Difference between revisions of "Andromeda Galaxy"

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{{Infobox galaxy
{{Infobox galaxy
| name = Andromeda Galaxy
| name = Andromeda Galaxy
| image = [[File:Andromeda Galaxy (with h-alpha).jpg|250px]]
| image = [[File:Andromeda Galaxy (with h-alpha).jpg|280px]]
| caption = The Andromeda Galaxy
| caption = The Andromeda Galaxy
| epoch = [[J2000]]
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| pronounce = {{IPAc-en|æ|n|ˈ|d|r|ɒ|m|ɨ|d|ə}}
| pronounce = {{IPAc-en|æ|n|ˈ|d|r|ɒ|m|ɨ|d|ə}}
| type = SA(s)b<ref name="ned" />
| type = SA(s)b<ref name="ned" />
| mass = ~1{{e|12}}<ref name="Karachentsevetal2006" /><ref name="Evans" />
| mass = ~1.5{{e|12}}<ref name="Jorge Peñarrubia2014"/>
| size = ~220 kly (diameter)<ref name="Chapman et al 2006"/>
| stars = 1 trillion (10<sup>12</sup>)<!-- exponential value to disambiguate "trillion" --><ref name="trillion-stars" />
| stars = 1 trillion (10<sup>12</sup>)<!-- exponential value to disambiguate "trillion" --><ref name="trillion-stars" />
| ra = {{RA|00|42|44.3}}<ref name="ned" />
| ra = {{RA|00|42|44.3}}<ref name="ned" />
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| constellation name = [[Andromeda (constellation)|Andromeda]]
| constellation name = [[Andromeda (constellation)|Andromeda]]
| absmag_v = −21.5{{efn|name=blue mag}}<ref name="Ribas2005" />
| absmag_v = −21.5{{efn|name=blue mag}}<ref name="Ribas2005" />
| names = [[Messier object|M]]31, [[New General Catalogue|NGC]] 224, [[Uppsala General Catalogue|UGC]] 454, [[Principal Galaxies Catalogue|PGC]] 2557, [[Second Cambridge Catalogue of Radio Sources|2C]] 56 (Core),<ref name="ned" /> LEDA 2557
| names = [[Messier object|M]]31, [[New General Catalogue|NGC]] 224, [[Uppsala General Catalogue|UGC]] 454, [[Principal Galaxies Catalogue|PGC]] 2557, [[Second Cambridge Catalogue of Radio Sources|2C]] 56 (Core),<ref name="ned" /> CGCG 535-17, MCG +07-02-016, IRAS 00400+4059, 2MASX J00424433+4116074, GC 116, h 50, Bode 3, Flamsteed 58, Hevelius 32, Ha 3.3, IRC +40013
}}
}}


The '''Andromeda Galaxy''' {{IPAc-en|æ|n|ˈ|d|r|ɒ|m|ɨ|d|ə}} is a [[spiral galaxy]] approximately 2.5 million [[light-year]]s (2.4{{e|19}}&nbsp;km) from [[Earth]]<ref name="Ribas2005" /> in the [[Andromeda (constellation)|Andromeda constellation]]. Also known as [[Messier object|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 [[List of nearest galaxies|closest galaxy]] overall. It gets its name from the area of the sky in which it appears, the [[Andromeda (constellation)|constellation of Andromeda]], which was named after the mythological princess [[Andromeda (mythology)|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. Although the largest, the Andromeda Galaxy may not be the most massive, as recent findings suggest that the Milky Way contains more [[dark matter]] and could be the most massive in the grouping.<ref name="DarkMatter" /> The 2006 observations by the [[Spitzer Space Telescope]] revealed that M31 contains [[Orders of magnitude (numbers)#1012|one trillion (10<sup>12</sup>)]]<!-- exponential value to disambiguate "trillion --> stars:<ref name="trillion-stars" /> at least twice the number of stars in the Milky Way galaxy, which is estimated to be 200–400 billion.<ref name="Frommert & Kronberg 2005" />
The '''Andromeda Galaxy''' {{IPAc-en|æ|n|ˈ|d|r|ɒ|m|ɨ|d|ə}} is a [[spiral galaxy]] approximately 780 [[Parsec#Parsecs and kiloparsecs|kiloparsecs]] (2.5 million [[light-year]]s; 2.4{{e|19}}&nbsp;km) from [[Earth]]<ref name="Ribas2005" /> in the [[Andromeda (constellation)|Andromeda constellation]]. Also known as [[Messier object|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 [[List of nearest galaxies|nearest galaxy]] overall. It gets its name from the area of the sky in which it appears, the [[Andromeda (constellation)|constellation of Andromeda]], which was named after the mythological princess [[Andromeda (mythology)|Andromeda]]. The Andromeda Galaxy is the largest [[galaxy]] of the [[Local Group]], which also contains the Milky Way, the [[Triangulum Galaxy]], and about 44 other smaller galaxies.


