The Andromeda Galaxy (also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Nebula in older texts) is a spiral galaxy approximately 2.5 million light-years away[4] in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way. As it is visible as a faint smudge on a moonless night, it is one of the farthest objects visible to the naked eye, and can be seen even from urban areas with binoculars. It is named after the princess Andromeda (Greek: Ανδρομέδη - Andromédē) in Greek mythology. Andromeda is the largest galaxy of the w:Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, it may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping.[7] The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars, greatly exceeding the number of stars in our own galaxy.[8] While the 2006 estimates put the mass of the Milky Way to be ~80% of the mass of Andromeda, which is estimated to be 7.1×1011 [solar masses,[2] a 2009 study concluded that Andromeda and the Milky Way are about equal in mass.[9]

Andromeda Galaxy
A visible light image of the Andromeda Galaxy.
Observation data (J2000 epoch)
Right ascension00h 42m 44.3s[1]
Declination+41° 16′ 9″[1]
Redshift−301 ± 1 km/s[2]
Distance2.54 ± 0.06 Mly
(778 ± 17 kpc)[3][2][4][5][6]a
Apparent dimensions (V)190′ × 60′[1]
Apparent magnitude (V)4.4[1]
Other designations
M31, NGC 224, UGC 454, PGC 2557, 2C 56 (Core)[1], LEDA 2557

At an apparent magnitude of 4.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,[10] making it easily visible to the naked eye even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full moon when photographed through a larger telescope, only the brighter central region is visible with the naked eye.

Observation history

The earliest recorded observation of the Andromeda Galaxy was in 964 CE by the Persian astronomer, Abd al-Rahman al-Sufi (Azophi),[11] who described it as a "small cloud" in his Book of Fixed Stars. Other star charts of that period have it labeled as the Little Cloud.[11] The first description of the object based on telescopic observation was given by Simon Marius[11] in 1612. Charles Messier catalogued it as object M31 in 1764 and incorrectly credited Marius as the discoverer, unaware of Al Sufi's earlier work. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of the M31. He believed it to be the nearest of all the "great nebulae" and, based on the color and magnitude of the nebula, he incorrectly estimated that it was no more than 2,000 times the distance of w:Sirius.[12]

w:William Huggins in 1864 observed the w:spectrum of M31 and noted that it differed from a gaseous nebula.[13] The spectra of M31 displayed a w:continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. The Andromeda nebula was very similar to the spectra of individual stars, and from this it was deduced that M31 had a stellar nature. In 1885, a supernova (known as "S Andromedae") was seen in M31, the first and so far only one observed in that galaxy. At the time M31 was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly "Nova 1885".[14]

Great Andromeda Nebula by w:Isaac Roberts.

The first photographs of M31 were taken in 1887 by w:Isaac Roberts from his private observatory in w:Sussex, England. The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time.[15] However, at the time this object was commonly believed to be a nebula within our galaxy, and Roberts mistakenly believed that M31 and similar spiral nebulae were actually solar systems being formed, with the satellites nascent planets. The w:radial velocity of this object with respect to our w:solar system was measured in 1912 by w:Vesto Slipher at the w:Lowell Observatory, using w:spectroscopy. The result was the largest velocity recorded at that time, at 300 kilometres per second (186 miles/s.), moving in the direction of the Sun.[16]

M31 in proximity to Pegasus constellation

Island universe

In 1917, w:Heber Curtis observed a nova within M31. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our Galaxy. As a result he was able to come up with a distance estimate of 500,000 light-years. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.[17]

In 1920 the w:Great Debate between w:Harlow Shapley and w:Heber Curtis took place, concerning the nature of the w:Milky Way, spiral nebulae, and the dimensions of the w:universe. To support his claim that Great Andromeda Nebula (M31) was an external galaxy, Curtis also noted the appearance of dark lanes resembling the dust clouds in our own Galaxy, as well as the significant w:Doppler shift. In 1922 w:Ernst Öpik presented a very elegant and simple astrophysical method to estimate the distance of M31, his result (450 kpc) put Andromeda Nebula far outside our Galaxy.[18] w:Edwin Hubble settled the debate in 1925 when he identified extragalactic w:Cepheid variable stars for the first time on astronomical photos of M31. These were made using the 2.5 metre (100 in) w: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 Galaxy, but an entirely separate galaxy located a significant distance from our own.[19]

This galaxy plays an important role in galactic studies, since it is the nearest giant spiral (although not the nearest galaxy). In 1943, w:Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Based on his observations of this galaxy, he was able to discern two distinct populations of stars based on their w: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 w:Jan Oort.)[20] Dr. Baade also discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to M31, as well as the remainder of the Universe.[21]

Radio emission from the Andromeda Galaxy was first detected by w:Grote Reber in 1940. The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group.[22] The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomy catalogue. In 2009, the first planet may have been discovered in the Andromeda Galaxy. This candidate was detected using a technique called w:microlensing, which is caused by the deflection of light by a massive object.[23]


