Andromeda Galaxy

From Wikipedia, the free encyclopedia - View original article

Andromeda Galaxy
Andromeda Galaxy (with h-alpha).jpg
The Andromeda Galaxy
Observation data (J2000 epoch)
Right ascension00h 42m 44.3s[1]
Declination+41° 16′ 9″[1]
Redshiftz = -0.001001
(minus sign
indicates blueshift)[1]
Helio radial velocity−301 ± 1 km/s[2]
Distance2.54 ± 0.11 Mly
(778 ± 33 kpc)[2][3][4][5][6][a]
Mass~1.5×1012[7] M
size (ly)~220kly diameter[8]
Number of stars1 trillion (1012)[9]
Apparent dimensions (V)190′ × 60′[1]
Apparent magnitude (V)3.44[10][11]
Absolute magnitude (V)−21.5[b][4]
Other designations
M31, NGC 224, UGC 454, PGC 2557, 2C 56 (Core),[1] LEDA 2557
See also: Galaxy, List of galaxies
Jump to: navigation, search
Andromeda Galaxy
Andromeda Galaxy (with h-alpha).jpg
The Andromeda Galaxy
Observation data (J2000 epoch)
Right ascension00h 42m 44.3s[1]
Declination+41° 16′ 9″[1]
Redshiftz = -0.001001
(minus sign
indicates blueshift)[1]
Helio radial velocity−301 ± 1 km/s[2]
Distance2.54 ± 0.11 Mly
(778 ± 33 kpc)[2][3][4][5][6][a]
Mass~1.5×1012[7] M
size (ly)~220kly diameter[8]
Number of stars1 trillion (1012)[9]
Apparent dimensions (V)190′ × 60′[1]
Apparent magnitude (V)3.44[10][11]
Absolute magnitude (V)−21.5[b][4]
Other designations
M31, NGC 224, UGC 454, PGC 2557, 2C 56 (Core),[1] LEDA 2557
See also: Galaxy, List of galaxies

The Andromeda Galaxy /ænˈdrɒmɨdə/ is a spiral galaxy approximately 2.5 million light-years (2.4×1019 km) from Earth[4] in the Andromeda constellation. Also known as Messier 31, M31, or NGC 224, it is often referred to as the Great Andromeda Nebula in older texts. The Andromeda Galaxy is the nearest spiral galaxy to our Milky Way galaxy, but not the nearest galaxy overall. It gets its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda. The Andromeda Galaxy is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 30 other smaller galaxies.

The Andromeda Galaxy is probably the most massive galaxy in the Local Group as well,[7] despite earlier findings that suggested that the Milky Way contains more dark matter and could be the most massive in the grouping.[12] The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars:[9] at least twice the number of stars in the Milky Way galaxy, which is estimated to be 200–400 billion.[13]

The Andromeda Galaxy is estimated to be 1.5×1012 solar masses.,[7] while the mass of the Milky Way is estimated to be 8.5×1011 solar masses. In comparison a 2009 study estimated that the Milky Way and M31 are about equal in mass,[14] while a 2006 study put the mass of the Milky Way at ~80% of the mass of the Andromeda Galaxy. The two galaxies are expected to collide in 3.75 billion years, eventually merging to form a giant elliptical galaxy [15] or perhaps a large disk galaxy.[16]

At 3.4, the apparent magnitude of Andromeda Galaxy is one of the brightest of any Messier objects,[17] making it visible to the naked eye on moonless nights 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 to the naked eye or when viewed using binoculars or a small telescope.

Observation history[edit]

Great Andromeda Nebula by Isaac Roberts, 1899

The Persian astronomer Abd al-Rahman al-Sufi wrote a line about the chained constellation in his Book of Fixed Stars around 964, describing it as a "small cloud".[18][19] Star charts of that period have it labeled as the Little Cloud.[19] The first description of the object based on telescopic observation was given by German astronomer Simon Marius on December 15, 1612.[20] 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 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 guessed that it was no more than 2,000 times the distance of Sirius.[21]

William Huggins in 1864 observed the spectrum of M31 and noted that it differed from a gaseous nebula.[22] The spectra of M31 displayed a 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".[23]

M31 seen with the naked-eye above the Very Large Telescope.[24]

The first photographs of M31 were taken in 1887 by Isaac Roberts from his private observatory in Sussex, England. The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time.[25] However, at the time this object was still 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.[citation needed] The radial velocity of this object with respect to our solar system was measured in 1912 by Vesto Slipher at the Lowell Observatory, using spectroscopy. The result was the largest velocity recorded at that time, at 300 kilometres per second (190 mi/s), moving in the direction of the Sun.[26]

Island universe[edit]

Location of M31 in the Andromeda constellation

In 1917, American astronomer 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 elsewhere in the sky. As a result he was able to come up with a distance estimate of 500,000 light-years (3.2×1010 AU). He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.[27]