The Andromeda Galaxy is estimated to be 7.1{{e|11}} [[solar mass]]es.<ref name="Karachentsevetal2006" /> In comparison a 2009 study estimated that the Milky Way and M31 are about equal in mass,<ref name="CfA" /> while a 2006 study put the mass of the Milky Way at ~80% of the mass of the Andromeda Galaxy. The two galaxies are [[Andromeda–Milky Way collision|expected to collide]] in 3.75 billion years, eventually merging to form a giant [[elliptical galaxy]].<ref name="milky-way-collide" />
The Andromeda Galaxy is probably the most massive galaxy in the Local Group as well,<ref name="Jorge Peñarrubia2014">{{cite journal
    | author=Jorge Peñarrubia, Yin-Zhe Ma, Matthew G. Walker, Alan McConnachie
    | title=A dynamical model of the local cosmic expansion
    | journal=Monthly Notices of the Royal Astronomical Society
    | volume=433
    | issue=3
    | pages=2204–2022
    | bibcode=
    | doi=10.1093/mnras/stu879}}</ref> despite earlier findings that suggested that the Milky Way contains more [[dark matter]] and could be the most massive in the grouping.<ref name="DarkMatter" /> The 2006 observations by the [[Spitzer Space Telescope]] revealed that M31 contains [[Orders of magnitude (numbers)#1012|one trillion (10<sup>12</sup>)]]<!-- exponential value to disambiguate "trillion --> stars:<ref name="trillion-stars" /> at least twice the number of stars in the Milky Way galaxy, which is estimated to be 200–400 billion.<ref name="Frommert & Kronberg 2005" />


At an [[apparent magnitude]] of 3.4, the Andromeda Galaxy is one of the brightest [[Messier object]]s,<ref name="Frommert & Kronberg 2007" /> 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.
The Andromeda Galaxy is estimated to be 1.5{{e|12}} [[solar mass]]es,<ref name="Jorge Peñarrubia2014"/> while the mass of the Milky Way is estimated to be 8.5{{e|11}} solar masses. In comparison a 2009 study estimated that the Milky Way and M31 are about equal in mass,<ref name="CfA" /> while a 2006 study put the mass of the Milky Way at ~80% of the mass of the Andromeda Galaxy. The two galaxies are [[Andromeda–Milky Way collision|expected to collide]] in 3.75 billion years, eventually merging to form a giant [[elliptical galaxy]] <ref name="milky-way-collide" /> or perhaps a large [[disk galaxy]].<ref name="Ueda2014">{{cite journal
    | author=Junko Ueda et al.
    | title=Cold molecular gas in merger remnants. I. Formation of molecular gas disks
    | journal=The Astrophysical Journal Supplement Series
    | volume=214
    | issue=1
    | pages=
    | bibcode=
    | doi=10.1088/0067-0049/214/1/1}}</ref>
 
At 3.4, the [[apparent magnitude]] of the Andromeda Galaxy is one of the brightest of any [[Messier object]]s,<ref name="Frommert & Kronberg 2007" /> 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 ==
== Observation history ==
[[File:Pic iroberts1.jpg|thumb|Great Andromeda Nebula by [[Isaac Roberts]]]]
[[File:Pic iroberts1.jpg|thumb|Great Andromeda Nebula by [[Isaac Roberts]], 1899]]
The [[Persian people|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".<ref name="Henbest & Couper 1994" /><ref name="NSOG" /> [[Star chart]]s of that period have it labeled as the ''Little Cloud''.<ref name="NSOG"/> The first description of the object based on telescopic observation was given by German astronomer [[Simon Marius]] on December 15, 1612.<ref name="Aati"/> [[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]].<ref name="Herschel 1785"/>
The [[Persian people|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".<ref name="Henbest & Couper 1994" /><ref name="NSOG" /> [[Star chart]]s of that period have it labeled as the ''Little Cloud''.<ref name="NSOG"/> The first description of the object based on telescopic observation was given by German astronomer [[Simon Marius]] on December 15, 1612.<ref name="Aati"/> [[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]].<ref name="Herschel 1785"/>


[[William Huggins]] in 1864 observed the [[spectrum]] of M31 and noted that it differed from a gaseous nebula.<ref name="Huggins & Miller 1864"/> The spectra of M31 displayed a [[wikt:continuum|continuum]] of [[frequency|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".<ref name="Backhouse 1888"/>
[[William Huggins]] in 1864 observed the [[spectrum]] of M31 and noted that it differed from a gaseous nebula.<ref name="Huggins & Miller 1864"/> The spectra of M31 displayed a [[wikt:continuum|continuum]] of [[frequency|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".<ref name="Backhouse 1888"/>


[[File:Над VLT – две галактики, видимые простым глазом.jpg|left|thumb|M31 seen with the naked-eye above the [[Very Large Telescope]].<ref>{{cite news|title=Two naked-eye galaxies above the VLT|url=http://www.eso.org/public/images/potw1342a/|accessdate=22 October 2013|newspaper=ESO Picture of the Week}}</ref> ]]
[[File:Над VLT – две галактики, видимые простым глазом.jpg|left|thumb|M31 above the [[Very Large Telescope]].<ref>{{cite news|title=Two naked-eye galaxies above the VLT|url=http://www.eso.org/public/images/potw1342a/|accessdate=22 October 2013|newspaper=ESO Picture of the Week}}</ref> ]]


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.<ref name="Roberts 1899" /> 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 | date = April 2012}}. 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 {{convert|300|km/s}}, moving in the direction of the Sun.<ref name="Slipher 1913"/>
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.<ref name="Roberts 1899" /> 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 | date = April 2012}} 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 {{convert|300|km/s}}, moving in the direction of the Sun.<ref name="Slipher 1913"/>