The Andromeda Galaxy is approaching the w:Sun at about 300 kilometers per second (186 miles/s.), so it is one of the few w:blue shifted galaxies. The Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometers per second (62–87 miles/s.; 223,200–313,200mph).[24] The collision is predicted to occur in about 2.5 billion years. In that case the two galaxies will likely merge to form a giant w:elliptical galaxy.[25] However, Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two, which creates uncertainty about the details of the collision.[26] Such events are frequent among the galaxies in w:galaxy groups. The fate of the w:Earth and the w:Solar System in the event of a collision are presently unknown, but there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.[27]

The measured distance to the Andromeda Galaxy was doubled in 1953 when it was discovered that there is another, dimmer type of w:Cepheid. In the 1990s, measurements of both standard w:red giants as well as w:red clump stars from the w:Hipparcos satellite measurements were used to calibrate the Cepheid distances.[28][29]

The Andromeda Galaxy pictured in w:ultraviolet light by w:GALEX

Recent distance estimates

At least four distinct techniques have been used to measure distances to the Andromeda Galaxy.

In 2003, using the infrared w:surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value of Freedman et al. 2001 and using a metallicity correction of -0.2 mag dex−1 in (O/H), an estimate of 2.57 ± 0.06 Mly (787 ± 18 kpc) was derived.

Using the Cepheid variable method, an estimate of 2.51 ± 0.13 Mly (770 ± 40 kpc) was achieved in 2004.[3][2]

In 2005, a group of astronomers consisting of w:Ignasi Ribas (CSIC, IEEC) and his colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. The binary star, designated M31VJ00443799+4129236,c has two luminous and hot blue stars of types O and B. By studying the eclipses of the stars, which occur every 3.54969 days, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars they were able to measure the w:absolute magnitude of the stars. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 2.52 ± 0.14 Mly (770 ± 40 kpc) and the whole Andromeda Galaxy at about 2.5 Mly.[4] This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

Andromeda is close enough that the w:Tip of the Red Giant Branch (TRGB) method may also be used to estimate its distance. The estimated distance to M31 using this technique in 2005 yielded 2.56 ± 0.08 Mly (785 ± 25 kpc).[5]

Averaged together, all these distance measurements give a combined distance estimate of 2.54 ± 0.06 Mly (778 ± 17 kpc).a Based upon the above distance, the diameter of M31 at the widest point is estimated to be 141 ± 3 kly.d

Mass and luminosity estimates

Mass estimates for the Andromeda halo (including w:dark matter) give a value of approximately 1.23×1012 M[30] (or 1.2 million million w:solar masses) compared to 1.9×1012 M for the Milky Way. Thus M31 may be less massive than our own galaxy, although the error range is still too large to say for certain. Even so, the masses of the Milky Way and M31 are comparable, and M31's spheroid actually has a higher stellar density than that of the Milky Way.[31]

In particular, M31 appears to have significantly more common stars than the Milky Way, and the estimated w:luminosity of M31, ~2.6×1010 L, is about 25% higher than that of our own galaxy.[32] However the rate of star formation in the Milky Way is much higher, with M31 only producing about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of w:supernovae in the Milky Way is also double that of M31.[33] This suggests that M31 has experienced a great star formation phase in its past, but is now relatively w:quiescent, whereas the Milky Way is experiencing more active star formation.[32] Should this continue, the luminosity in the Milky Way may overtake that of M31 in the future.


Based on its appearance in visible light, the Andromeda galaxy is classified as an SA(s)b galaxy in the de Vaucouleurs-Sandage extended classification system of spiral galaxies.[1] However, data from the w:2MASS survey showed that the bulge of M31 has a box-like appearance, which implies that the galaxy is actually a barred galaxy with the bar viewed almost directly along its long axis.[34]

The Andromeda Galaxy seen in w:infrared by the w:Spitzer Space Telescope, one of w:NASA's four Great Space Observatories

In 2005, astronomers used the w:Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself.[35] This means that the spiral disk of stars in Andromeda is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years in diameter. Previously, estimates of Andromeda's size ranged from 70,000 to 120,000 light-years across.

The galaxy is inclined an estimated 77° relative to the Earth (where an angle of 90° would be viewed directly from the side). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk.[36] A possible cause of such a warp could be gravitational interaction with the satellite galaxies near M31. The galaxy M33 could be responsible for some warp in M31's arms, though more precise distances and radial velocities are required.

