5×10⁶ ly (1.6×10¹¹ AU).^[6] This new value is in excellent agreement with the previous, independent Cepheid-based distance value. The TRGB method was also used in 2005 giving a distance of 2.56×10⁶ ± 0.08×10⁶ ly (1.619×10¹¹ ± 5.1×10⁹ AU).^[46] Averaged together, these distance estimates give a value of 2.54×10⁶ ± 0.11×10⁶ ly (1.606×10¹¹ ± 7.0×10⁹ AU).^[d]

Mass estimates[edit]

Until 2018, mass estimates for the Andromeda Galaxy’s halo (including dark matter) gave a value of approximately 1.5×10¹² M_☉,^[48] compared to 8×10¹¹ M_☉ for the Milky Way. This contradicted earlier measurements that seemed to indicate that the Andromeda Galaxy and Milky Way are almost equal in mass.

In 2018, the equality of mass was re-established by radio results as approximately 8×10¹¹ M_☉.^{[49][50][51][52]} In 2006, the Andromeda Galaxy’s spheroid was determined to have a higher stellar density than that of the Milky Way,^[53] and its galactic stellar disk was estimated at about twice the diameter of that of the Milky Way.^[9] The total mass of the Andromeda Galaxy is estimated to be between 8×10¹¹ M_☉^[49] and 1.1×10¹² M_☉.^[54][55] The stellar mass of M31 is 10–15×10¹⁰ M_☉, with 30% of that mass in the central bulge, 56% in the disk, and the remaining 14% in the stellar halo.^[56] 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 a number of research groups worldwide.

As of 2019, current calculations based on escape velocity and dynamical mass measurements put the Andromeda Galaxy at 0.8×10¹² M_☉,^[57] which is only half of the Milky Way’s newer mass, calculated in 2019 at 1.5×10¹² M_☉.^[58][59][60]

In addition to stars, the Andromeda Galaxy’s interstellar medium contains at least 7.2×10⁹ M_☉^[61] in the form of neutral hydrogen, at least 3.4×10⁸ M_☉ as molecular hydrogen (within its innermost 10 kiloparsecs), and 5.4×10⁷ M_☉ of dust.^[62]

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 supernovae, and its properties are those expected for a galaxy that lies in the “green valley” of the Galaxy color–magnitude diagram (see below). Supernovae erupt in 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.^{[63][64][65][66]}

Luminosity estimates[edit]

Compared to the Milky Way, the Andromeda Galaxy appears to have predominantly older stars with ages >7×10⁹ years.^{[56][clarification needed]} The estimated luminosity of the Andromeda Galaxy, ~2.6×10¹⁰ L_☉, is about 25% higher than that of our own galaxy.^[67][11] However, the galaxy has a high inclination as seen from Earth and its interstellar dust absorbs an unknown amount of light, so it is difficult to estimate its actual brightness and other authors have given other values for the luminosity of the Andromeda Galaxy (some authors even propose it is the second-brightest galaxy within a radius of 10 megaparsecs of the Milky Way, after the Sombrero Galaxy,^[68] with an absolute magnitude of around −22.21^[e] or close^[69]).

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,^[a] compared to −20.9 for the Milky Way), and a total luminosity in that wavelength of 3.64×10¹⁰ L_☉.^[70]

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 novae in the Milky Way is also double that of the Andromeda Galaxy.^[71] 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.^[67] 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 in the Galaxy color–magnitude diagram is known as the “green valley”, a region populated by galaxies like the Milky Way in transition from the “blue cloud” (galaxies actively forming new stars) to the “red sequence” (galaxies that lack star formation). Star formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties to 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.^[72]

Structure