=== Island universe ===
=== Island universe ===
[[File:Andromeda constellation map (1).png|thumb|Location of M31 in the Andromeda constellation]]
[[File:Andromeda constellation map.svg|thumb|Location of M31 in the Andromeda constellation]]


In 1917, American astronomer [[Heber Doust Curtis|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 [[Magnitude (astronomy)|magnitudes]] fainter than those that occurred elsewhere in the sky. As a result he was able to come up with a distance estimate of {{convert|500000|ly}}. He became a proponent of the so-called "island universes" hypothesis, which held that [[spiral galaxy|spiral nebulae]] were actually independent galaxies.<ref name="Curtis 1988"/>
In 1917, American astronomer [[Heber Doust Curtis|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 [[Magnitude (astronomy)|magnitudes]] fainter than those that occurred elsewhere in the sky. As a result he was able to come up with a distance estimate of {{convert|500000|ly}}. He became a proponent of the so-called "island universes" hypothesis, which held that [[Spiral galaxy#Spiral nebula| spiral nebulae]] were actually independent galaxies.<ref name="Curtis 1988"/>


In 1920, [[Great Debate (astronomy)|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 [[lightyear|ly]].<ref name="Öpik 1922" /> [[Edwin Hubble]] settled the debate in 1925 when he identified extragalactic [[Cepheid variable star]]s 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.<ref name="Hubble 1929"/>
In 1920, the [[Great Debate (astronomy)|Great Debate]] between [[Harlow Shapley]] and Curtis took place, concerning the nature of the [[Milky Way]], spiral nebulae, and the dimensions of the [[universe]]. To support his claim that the Great Andromeda Nebula 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 method to estimate the distance of M31 using the measured velocities of its stars. His result put the Andromeda Nebula far outside our Galaxy at a distance of about {{convert|450,000|pc|ly}}.<ref name="Öpik 1922" /> [[Edwin Hubble]] settled the debate in 1925 when he identified extragalactic [[Cepheid variable star]]s 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.<ref name="Hubble 1929"/>


[[File:Stars in the Andromeda Galaxy's disc.jpg|thumb|left|Stars in the Andromeda Galaxy's disc<ref name="spacetelescope 2011-07-21"/>]]
[[File:Stars in the Andromeda Galaxy's disc.jpg|thumb|left|Stars in the Andromeda Galaxy's disc<ref name="spacetelescope 2011-07-21"/>]]


M31 plays an important role in galactic studies, since it is the nearest spiral galaxy (although not the [[List of nearest galaxies|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]].)<ref name="Baade 1944"/> 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.<ref name="Gribbin 2001"/>
M31 plays an important role in galactic studies, since it is the nearest spiral galaxy (although not the [[List of nearest galaxies|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]].)<ref name="Baade 1944"/> 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.<ref name="Gribbin 2001"/>


Radio emission from the Andromeda Galaxy was first detected by [[Robert Hanbury Brown|Hanbury Brown]] and [[Cyril Hazard]] at [[Jodrell Bank Observatory]] using the 218-ft [[Transit Telescope#Transit Telescope|Transit Telescope]], and was announced in 1950<ref name="Brown & Hazard 1950"/><ref name="Brown & Hazard 1951"/> (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 astronomy|radio maps]] of the galaxy were made in the 1950s by [[John E. Baldwin|John Baldwin]] and collaborators at the [[Cavendish Astrophysics Group|Cambridge Radio Astronomy Group]].<ref name="van der Kruit & Allen 1976"/> The core of the Andromeda Galaxy is called 2C 56 in the [[Second Cambridge Catalogue of Radio Sources|2C]] radio astronomy 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.<ref name="Ingrosso 2009"/>
Radio emission from the Andromeda Galaxy was first detected by [[Robert Hanbury Brown|Hanbury Brown]] and [[Cyril Hazard]] at [[Jodrell Bank Observatory]] using the 218-ft [[Transit Telescope#Transit Telescope|Transit Telescope]], and was announced in 1950<ref name="Brown & Hazard 1950"/><ref name="Brown & Hazard 1951"/> (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 astronomy|radio maps]] of the galaxy were made in the 1950s by [[John E. Baldwin|John Baldwin]] and collaborators at the [[Cavendish Astrophysics Group|Cambridge Radio Astronomy Group]].<ref name="van der Kruit & Allen 1976"/> The core of the Andromeda Galaxy is called 2C 56 in the [[Second Cambridge Catalogue of Radio Sources|2C]] radio astronomy 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.<ref name="Ingrosso 2009"/>
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| last8 = Rich
| last8 = Rich
| first8 = R. M.
| first8 = R. M.
| year = 2012
| date = 2012
| title = The Recent Stellar Archeology of M31 – The Nearest Red Disk Galaxy
| title = The Recent Stellar Archeology of M31&nbsp;– The Nearest Red Disk Galaxy
| journal = [[The Astrophysical Journal]]
| journal = [[The Astrophysical Journal]]
| volume = 751
| volume = 751
Line 88: Line 107:
The most important event in M31's past history was the [[Galactic merger|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 [[Starburst galaxy|very high]], to the point of becoming a [[luminous infrared galaxy]] for roughly 100 million years.
The most important event in M31's past history was the [[Galactic merger|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 [[Starburst galaxy|very high]], to the point of becoming a [[luminous infrared galaxy]] for roughly 100 million years.