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 (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 Doppler shift. In 1922 Ernst Öpik presented a very elegant and simple astrophysical method to estimate the distance of M31. His result put the Andromeda Nebula far outside our Galaxy at a distance of about 450,000 parsec, which is about 1,500,000 ly.[28] Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of M31. These were made using the 2.5-metre (100-in) 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.[29]

Stars in the Andromeda Galaxy's disc[30]

M31 plays an important role in galactic studies, since it is the nearest spiral galaxy (although not the nearest galaxy). In 1943 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 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.)[31] 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.[32]

Radio emission from the Andromeda Galaxy was first detected by Hanbury Brown and Cyril Hazard at Jodrell Bank Observatory using the 218-ft Transit Telescope, and was announced in 1950[33][34] (Earlier observations were made by radio astronomy pioneer Grote Reber in 1940, but were inconclusive, and were later shown to be an order of magnitude too high). The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group.[35] 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 microlensing, which is caused by the deflection of light by a massive object.[36]


The Andromeda Galaxy as seen by NASA's Wide-field Infrared Survey Explorer

The measured distance to the Andromeda Galaxy was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid. 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.[37][38]

Formation and history[edit]

According to a team of astronomers reporting in 2010, M31 was formed out of the collision of two smaller galaxies between 5 and 9 billion years ago.[39]

A paper published in 2012[40] has outlined M31's basic history since its birth. According to it, Andromeda was born roughly 10 billion years ago from the merger of many smaller protogalaxies, leading to a galaxy smaller than the one we see today.

The most important event in M31's past history was the merger mentioned above that took place 8 billion years ago. This violent collision formed most of its (metal-rich) galactic halo and extended disk and during that epoch Andromeda's star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years.

M31 and the Triangulum Galaxy (M33) had a very close passage 2–4 billion years ago. This event produced high levels of star formation across the Andromeda Galaxy's disk – even some globular clusters – and disturbed M33's outer disk.

While there has been activity during the last 2 billion years, this has been much lower than during the past. During this epoch, star formation throughout M31's disk decreased to the point of nearly shutting down, then increased again relatively recently. There have been interactions with satellite galaxies like M32, M110, or others that have already been absorbed by M31. These interactions have formed structures like Andromeda's Giant Stellar Stream. A merger roughly 100 million years ago is believed to be responsible for a counter-rotating disk of gas found in the center of M31 as well as the presence there of a relatively young (100 million years old) stellar population.

Recent distance estimate[edit]

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

In 2003, using the infrared 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 megalight-years (788 ± 18 kpc) was derived.

The Andromeda Galaxy pictured in ultraviolet light by GALEX

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

In 2005 Ignasi Ribas (CSIC, Institute for Space Studies of Catalonia (IEEC)) and 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 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 a distance of 2.52 ± 0.14 Mly (773 ± 43 kpc) and the whole Andromeda Galaxy at about 2.5 Mly (770 kpc).[4] This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

M31 is close enough that the 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 ± 0.11 Mly (779 ± 34 kpc).[a] Based upon the above distance, the diameter of M31 at the widest point is estimated to be 141 ± 3 kly (43,230 ± 920 pc).[d] Applying trigonometry (arctangent), that figures to extending at an apparent 3.18° angle in the sky.

Mass and luminosity estimates[edit]


Mass estimates for the Andromeda Galaxy's halo (including dark matter) give a value of approximately 1.5×1012 M[7] (or 1.5 trillion solar masses) compared to 8×1011 M for the Milky Way. This contradicts earlier measurements, that seem to indicate that Andromeda and the Milky Way are almost equal in mass. Even so, M31's spheroid actually has a higher stellar density than that of the Milky Way[41] and it's galactic stellar disk is about twice the size of that of the Milky Way.[8]


M31 appears to have significantly more common stars than the Milky Way, and the estimated luminosity of M31, ~2.6×1010 L, is about 25% higher than that of our own galaxy.[42] 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 (including to propose it is the second brightest galaxy within a radius of 10 megaparsecs of the Milky Way, after the Sombrero Galaxy[43]) , the most recent estimation (done in 2010 with the help of Spitzer Space Telescope) suggesting 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,[b] compared to −20.9 for the Milky Way), and a total luminosity in that wavelength of 3.64×1010L[44]

The rate of star formation in the Milky Way is much higher, with M31 producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of supernovae in the Milky Way is also double that of M31.[45] This suggests that M31 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.[42] Should this continue, the luminosity in the Milky Way may eventually overtake that of M31.