Spectroscopic studies have provided detailed measurements of the rotational velocity of M31 at various radii from the core. In the vicinity of the core, the rotational velocity climbs to a peak of 225 kilometres per second (140 miles/s.) at a radius of 1,300 w:light-years, then descends to a minimum at 7,000 light-years where the rotation velocity may be as low as 50 kilometres per second (31 miles/s.). Thereafter the velocity steadily climbs again out to a radius of 33,000 light-years, where it reaches a peak of 250 kilometres per second (155 miles/s.). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (124 miles/s.) at 80,000 light-years. These velocity measurements imply a concentrated mass of about 6×109 M in the nucleus. The total mass of the galaxy increases w:linearly out to 45,000 light-years, then more slowly beyond that radius.[37]

The w:spiral arms of Andromeda are outlined by a series of w:H II regions that Baade described as resembling "beads on a string". They appear to be tightly wound, although they are more widely spaced than in our galaxy.[38] Rectified images of the galaxy show a fairly normal spiral galaxy with the arms wound up in a clockwise direction. There are two continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years. These can be followed outward from a distance of roughly 1,600 light-years from the core. The most likely cause of the spiral pattern is thought to be interaction with M32. This can be seen by the displacement of the neutral hydrogen clouds from the stars.[39]

Image of Andromeda Galaxy (M31) taken by Spitzer in infrared, 24 w:micrometres (Credit:w:NASA/w:JPL-w:Caltech/K. Gordon (w:University of Arizona)

In 1998, images from the w:European Space Agency's w:Infrared Space Observatory demonstrated that the overall form of the Andromeda galaxy may be transitioning into a w:ring galaxy. The gas and dust within Andromeda is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 light-years from the core.[40] This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust.

Close examination of the inner region of Andromeda 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 Andromeda 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.[41]

Studies of the extended halo of M31 show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally "metal-poor", and increasingly so with greater distance.[42] This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 1–200 low-mass galaxies during the past 12 billion years.[43] The stars in the extended halos of M31 and the Milky Way may extend nearly one-third the distance separating the two galaxies.


HST image of Andromeda galaxy core showing possible double structure. w:NASA/w:ESA photo.
Artist's concept of Andromeda galaxy core showing a view across a mysterious disk of young, blue stars encircling a supermassive black hole. w:NASA/w:ESA photo.

M31 is known to harbor a dense and compact star cluster at its very center. In a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. The luminosity of the nucleus is in excess of the most luminous globular clusters.[citation needed]

In 1991 w:Tod R. Lauer used WFPC, then on board the w:Hubble Space Telescope, to image Andromeda's inner nucleus. The nucleus consists of two concentrations separated by 1.5 w:parsecs. 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 a 108 M w:black hole.[44]

w:Scott Tremaine has proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an eccentric orbit around the central black hole.[45] The eccentricity is such that stars linger at the orbital apocenter, creating a concentration of stars. P2 also contains a compact disk of hot, spectral class A stars. The A stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.[46]

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus was the remnant of a small galaxy "cannibalized" by Andromeda,[47] this is no longer considered to be a viable explanation. The primary reason is that such a nucleus would have an exceedingly short lifetime due to 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.[45]

Discrete sources

Multiple X-ray sources have been detected in the Andromeda Galaxy, using observations from the ESA's w:XMM-Newton orbiting observatory. w:Robin Barnard et al. hypothesized that these are candidate black holes or w:neutron stars, which are heating incoming gas to millions of kelvins and emitting X-rays. The spectrum of the neutron stars is the same as the hypothesized black holes, but can be distinguished by their masses.[48]

There are approximately 460 w:globular clusters associated with the Andromeda galaxy.[49] The most massive of these clusters, identified as w:Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the w:local group of galaxies.[50] It contains several million stars, and is about twice as luminous as w:Omega Centauri, the brightest known globular cluster in the w:Milky Way. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a w:dwarf galaxy that was consumed by M31 in the distant past.[51] The globular with the greatest apparent brightness is w:G76 which is located in the south-west arm's eastern half.[11]

In 2005, astronomers discovered a completely new type of star cluster in M31. 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.[52]


Like the Milky Way, Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The best known and most readily observed satellite galaxies are M32 and M110. Based on current evidence, it appears that M32 underwent a close encounter with M31 (Andromeda) 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 w:star formation in the core region, which lasted until the relatively recent past.[53]

M110 also appears to be interacting with M31, and astronomers have found a stream of metal-rich stars in the halo of M31 that appears to have been stripped from these satellite galaxies.[54] M110 does contain a dusty lane, which may indicate recent or ongoing star formation.[55]

In 2006 it was discovered that nine of these galaxies lay along a plane that intersects the core of the Andromeda Galaxy, rather than being randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.[56]


^a average(787 ± 18, 770 ± 40, 772 ± 44, 783 ± 25) = ((787 + 770 + 772 + 783) / 4) ± ((182 + 402 + 442 + 252)0.5 / 4) = 778 ± 17
^b Apparent Magnitude of 4.36 - w:distance modulus of 24.4 = −20.0
^c J00443799+4129236 is at celestial coordinates R.A. 00h 44m 37.99s, Dec. +41° 29′ 23.6″.
^d distance × tan( diameter_angle = 190′ ) = 141 ± 3 kly diameter


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