M31 and the [[Triangulum Galaxy|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.
M31 and the [[Triangulum Galaxy|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&nbsp;– even some globular clusters&nbsp;– 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 [[List of stellar streams#Andromeda Galaxy streams|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.
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 [[List of stellar streams#Andromeda Galaxy streams|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.
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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 {{convert|2.56|+/-|0.08|Mly|abbr=on}}.<ref name="McConnachieetal2005" />
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 {{convert|2.56|+/-|0.08|Mly|abbr=on}}.<ref name="McConnachieetal2005" />


Averaged together, all these distance measurements give a combined distance estimate of {{convert|2.54|+/-|0.11|Mly|abbr=on}}.{{efn|name=avg dist}} Based upon the above distance, the diameter of M31 at the widest point is estimated to be {{convert|141|+/-|3|kly|abbr=on}}.{{efn|name=dia calc}} Applying [[trigonometry]] ([[inverse trigonometric functions|arctangent]]), that figures to extending at an apparent 3.18[[Degree (angle)|°]] angle in the sky.
Averaged together, all these distance measurements give a combined distance estimate of {{convert|2.54|+/-|0.11|Mly|abbr=on}}.{{efn|name=avg dist}} Based upon the above distance, the diameter of M31 at the widest point is estimated to be {{convert|220|+/-|3|kly|abbr=on}}. Applying [[trigonometry]] ([[inverse trigonometric functions|arctangent]]), that figures to extending at an apparent 3.18[[Degree (angle)|°]] angle in the sky.


=== Mass and luminosity estimates ===
=== Mass and luminosity estimates ===
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==== Mass ====
==== Mass ====


Mass estimates for the Andromeda Galaxy's halo (including [[dark matter]]) give a value of approximately 1.23{{e|12}}&nbsp;[[Solar mass|''M''<sub>☉</sub>]]<ref name="Evans" /> (or 1.2 [[1000000000000 (number)|trillion]] [[solar mass]]es) compared to 1.9{{e|12}}&nbsp;''M''<sub>☉</sub> 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 [[Spiral galaxy#Galactic spheroid|spheroid]] actually has a higher stellar density than that of the Milky Way.<ref name="Kalirai et al 2006" />
Mass estimates for the Andromeda Galaxy's halo (including [[dark matter]]) give a value of approximately {{Solar mass|1.5{{e|12}}|link=y}}<ref name="Jorge Peñarrubia2014"/> (or 1.5 [[1000000000000 (number)|trillion]] [[solar mass]]es) compared to {{Solar mass|8{{e|11}}}} 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 [[Spiral galaxy#Galactic spheroid|spheroid]] actually has a higher stellar density than that of the Milky Way<ref name="Kalirai et al 2006"/> and its galactic stellar disk is about twice the size of that of the Milky Way.<ref name="Chapman et al 2006" />


==== Luminosity ====
==== Luminosity ====


M31 appears to have significantly more common stars than the Milky Way, and the estimated [[luminosity]] of M31, ~2.6{{e|10}}&nbsp;[[Solar luminosity|''L''<sub>☉</sub>]], is about 25% higher than that of our own galaxy.<ref name="vdb" /> 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's the second brightest galaxy within a radius of 10 [[mega-|mega]][[parsec]]s of the Milky Way, after the [[Sombrero Galaxy]]<ref name="Karachentsev2004">{{cite journal
M31 appears to have significantly more common stars than the Milky Way, and the estimated [[luminosity]] of M31, {{Solar luminosity|~2.6{{e|10}}|link=y}}, is about 25% higher than that of our own galaxy.<ref name="vdb" /> 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 [[mega-|mega]][[parsec]]s of the Milky Way, after the [[Sombrero Galaxy]]<ref name="Karachentsev2004">{{cite journal
     | author=Karachentsev, Igor D.; Karachentseva, Valentina E.; Huchtmeier, Walter K.; Makarov, Dmitry I.
     | author=Karachentsev, Igor D.; Karachentseva, Valentina E.; Huchtmeier, Walter K.; Makarov, Dmitry I.
     | title=A Catalog of Neighboring Galaxies
     | title=A Catalog of Neighboring Galaxies
     | journal=The Astronomical Journal
     | journal=The Astronomical Journal
     | year=2003
     | date=2003
     | volume=127
     | volume=127
     | issue=4
     | issue=4
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     | bibcode=2004AJ....127.2031K
     | bibcode=2004AJ....127.2031K
     | doi=10.1086/382905}}</ref>)
     | doi=10.1086/382905}}</ref>)
, 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{{efn|name=blue mag}}, compared to −20.9 for the Milky Way), and a total luminosity in that [[wavelength]] of 3.64{{e|10}}''L''<sub>☉</sub><ref name="Tempel2010">{{cite journal
, 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,{{efn|name=blue mag}} compared to −20.9 for the Milky Way), and a total luminosity in that [[wavelength]] of {{Solar luminosity|3.64{{e|10}}}}<ref name="Tempel2010">{{cite journal
     | author=Tempel, E.; Tamm, A.; Tenjes, P.
     | author=Tempel, E.; Tamm, A.; Tenjes, P.
     | title=Dust-corrected surface photometry of M 31 from Spitzer far-infrared observations
     | title=Dust-corrected surface photometry of M 31 from Spitzer far-infrared observations
     | journal=Astronomy and Astrophysics
     | journal=Astronomy and Astrophysics
     | year=2010
     | date=2010
     | volume=509
     | volume=509
     | id=A91
     | id=wA91
     | bibcode=2010A&A...509A..91T
     | bibcode=2010A&A...509A..91T
     | doi=10.1051/0004-6361/200912186|arxiv = 0912.0124
     | doi=10.1051/0004-6361/200912186|arxiv = 0912.0124
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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 [[supernova]]e in the Milky Way is also double that of M31.<ref name="Liller & Mayer 1987" /> 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.<ref name="vdb" /> Should this continue, the luminosity in the Milky Way may eventually overtake that of M31.
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 [[supernova]]e in the Milky Way is also double that of M31.<ref name="Liller & Mayer 1987" /> 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.<ref name="vdb" /> 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 both Andromeda and the Milky Way.<ref>{{cite journal | doi=10.1088/0004-637X/736/2/84 | title=The Mid-life Crisis of the Milky Way and M31 | year=2011 | author=Mutch, S.J. | journal=The Astrophysical Journal | volume=736 | issue=2 | last2=Croton | first2=D.J. | last3=Poole | first3=G.B. |bibcode = 2011ApJ...736...84M|arxiv = 1105.2564 | pages=84 }}</ref>
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.<ref>{{cite journal | doi=10.1088/0004-637X/736/2/84 | title=The Mid-life Crisis of the Milky Way and M31 | date=2011 | author=Mutch, S.J. | journal=The Astrophysical Journal | volume=736 | issue=2 | last2=Croton | first2=D.J. | last3=Poole | first3=G.B. |bibcode = 2011ApJ...736...84M|arxiv = 1105.2564 | pages=84 }}</ref>