According to recent studies, like the Milky Way, the Andromeda Galaxy lies in what in the galaxy color–magnitude diagram is known as the green valley, a region populated by galaxies 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, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between Andromeda and the Milky Way.[46]


The Andromeda Galaxy seen in infrared by the Spitzer Space Telescope, one of NASA's four Great Space Observatories
Image of the Andromeda Galaxy taken by Spitzer in infrared, 24 micrometres (Credit:NASA/JPLCaltech/K. Gordon, University of Arizona)
A Swift Tour of Andromeda Galaxy
A Galaxy Evolution Explorer image of the Andromeda Galaxy. The bands of blue-white making up the galaxy's striking rings are neighborhoods that harbor hot, young, massive stars. Dark blue-grey lanes of cooler dust show up starkly against these bright rings, tracing the regions where star formation is currently taking place in dense cloudy cocoons. When observed in visible light, Andromeda’s rings look more like spiral arms. The ultraviolet view shows that these arms more closely resemble the ring-like structure previously observed in infrared wavelengths with NASA’s Spitzer Space Telescope. Astronomers using Spitzer interpreted these rings as evidence that the galaxy was involved in a direct collision with its neighbor, M32, more than 200 million years ago.

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 2MASS survey showed that the bulge of M31 has a box-like appearance, which implies that the galaxy is actually a barred spiral galaxy like the Milky Way, with the Andromeda Galaxy's bar viewed almost directly along its long axis.[47]

In 2005, astronomers used the Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself.[8] This means that the spiral disk of stars in M31 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 (67,000 pc) in diameter. Previously, estimates of the Andromeda Galaxy's size ranged from 70,000 to 120,000 light-years (21,000 to 37,000 pc) 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.[48] 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 mi/s) at a radius of 1,300 light-years (82,000,000 AU), then descends to a minimum at 7,000 light-years (440,000,000 AU) where the rotation velocity may be as low as 50 kilometres per second (31 mi/s). Thereafter the velocity steadily climbs again out to a radius of 33,000 light-years (2.1×109 AU), where it reaches a peak of 250 kilometres per second (160 mi/s). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (120 mi/s) at 80,000 light-years (5.1×109 AU). These velocity measurements imply a concentrated mass of about 6×109 M in the nucleus. The total mass of the galaxy increases linearly out to 45,000 light-years (2.8×109 AU), then more slowly beyond that radius.[49]

The spiral arms of M31 are outlined by a series of 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.[50] Since the Andromeda Galaxy is seen close to edge-on, however, the studies of its spiral structure are difficult. While rectified images of the galaxy seem to show a fairly normal spiral galaxy with the arms wound up in a clockwise direction, exhibiting two continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years (820,000,000 AU) and that can be followed outward from a distance of roughly 1,600 light-years (100,000,000 AU) from the core, other alternative spiral structures have been proposed such as a single spiral arm[51] or a flocculent[52] pattern of long, filamentary, and thick spiral arms.[1][53]

The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellites M32 and M110.[54] This can be seen by the displacement of the neutral hydrogen clouds from the stars.[55]

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 M31 is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 light-years (2.0×109 AU) from the core.[56] This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust.

Later studies with the help of the Spitzer Space Telescope showed how Andromeda'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.[54]

Close examination of the inner region of M31 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 galaxy in 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 M31.[57] 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 encounter.[58]

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.[41] 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 billion years.[59] 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 the Andromeda Galaxy core showing possible double structure. NASA/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]

Chandra X-ray telescope image of the center of M31. A number of X-ray sources, likely X-ray binary stars, within Andromeda's central region appear as yellowish dots. The blue source at the center is at the position of the supermassive black hole.

In 1991 Tod R. Lauer used WFPC, then on board the Hubble Space Telescope, to image M31's inner nucleus. The nucleus consists of two concentrations separated by 1.5 parsecs (4.9 ly). 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 black hole measured at 3–5 × 107 M in 1993,[60] and at 1.1–2.3 × 108 M in 2005.[61] The velocity dispersion of material around it is measured to be ≈ 160 km/s.[62]

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.[63] 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.[64]

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 M31,[65] this is no longer considered a viable explanation, largely because 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.[63]

Discrete sources[edit]

Artist's concept of the Andromeda Galaxy core showing a view across a disk of young, blue stars encircling a supermassive black hole. NASA/ESA photo

Apparently, by late 1968, no X-rays had been detected from the Andromeda Galaxy.[66] A balloon flight on October 20, 1970, set an upper limit for detectable hard X-rays from M31.[67]

Multiple X-ray sources have since been detected in the Andromeda Galaxy, using observations from the ESA's XMM-Newton orbiting observatory. Robin Barnard et al. hypothesized that these are candidate black holes or 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.[68]

There are approximately 460 globular clusters associated with the Andromeda Galaxy.[69] 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.[70] It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the 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 dwarf galaxy that was consumed by M31 in the distant past.[71] The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half.[19] Another massive globular cluster -named 037-B327-, discovered in 2006 as is heavily reddened by the Andromeda Galaxy's interstellar dust, was thought to be more massive than G1 and the largest cluster of the Local Group;[72] however other studies have shown is actually similar in properties to G1.[73]

Unlike the globular clusters of the Milky Way, which show a relatively low age dispersion, Andromeda'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[74]

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.[75]

In the year 2012, a microquasar, a radio burst emanating from a smaller black hole, was detected in the Andromeda Galaxy. The progenitor black hole was located near the galactic center and had about 10 \begin{smallmatrix}M_\odot\end{smallmatrix}. Discovered through a data collected by the ESA's XMM-Newton probe, and subsequently observed by NASA's Swift and Chandra, 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.[76]


Like the Milky Way, the 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 star formation in the core region, which lasted until the relatively recent past.[77]

M110 also appears to be interacting with M31, and astronomers have found in the halo of M31 a stream of metal-rich stars that appear to have been stripped from these satellite galaxies.[78] M110 does contain a dusty lane, which may indicate recent or ongoing star formation.[79]

In 2006 it was discovered that nine of these galaxies lie 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.[80]

Future collision with the Milky Way[edit]

The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second (68 mi/s).[81] It has been measured approaching relative to our sun at around 300 kilometres per second (190 mi/s)[1] as the sun orbits around the center of our galaxy at a speed of approximately 225 kilometres per second (140 mi/s). This makes Andromeda one of the few blueshifted galaxies that we observe. Andromeda's tangential or side-ways velocity with respect to the Milky Way is relatively much smaller than the approaching velocity and therefore it is expected to directly collide with the Milky Way in about 4 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy[82] or perhaps even a large disk galaxy.[16] Such events are frequent among the galaxies in galaxy groups. The fate of the 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 M31.[83]

See also[edit]

The Andromeda Galaxy on a German postage stamp of 1999


  1. ^ a b average(787 ± 18, 770 ± 40, 772 ± 44, 783 ± 25) = ((787 + 770 + 772 + 783) / 4) ± (182 + 402 + 442 + 252)0.5 / 2 = 778 ± 33.
  2. ^ a b Blue absolute magnitude of -20.89 - Color index of 0.63 = -21.52
  3. ^ J00443799+4129236 is at celestial coordinates R.A. 00h 44m 37.99s, Dec. +41° 29′ 23.6″.
  4. ^ distance × tan( diameter_angle = 190′ ) = 141 ± 3 kly diameter.