== Structure ==
== Structure ==
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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.<ref name="UC Santa Cruz 2001" /> A possible cause of such a warp could be gravitational interaction with the satellite galaxies near M31. The galaxy [[Triangulum Galaxy|M33]] could be responsible for some warp in M31's arms, though more precise distances and radial velocities are required.
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.<ref name="UC Santa Cruz 2001" /> A possible cause of such a warp could be gravitational interaction with the satellite galaxies near M31. The galaxy [[Triangulum 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 [[Galaxy rotation curve|rotational velocity of M31]] at various radii from the core. In the vicinity of the core, the rotational velocity climbs to a peak of {{convert|225|km/s}} at a radius of {{convert|1300|ly|lk=on}}, then descends to a minimum at {{convert|7000|ly|lk=on}} where the rotation velocity may be as low as {{convert|50|km/s}}. Thereafter the velocity steadily climbs again out to a radius of {{convert|33000|ly|lk=on}}, where it reaches a peak of {{convert|250|km/s}}. The velocities slowly decline beyond that distance, dropping to around {{convert|200|km/s}} at {{convert|80000|ly|lk=on}}. These velocity measurements imply a concentrated mass of about 6{{e|9}}&nbsp;[[Solar mass|M<sub>☉</sub>]] in the [[Galaxy nucleus|nucleus]]. The total mass of the galaxy increases [[linear]]ly out to {{convert|45000|ly|lk=on}}, then more slowly beyond that radius.<ref name="Rubin & Ford 1970" />
Spectroscopic studies have provided detailed measurements of the [[Galaxy rotation curve|rotational velocity of M31]] at various radii from the core. In the vicinity of the core, the rotational velocity climbs to a peak of {{convert|225|km/s}} at a radius of {{convert|1300|ly|lk=on}}, then descends to a minimum at {{convert|7000|ly|lk=on}} where the rotation velocity may be as low as {{convert|50|km/s}}. Thereafter the velocity steadily climbs again out to a radius of {{convert|33000|ly|lk=on}}, where it reaches a peak of {{convert|250|km/s}}. The velocities slowly decline beyond that distance, dropping to around {{convert|200|km/s}} at {{convert|80000|ly|lk=on}}. These velocity measurements imply a concentrated mass of about {{Solar mass|6{{e|9}}|link=y}} in the [[Galaxy nucleus|nucleus]]. The total mass of the galaxy increases [[linear]]ly out to {{convert|45000|ly|lk=on}}, then more slowly beyond that radius.<ref name="Rubin & Ford 1970" />
 