  1. ^ a b c d e f g h i "Results for Messier 31". NASA/IPAC Extragalactic Database. NASA/IPAC. Retrieved 2006-11-01. 
  2. ^ a b c Karachentsev, I. D.; Kashibadze, O. G. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics 49 (1): 3–18. Bibcode:2006Ap.....49....3K. doi:10.1007/s10511-006-0002-6. 
  3. ^ a b Karachentsev, I. D. et al. (2004). "A Catalog of Neighboring Galaxies". Astronomical Journal 127 (4): 2031–2068. Bibcode:2004AJ....127.2031K. doi:10.1086/382905. 
  4. ^ a b c d Ribas, I. et al. (2005). "First Determination of the Distance and Fundamental Properties of an Eclipsing Binary in the Andromeda Galaxy". Astrophysical Journal Letters 635 (1): L37–L40. arXiv:astro-ph/0511045. Bibcode:2005ApJ...635L..37R. doi:10.1086/499161. 
  5. ^ a b McConnachie, A. W. et al. (2005). "Distances and metallicities for 17 Local Group galaxies". Monthly Notices of the Royal Astronomical Society 356 (4): 979–997. arXiv:astro-ph/0410489. Bibcode:2005MNRAS.356..979M. doi:10.1111/j.1365-2966.2004.08514.x. 
  6. ^ Jensen, J. B. et al. (2003). "Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations". Astrophysical Journal 583 (2): 712–726. arXiv:astro-ph/0210129. Bibcode:2003ApJ...583..712J. doi:10.1086/345430. 
  7. ^ a b c d Jorge Peñarrubia, Yin-Zhe Ma, Matthew G. Walker, Alan McConnachie. "A dynamical model of the local cosmic expansion". Monthly Notices of the Royal Astronomical Society 433 (3): 2204–2022. doi:10.1093/mnras/stu879. 
  8. ^ a b c Chapman, S. C. et al. (2006). "A kinematically selected, metal-poor spheroid in the outskirts of M31". Astrophysical Journal 653 (1): 255. arXiv:astro-ph/0602604. Bibcode:2006ApJ...653..255C. doi:10.1086/508599.  Also see the press release, "Andromeda's Stellar Halo Shows Galaxy's Origin to Be Similar to That of Milky Way" (Press release). Caltech Media Relations. February 27, 2006. Archived from the original on 9 May 2006. Retrieved 2006-05-24. 
  9. ^ a b Young, K. (June 6, 2006). "The Andromeda galaxy hosts a trillion stars". New Scientist. Retrieved 2014-10-06. 
  10. ^ "SIMBAD-M31". SIMBAD Astronomical Database. Retrieved 2009-11-29. 
  11. ^ Armando, G. P. et al. (2007). "The GALEX Ultraviolet Atlas of Nearby Galaxies". Astrophysical Journal 173 (2): 185–255. arXiv:astro-ph/0606440. Bibcode:2007ApJS..173..185G. doi:10.1086/516636. 
  12. ^ Amos, J. (February 5, 2006). "Dark matter comes out of the cold". BBC News. Archived from the original on 21 June 2006. Retrieved 2006-05-24. 
  13. ^ Frommert, H.; Kronberg, C. (August 25, 2005). "The Milky Way Galaxy". SEDS. Archived from the original on 12 May 2007. Retrieved 2007-05-09. 
  14. ^ "Milky Way a Swifter Spinner, More Massive, New Measurements Show". CfA Press Release No. 2009-03. Harvard-Smithsonian Center for Astrophysics. January 5, 2009. Archived from the original on 8 October 2014. Retrieved 2014-10-06. 
  15. ^ "NASA's Hubble Shows Milky Way is Destined for Head-On Collision". NASA. 31 May 2012. Archived from the original on 4 June 2014. Retrieved 12 July 2012. 
  16. ^ a b Junko Ueda et al. "Cold molecular gas in merger remnants. I. Formation of molecular gas disks". The Astrophysical Journal Supplement Series 214 (1). doi:10.1088/0067-0049/214/1/1. 
  17. ^ Frommert, H.; Kronberg, C. (August 22, 2007). "Messier Object Data, sorted by Apparent Visual Magnitude". SEDS. Archived from the original on 12 July 2007. Retrieved 2007-08-27. 
  18. ^ Henbest, N.; Couper, H. (1994). The guide to the galaxy. p. 31. ISBN 978-0-521-45882-5. 
  19. ^ a b c Kepple, G. R.; Sanner, G. W. (1998). The Night Sky Observer's Guide. Vol. 1. Willmann-Bell. p. 18. ISBN 978-0-943396-58-3. 
  20. ^ Davidson, Norman (1985). Astronomy and the imagination: a new approach to man's experience of the stars. Routledge Kegan & Paul. p. 203. ISBN 978-0-7102-0371-7. 
  21. ^ Herschel, W. (1785). "On the Construction of the Heavens". Philosophical Transactions of the Royal Society of London 75 (0): 213–266. doi:10.1098/rstl.1785.0012. 
  22. ^ Huggins, W.; Miller, W. A. (1864). "On the Spectra of Some of the Nebulae". Philosophical Transactions of the Royal Society of London 154 (0): 437–444. Bibcode:1864RSPT..154..437H. doi:10.1098/rstl.1864.0013. 
  23. ^ Backhouse, T. W. (1888). "nebula in Andromeda and Nova, 1885". Monthly Notices of the Royal Astronomical Society 48: 108. Bibcode:1888MNRAS..48..108B. doi:10.1093/mnras/48.3.108. 
  24. ^ "Two naked-eye galaxies above the VLT". ESO Picture of the Week. Retrieved 22 October 2013. 
  25. ^ Roberts, I. (1899). A Selection of Photographs of Stars, Star-clusters and Nebulae II. London: Universal Press. 
  26. ^ Slipher, V. M. (1913). "The Radial Velocity of the Andromeda Nebula". Lowell Observatory Bulletin 1: 56–57. Bibcode:1913LowOB...1b..56S. 
  27. ^ Curtis, H. D. (1988). "Novae in Spiral Nebulae and the Island Universe Theory". Publications of the Astronomical Society of the Pacific 100: 6. Bibcode:1988PASP..100....6C. doi:10.1086/132128. 
  28. ^ Öpik, E. (1922). "An estimate of the distance of the Andromeda Nebula". Astrophysical Journal 55: 406–410. Bibcode:1922ApJ....55..406O. doi:10.1086/142680. 
  29. ^ Hubble, E. P. (1929). "A spiral nebula as a stellar system, Messier 31". Astrophysical Journal 69: 103–158. Bibcode:1929ApJ....69..103H. doi:10.1086/143167. 
  30. ^ "Four Unusual Views of the Andromeda Galaxy". ESA/Hubble. 21 July 2011. Retrieved 21 July 2011. 
  31. ^ Baade, W. (1944). "The Resolution of Messier 32, NGC 205, and the Central Region of the Andromeda Nebula". Astrophysical Journal 100: 137. Bibcode:1944ApJ...100..137B. doi:10.1086/144650. 
  32. ^ Gribbin, J. R. (2001). The Birth of Time: How Astronomers Measure the Age of the Universe. Yale University Press. p. 151. ISBN 978-0-300-08914-1. 
  33. ^ Brown, R. Hanbury; Hazard, C. (1950). "Radio-frequency Radiation from the Great Nebula in Andromeda (M.31)". Nature 166 (4230): 901. Bibcode:1950Natur.166..901B. doi:10.1038/166901a0. 
  34. ^ Brown, R. Hanbury; Hazard, C. (1951). "Radio emission from the Andromeda nebula". MNRAS 111: 357. Bibcode:1951MNRAS.111..357B. doi:10.1093/mnras/111.4.357. 
  35. ^ van der Kruit, P. C.; Allen, R. J. (1976). "The Radio Continuum Morphology of Spiral Galaxies". Annual Review of Astronomy and Astrophysics 14 (1): 417–445. Bibcode:1976ARA&A..14..417V. doi:10.1146/annurev.aa.14.090176.002221. 
  36. ^ Ingrosso, G. et al. (2009). "Pixel-lensing as a way to detect extrasolar planets in M31". Monthly Notices of the Royal Astronomical Society 399 (1): 219–228. arXiv:0906.1050. Bibcode:2009MNRAS.399..219I. doi:10.1111/j.1365-2966.2009.15184.x. 
  37. ^ Holland, S. (1998). "The Distance to the M31 Globular Cluster System". Astronomical Journal 115 (5): 1916–1920. arXiv:astro-ph/9802088. doi:10.1086/300348. 
  38. ^ Stanek, K. Z.; Garnavich, P. M (1998). "Distance to M31 With the HST and Hipparcos Red Clump Stars". Astrophysical Journal Letters 503 (2): 131–141. arXiv:astro-ph/9802121. Bibcode:1998ApJ...503L.131S. doi:10.1086/311539. 
  39. ^ Moskvitch, Katia (25 November 2010). "Andromeda 'born in a collision'". BBC News. Archived from the original on 26 November 2010. Retrieved 25 November 2010. 
  40. ^ Davidge, T. J.; McConnachie, A. W.; Fardal, M. A.; Fliri, J.; Valls-Gabaud, D.; Chapman, S. C.; Lewis, G. F.; Rich, R. M. (2012). "The Recent Stellar Archeology of M31 – The Nearest Red Disk Galaxy". The Astrophysical Journal 751 (1): 74. arXiv:1203.6081. Bibcode:2012ApJ...751...74D. doi:10.1088/0004-637X/751/1/74. 
  41. ^ a b Kalirai, J. S. et al. (2006). "The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)". Astrophysical Journal 648 (1): 389–404. arXiv:astro-ph/0605170. doi:10.1086/505697. 
  42. ^ a b van den Bergh, S. (1999). "The local group of galaxies". Astronomy and Astrophysics Review 9 (3–4): 273–318. Bibcode:1999A&ARv...9..273V. doi:10.1007/s001590050019. 
  43. ^ Karachentsev, Igor D.; Karachentseva, Valentina E.; Huchtmeier, Walter K.; Makarov, Dmitry I. (2003). "A Catalog of Neighboring Galaxies". The Astronomical Journal 127 (4): 2031–2068. Bibcode:2004AJ....127.2031K. doi:10.1086/382905. 
  44. ^ Tempel, E.; Tamm, A.; Tenjes, P. (2010). "Dust-corrected surface photometry of M 31 from Spitzer far-infrared observations". Astronomy and Astrophysics 509: A91. arXiv:0912.0124. Bibcode:2010A&A...509A..91T. doi:10.1051/0004-6361/200912186. wA91. 
  45. ^ Liller, W.; Mayer, B. (1987). "The Rate of Nova Production in the Galaxy". Publications of the Astronomical Society of the Pacific 99: 606–609. Bibcode:1987PASP...99..606L. doi:10.1086/132021. 
  46. ^ Mutch, S.J.; Croton, D.J.; Poole, G.B. (2011). "The Mid-life Crisis of the Milky Way and M31". The Astrophysical Journal 736 (2): 84. arXiv:1105.2564. Bibcode:2011ApJ...736...84M. doi:10.1088/0004-637X/736/2/84. 