The [[spiral arm]]s of M31 are outlined by a series of [[H II region]]s, first studied in great detail by [[Walter Baade]] and described by him as resembling "beads on a string". his studies show two spiral arms that appear to be tightly wound, although they are more widely spaced than in our galaxy.<ref name="Arp 1964" /> His descriptions of the spiral structure, as each arm crosses the major axis of M31, are as follows<ref name="Bergh1991">{{cite journal
    |last1=van den Bergh
    |first1=Sidney
    | title=The stellar populations of M31
    | journal=Astronomical Society of the Pacific
    | date=1991
    | volume=103
    | pages=1053–1068
    | doi=10.1086/132925
    | bibcode=1991PASP..103.1053V}}</ref><sup>§pp1062</sup><ref name="Hodge1966">{{cite book
    | last=Hodge
    | first=P .W.
    | authorlink=Paul W. Hodge
    | title=Galaxies and Cosmology
    | date=1966
    | publisher=McGraw Hill
    | isbn=
    | url=https://archive.org/stream/GalaxiesCosmology/Hodge-GalaxiesCosmology#page/n53/mode/1up}}</ref><sup>§pp92</sup>:
{| class="wikitable"
|+ Baade's spiral arms of M31
! Arms (N=cross M31's major axis at north, S=cross M31's major axis at south)
! Distance from center ([[arcminute]]s) (N*/S*)
! Distance from center (kpc) (N*/S*)
! Notes
|-
|align=left|N1/S1|| 3.4/1.7|| 0.7/0.4|| Dust arms with no OB associations of HII regions.
|-
|align=left|N2/S2||8.0/10.0||1.7/2.1|| Dust arms with some OB associations.
|-
|align=left|N3/S3||25/30||5.3/6.3|| As per N2/S2, but with some HII regions too.
|-
|align=left|N4/S4||50/47||11/9.9|| Large numbers of OB associations, HII regions, and little dust.
|-
|align=left|N5/S5||70/66||15/14|| As per N4/S4 but much fainter.
|-
|align=left|N6/S6||91/95||19/20|| Loose OB associations. No dust visible.
|-
|align=left|N7/S7||110/116||23/24|| As per N6/S6 but fainter and inconspicuous.
|}


The [[spiral arm]]s of M31 are outlined by a series of [[H II region]]s 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.<ref name="Arp 1964" /> 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, existing two continuous trailing arms that are separated from each other by a minimum of about {{convert|13000|ly|lk=on}} and that can be followed outward from a distance of roughly {{convert|1600|ly|lk=on}} from the core, other alternative spiral structures have been proposed such as a single spiral arm<ref name="Simien1978">{{cite journal
Since the Andromeda Galaxy is seen close to edge-on, however, the studies of its spiral structure are difficult. While as stated above 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 {{convert|13000|ly|lk=on}} and that can be followed outward from a distance of roughly {{convert|1600|ly|lk=on}} from the core, other alternative spiral structures have been proposed such as a single spiral arm<ref name="Simien1978">{{cite journal
    |last1=Simien|first1=F.|last2=Pellet|first2=A.|last3=Monnet|first3=G.|last4=Athanassoula|first4=E.|last5=Maucherat|first5=A.|last6=Courtes|first6=G.
|last1=Simien|first1=F.|last2=Pellet|first2=A.|last3=Monnet|first3=G.|last4=Athanassoula|first4=E.|last5=Maucherat|first5=A.|last6=Courtes|first6=G.
     | title=The spiral structure of M31 - A morphological approach
     | title=The spiral structure of M31 - A morphological approach
     | journal=Astronomy and Astrophysics
     | journal=Astronomy and Astrophysics
     | year=1978
     | date=1978
     | volume=67
     | volume=67
     | pages=73–79
     | pages=73–79
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     | title=Cold dust in M31 as mapped by ISO
     | title=Cold dust in M31 as mapped by ISO
     | journal=The interstellar medium in M31 and M33. Proceedings 232. WE-Heraeus Seminar
     | journal=The interstellar medium in M31 and M33. Proceedings 232. WE-Heraeus Seminar
     | year=2000
     | date=2000
     | pages=69–72
     | pages=69–72
     | bibcode=2000immm.proc...69H}}</ref> pattern of long, filamentary, and thick spiral arms.<ref name="ned"/><ref name="Walterbos1988">{{cite journal
     | bibcode=2000immm.proc...69H}}</ref> pattern of long, filamentary, and thick spiral arms.<ref name="ned"/><ref name="Walterbos1988">{{cite journal
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     | title=An optical study of stars and dust in the Andromeda galaxy
     | title=An optical study of stars and dust in the Andromeda galaxy
     | journal=Astronomy and Astrophysics
     | journal=Astronomy and Astrophysics
     | year=1988
     | date=1988
     | volume=198
     | volume=198
     | pages=61–86
     | pages=61–86
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     | title=Spitzer MIPS Infrared Imaging of M31: Further Evidence for a Spiral-Ring Composite Structure
     | title=Spitzer MIPS Infrared Imaging of M31: Further Evidence for a Spiral-Ring Composite Structure
     | journal=The Astrophysical Journal
     | journal=The Astrophysical Journal
     | year=2006
     | date=2006
     | volume=638
     | volume=638
     | pages=L87–L92
     | pages=L87–L92
Line 192: Line 251:


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&nbsp;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.<ref name="Harvard-Smithsonian 2006-10-18"/>
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&nbsp;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.<ref name="Harvard-Smithsonian 2006-10-18"/>
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.<ref name="Block2006">{{cite journal
    |last1=Block|first1=D. L.|last2=Bournaud|first2=F.|last3=Combes|first3=F.|last4=Groess|first4= R.|last5=Barmby|first5=P.|last6=Ashby|first6=M. L. N.|last7=Fazio|first7=G. G.|last8=Pahre|first8=M. A. |last9=Willner|first9=S. P.
    | title=An almost head-on collision as the origin of the two off-centre rings in the Andromeda galaxy
    | journal=Nature
    | date=2006
    | volume=443
    | pages=832–834
    | bibcode=2006Natur.443..832B
    | doi=10.1038/nature05184|arxiv = astro-ph/0610543
    | issue=1 }}</ref>


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 "[[Metallicity|metal-poor]]", and increasingly so with greater distance.<ref name="Kalirai et al 2006"/> 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&nbsp;billion years.<ref name="Bullock & Johnston 2005"/> The stars in the extended halos of M31 and the Milky Way may extend nearly one-third the distance separating the two galaxies.
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 "[[Metallicity|metal-poor]]", and increasingly so with greater distance.<ref name="Kalirai et al 2006"/> 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&nbsp;billion years.<ref name="Bullock & Johnston 2005"/> The stars in the extended halos of M31 and the Milky Way may extend nearly one-third the distance separating the two galaxies.