  47. ^ Beaton, R. L. et al. (2006). "Unveiling the Boxy Bulge and Bar of the Andromeda Spiral Galaxy". Astrophysical Journal Letters 658 (2): L91. arXiv:astro-ph/0605239. doi:10.1086/514333. 
  48. ^ "Astronomers Find Evidence of an Extreme Warp in the Stellar Disk of the Andromeda Galaxy" (Press release). UC Santa Cruz. January 9, 2001. Archived from the original on 19 May 2006. Retrieved 2006-05-24. 
  49. ^ Rubin, V. C.; Ford, W. K. J. (1970). "Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission". Astrophysical Journal 159: 379. Bibcode:1970ApJ...159..379R. doi:10.1086/150317. 
  50. ^ Arp, H. (1964). "Andromeda Nebula from a Spectroscopic Survey of Emission". Astrophysical Journal 139: 1045. Bibcode:1964ApJ...139.1045A. doi:10.1086/147844. 
  51. ^ Simien, F.; Pellet, A.; Monnet, G.; Athanassoula, E.; Maucherat, A.; Courtes, G. (1978). "The spiral structure of M31 - A morphological approach". Astronomy and Astrophysics 67: 73–79. Bibcode:1978A&A....67...73S. 
  52. ^ Haas, M. (2000). "Cold dust in M31 as mapped by ISO". The interstellar medium in M31 and M33. Proceedings 232. WE-Heraeus Seminar: 69–72. Bibcode:2000immm.proc...69H. 
  53. ^ Walterbos, R. A. M.; Kennicutt, R. C., Jr. (1988). "An optical study of stars and dust in the Andromeda galaxy". Astronomy and Astrophysics 198: 61–86. Bibcode:1988A&A...198...61W. 
  54. ^ a b Gordon, K. D.; Bailin, J.; Engelbracht, C. W.; Rieke, G. H.; Misselt, K. A.; Latter, W. B.; Young, E. T.; Ashby, M. L. N.; Barmby, P.; Gibson, B. K.; Hines, D. C.; Hinz, J.; Krause, O.; Levine, D. A.; Marleau, F. R.; Noriega-Crespo, A.; Stolovy, S.; Thilker, D. A.; Werner, M. W. (2006). "Spitzer MIPS Infrared Imaging of M31: Further Evidence for a Spiral-Ring Composite Structure". The Astrophysical Journal 638 (2): L87–L92. arXiv:astro-ph/0601314. Bibcode:2006ApJ...638L..87G. doi:10.1086/501046. 
  55. ^ Braun, R. (1991). "The distribution and kinematics of neutral gas, HI region in M31". Astrophysical Journal 372: 54–66. Bibcode:1991ApJ...372...54B. doi:10.1086/169954. 
  56. ^ "ISO unveils the hidden rings of Andromeda" (Press release). European Space Agency. October 14, 1998. Retrieved 2006-05-24. 
  57. ^ "Busted! Astronomers Nab Culprit in Galactic Hit-and-Run". Harvard-Smithsonian Center for Astrophysics. October 18, 2006. Archived from the original on 8 October 2014. Retrieved 2014-10-06. 
  58. ^ Block, D. L.; Bournaud, F.; Combes, F.; Groess, R.; Barmby, P.; Ashby, M. L. N.; Fazio, G. G.; Pahre, M. A.; Willner, S. P. (2006). "An almost head-on collision as the origin of the two off-centre rings in the Andromeda galaxy". Nature 443 (1): 832–834. arXiv:astro-ph/0610543. Bibcode:2006Natur.443..832B. doi:10.1038/nature05184. 
  59. ^ Bullock, J. S.; Johnston, K.V. (2005). "Tracing Galaxy Formation with Stellar Halos I: Methods". Astrophysical Journal 635 (2): 931–949. arXiv:astro-ph/0506467. Bibcode:2005ApJ...635..931B. doi:10.1086/497422. 
  60. ^ Lauer, T. R. et al. (1993). "Planetary camera observations of the double nucleus of M31". Astronomical Journal 106 (4): 1436–1447, 1710–1712. Bibcode:1993AJ....106.1436L. doi:10.1086/116737. 
  61. ^ Bender, Ralf et al. (2005). "HST STIS Spectroscopy of the Triple Nucleus of M31: Two Nested Disks in Keplerian Rotation around a Supermassive Black Hole". Astrophysical Journal 631 (1): 280–300. arXiv:astro-ph/0509839. Bibcode:2005ApJ...631..280B. doi:10.1086/432434. 
  62. ^ Gebhardt, Karl; Bender, Ralf; Bower, Gary; Dressler, Alan; Faber, S. M.; Filippenko, Alexei V.; Green, Richard; Grillmair, Carl; Ho, Luis C.; Kormendy, John; Lauer, Tod R.; Magorrian, John; Pinkney, Jason; Richstone, Douglas; Tremaine, Scott (June 2000). "A Relationship between Nuclear Black Hole Mass and Galaxy Velocity Dispersion" (PDF). The Astrophysical Journal (Chicago, Illinois, USA: The University of Chicago Press) 539 (1): L13–L16. arXiv:astro-ph/0006289. Bibcode:2000ApJ...539L..13G. doi:10.1086/312840. Retrieved March 10, 2010. 
  63. ^ a b Tremaine, S. (1995). "An Eccentric-Disk Model for the Nucleus of M31". Astronomical Journal 110: 628–633. arXiv:astro-ph/9502065. Bibcode:1995AJ....110..628T. doi:10.1086/117548. 
  64. ^ "Hubble Space Telescope Finds a Double Nucleus in the Andromeda Galaxy" (Press release). Hubble News Desk. July 20, 1993. Retrieved 2006-05-26. 