== Nucleus ==
== Nucleus ==
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[[File:M31 Core in X-rays.jpg|thumb|300px|[[Chandra X-ray Observatory|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.]]
[[File:M31 Core in X-rays.jpg|thumb|300px|[[Chandra X-ray Observatory|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 [[Wide Field and Planetary Camera|WFPC]], then on board the [[Hubble Space Telescope]], to image M31's inner nucleus. The nucleus consists of two concentrations separated by {{convert|1.5|pc|lk=on}}. 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 × 10<sup>7</sup> [[Solar mass|M<sub>☉</sub>]] in 1993,<ref name="Lauer" /> and at 1.1–2.3 × 10<sup>8</sup> M<sub>☉</sub> in 2005.<ref name="Bender et al 2005" /> The [[velocity dispersion]] of material around it is measured to be ≈ 160&nbsp;km/s.<ref name="Gebhardt et al 2000" />
In 1991 [[Tod R. Lauer]] used [[Wide Field and Planetary Camera|WFPC]], then on board the [[Hubble Space Telescope]], to image M31's inner nucleus. The nucleus consists of two concentrations separated by {{convert|1.5|pc|lk=on}}. 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 × 10<sup>7</sup> {{Solar mass|link=y}} in 1993,<ref name="Lauer" /> and at 1.1–2.3 × 10<sup>8</sup> {{Solar mass}} in 2005.<ref name="Bender et al 2005" /> The [[velocity dispersion]] of material around it is measured to be ≈ 160&nbsp;km/s.<ref name="Gebhardt et al 2000" />


[[Scott Tremaine]] has proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an [[orbital eccentricity|eccentric orbit]] around the central black hole.<ref name="Tremaine 1995" /> The eccentricity is such that stars linger at the orbital [[apsis|apocenter]], creating a concentration of stars. P2 also contains a compact disk of hot, [[stellar classification|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.<ref name="hubblesite 1993-07-20" />
[[Scott Tremaine]] has proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an [[orbital eccentricity|eccentric orbit]] around the central black hole.<ref name="Tremaine 1995" /> The eccentricity is such that stars linger at the orbital [[apsis|apocenter]], creating a concentration of stars. P2 also contains a compact disk of hot, [[stellar classification|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.<ref name="hubblesite 1993-07-20" />
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There are approximately 460 [[globular cluster]]s associated with the Andromeda Galaxy.<ref name="Barmby & Huchra 2001" /> 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.<ref name="hubblesite 1996-04-24" /> 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.<ref name="Meylan et al 2001" /> The globular with the greatest apparent brightness is [[G76]] which is located in the south-west arm's eastern half.<ref name="NSOG" />
There are approximately 460 [[globular cluster]]s associated with the Andromeda Galaxy.<ref name="Barmby & Huchra 2001" /> 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.<ref name="hubblesite 1996-04-24" /> 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.<ref name="Meylan et al 2001" /> The globular with the greatest apparent brightness is [[G76]] which is located in the south-west arm's eastern half.<ref name="NSOG" />
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;<ref name=Ma06>{{cite journal
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;<ref name=Ma06>{{cite journal
|year = 2006
|date = 2006
|title  = A 'super' star cluster grown old: the most massive star cluster in the Local Group
|title  = A 'super' star cluster grown old: the most massive star cluster in the Local Group
|journal = Monthly Notices of the Royal Astronomical Society
|journal = Monthly Notices of the Royal Astronomical Society
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|arxiv = astro-ph/0602608}}</ref> however other studies have shown is actually similar in properties to G1.<ref name=Cohen06>{{cite journal
|arxiv = astro-ph/0602608}}</ref> however other studies have shown is actually similar in properties to G1.<ref name=Cohen06>{{cite journal
|last = Cohen|first=Judith G.
|last = Cohen|first=Judith G.
|year = 2006
|date = 2006
|title = The Not So Extraordinary Globular Cluster 037-B327 in M31
|title = The Not So Extraordinary Globular Cluster 037-B327 in M31
|journal = The Astrophysical Journal
|journal = The Astrophysical Journal
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|arxiv = astro-ph/0610863}}</ref>
|arxiv = astro-ph/0610863}}</ref>