  65. ^ Schewe, Phillip F.; Stein, Ben (July 26, 1993). "The Andromeda Galaxy has a Double Nucleus". Physics News Update (American Institute of Physics). Retrieved 2009-07-10. 
  66. ^ Fujimoto, M.; Hayakawa, S.; Kato, T. (1969). "Correlation between the Densities of X-Ray Sources and Interstellar Gas". Astrophysics and Space Science 4 (1): 64–83. Bibcode:1969Ap&SS...4...64F. doi:10.1007/BF00651263. 
  67. ^ Peterson, L. E. (1973). "Hard Cosmic X-Ray Sources". In Bradt, H.; Giacconi, R. X- and Gamma-Ray Astronomy, Proceedings of IAU Symposium no. 55 held in Madrid, Spain, 11–13 May 1972. International Astronomical Union. pp. 51–73. Bibcode:1973IAUS...55...51P. 
  68. ^ Barnard, R.; Kolb, U.; Osborne, J. P. (2005). "Timing the bright X-ray population of the core of M31 with XMM-Newton". arXiv:astro-ph/0508284 [astro-ph].
  69. ^ Barmby, P.; Huchra, J. P. (2001). "M31 Globular Clusters in the Hubble Space Telescope Archive. I. Cluster Detection and Completeness". Astronomical Journal 122 (5): 2458–2468. arXiv:astro-ph/0107401. Bibcode:2001AJ....122.2458B. doi:10.1086/323457. 
  70. ^ "Hubble Spies Globular Cluster in Neighboring Galaxy" (Press release). Hubble news desk STSci-1996-11. April 24, 1996. Archived from the original on 1 July 2006. Retrieved 2006-05-26. 
  71. ^ Meylan, G. et al. (2001). "G1 in M31 – Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy?". Astronomical Journal 122 (2): 830–841. arXiv:astro-ph/0105013. Bibcode:2001AJ....122..830M. doi:10.1086/321166. 
  72. ^ Ma, J.; De Grijs, R.; Yang, Y.; Zhou, X.; Chen, J.; Jiang, Z.; Wu, Z.; Wu, J. (2006). "A 'super' star cluster grown old: the most massive star cluster in the Local Group". Monthly Notices of the Royal Astronomical Society 368 (3): 1443. arXiv:astro-ph/0602608. Bibcode:2006MNRAS.368.1443M. doi:10.1111/j.1365-2966.2006.10231.x. 
  73. ^ Cohen, Judith G. (2006). "The Not So Extraordinary Globular Cluster 037-B327 in M31". The Astrophysical Journal 653: L21–L23. arXiv:astro-ph/0610863. Bibcode:2006ApJ...653L..21C. doi:10.1086/510384. 
  74. ^ Burstein, David; Li, Yong; Freeman, Kenneth C.; Norris, John E.; Bessell, Michael S.; Bland-Hawthorn, Joss; Gibson, Brad K.; Beasley, Michael A.; Lee, Hyun-chul; Barbuy, Beatriz; Huchra, John P.; Brodie, Jean P.; Forbes, Duncan A. (2004). "Globular Cluster and Galaxy Formation: M31, the Milky Way, and Implications for Globular Cluster Systems of Spiral Galaxies". Astrophysical Journal 614: 158–166. arXiv:astro-ph/0406564. Bibcode:2004ApJ...614..158B. doi:10.1086/423334. 
  75. ^ Huxor, A. P. et al. (2005). "A new population of extended, luminous, star clusters in the halo of M31". Monthly Notices of the Royal Astronomical Society 360 (3): 993–1006. arXiv:astro-ph/0412223. Bibcode:2005MNRAS.360.1007H. doi:10.1111/j.1365-2966.2005.09086.x. 
  76. ^ Prostak, Sergio (2012-12-14). "Microquasar in Andromeda Galaxy Amazes Astronomers". 
  77. ^ Bekki, K. et al. (2001). "A New Formation Model for M32: A Threshed Early-type Spiral?". Astrophysical Journal Letters 557 (1): L39–L42. arXiv:astro-ph/0107117. Bibcode:2001ApJ...557L..39B. doi:10.1086/323075. 
  78. ^ Ibata, R. et al. (2001). "A giant stream of metal-rich stars in the halo of the galaxy M31". Nature 412 (6842): 49–52. doi:10.1038/35083506. PMID 11452300. 
  79. ^ Young, L. M. (2000). "Properties of the Molecular Clouds in NGC 205". Astronomical Journal 120 (5): 2460–2470. arXiv:astro-ph/0007169. Bibcode:2000AJ....120.2460Y. doi:10.1086/316806. 
  80. ^ Koch, A.; Grebel, E. K. (March 2006). "The Anisotropic Distribution of M31 Satellite Galaxies: A Polar Great Plane of Early-type Companions". Astronomical Journal 131 (3): 1405–1415. arXiv:astro-ph/0509258. doi:10.1086/499534.  edit
  81. ^ Cowen, Ron (2012). "Andromeda on collision course with the Milky Way". Nature International Weekly Journal of Science. Nature (journal). Retrieved 2014-10-06. 
  82. ^ Cox, T. J.; Loeb, A. (2008). "The collision between the Milky Way and Andromeda". Monthly Notices of the Royal Astronomical Society 386 (1): 461–474. Bibcode:2008MNRAS.tmp..333C. doi:10.1111/j.1365-2966.2008.13048.x. 
  83. ^ Cain, F. (2007). "When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?". Universe Today. Archived from the original on 17 May 2007. Retrieved 2007-05-16. 

External links[edit]

Coordinates: Sky map 00h 42m 44.3s, +41° 16′ 10″