Unlike the globular clusters of the Milky Way, that 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<ref name=Bu06>{{cite journal
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<ref name=Bu06>{{cite journal
|year = 2004
|date= 2004
|title = Globular Cluster and Galaxy Formation: M31, the Milky Way, and Implications for Globular Cluster Systems of Spiral Galaxies
|title = Globular Cluster and Galaxy Formation: M31, the Milky Way, and Implications for Globular Cluster Systems of Spiral Galaxies
|journal = Astrophysical Journal
|journal = Astrophysical Journal
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|volume = 614
|volume = 614
|bibcode = 2004ApJ...614..158B
|bibcode = 2004ApJ...614..158B
|last1=Burstein|first1=David
|last1= Burstein|first1=David
|last2= Li|first2=Yong
|last2= Li|first2=Yong
|last3= Freeman|first3=Kenneth C.
|last3= Freeman|first3=Kenneth C.
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|arxiv = astro-ph/0406564 }}</ref>
|arxiv = astro-ph/0406564 }}</ref>


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.<ref name="Huxor et al 2005" />
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&nbsp;— several hundred light-years across&nbsp;— and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.<ref name="Huxor et al 2005" />


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 [[solar masses|<math>\begin{smallmatrix}M_\odot\end{smallmatrix}</math>]]. Discovered through a data collected by the [[ESA]]'s [[XMM-Newton]] probe, and subsequently observed by [[NASA]]'s [[Swift]] and [[Chandra X-Ray Observatory|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.<ref name=prostak-2012>{{cite news | title=Microquasar in Andromeda Galaxy Amazes Astronomers | url=http://www.sci-news.com/astronomy/article00779.html | agency= Sci-News.com | date=2012-12-14 | first = Sergio | last= Prostak}}</ref>
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 [[solar masses|<math>\begin{smallmatrix}M_\odot\end{smallmatrix}</math>]]. Discovered through a data collected by the [[ESA]]'s [[XMM-Newton]] probe, and subsequently observed by [[NASA]]'s [[Swift]] and [[Chandra X-Ray Observatory|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.<ref name=prostak-2012>{{cite news | title=Microquasar in Andromeda Galaxy Amazes Astronomers | url=http://www.sci-news.com/astronomy/article00779.html | agency= Sci-News.com | date=2012-12-14 | first = Sergio | last= Prostak}}</ref>
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{{main|Andromeda–Milky Way collision}}
{{main|Andromeda–Milky Way collision}}


The Andromeda Galaxy is approaching the [[Milky Way]] at about {{convert|300|km/s}},<ref name="ned" /> making it one of the few [[blueshift]]ed galaxies. The Andromeda Galaxy and the Milky Way are thus expected to collide in about 3.75 or 4.5&nbsp;billion years, although the details are uncertain since Andromeda's tangential velocity with respect to the Milky Way is known to only within about a factor of two.<ref name="The Sky At Night 2007-11-05" /> A likely outcome of the collision is that the [[Galaxy merger|galaxies will merge]] to form a giant [[elliptical galaxy]].<ref name="Cox & Loeb 2008" /> Such events are frequent among the galaxies in [[galaxy group]]s. 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.<ref name="Cain 2007" />
The Andromeda Galaxy is approaching the [[Milky Way]] at about {{convert|110|km/s}}.<ref name="nature" /> It has been measured approaching relative to our sun at around {{convert|300|km/s}}<ref name="ned" /> as the sun orbits around the center of our galaxy at a speed of approximately {{convert|225|km/s}}. This makes Andromeda one of the few [[blueshift]]ed 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&nbsp;billion years. A likely outcome of the collision is that the [[Galaxy merger|galaxies will merge]] to form a giant [[elliptical galaxy]]<ref name="Cox & Loeb 2008" /> or perhaps even a large disk galaxy.<ref name="Ueda2014"/> Such events are frequent among the galaxies in [[galaxy group]]s. 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.<ref name="Cain 2007" />


== See also ==
== See also ==
{{Portal|Astronomy}}
[[File:DPAG 1999 2077 Andromeda-Galaxie.jpg|thumb|220px|The Andromeda Galaxy on a German postage stamp of 1999]]
[[File:DPAG 1999 2077 Andromeda-Galaxie.jpg|thumb|220px|The Andromeda Galaxy on a German postage stamp of 1999]]
* [[Nebulae in fiction#Andromeda Nebula (Great Nebula in Andromeda)|Andromeda Nebula in fiction]]
* [[Nebulae in fiction#Andromeda Nebula (Great Nebula in Andromeda)|Andromeda Nebula in fiction]]
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* [[List of galaxies]]
* [[List of galaxies]]
* [[New General Catalogue]]
* [[New General Catalogue]]
* [[NGC 206]] – the brightest [[star cloud]] in the Andromeda Galaxy
* [[NGC 206]]&nbsp;– the brightest [[star cloud]] in the Andromeda Galaxy


== Notes ==
== Notes ==
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| name = M31VJ
| name = M31VJ
| 1 = J00443799+4129236 is at [[Celestial coordinate system|celestial coordinates]] [[Right Ascension|R.A.]] {{RA|00|44|37.99}}, [[Declination|Dec.]] {{DEC|+41|29|23.6}}.
| 1 = J00443799+4129236 is at [[Celestial coordinate system|celestial coordinates]] [[Right Ascension|R.A.]] {{RA|00|44|37.99}}, [[Declination|Dec.]] {{DEC|+41|29|23.6}}.
}}
{{efn
| name = dia calc
| 1= distance × tan( diameter_angle = 190′ ) = 141 ± 3 kly diameter.
}}
}}


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| publisher = [[NASA]]/[[Infrared Processing and Analysis Center|IPAC]]
| publisher = [[NASA]]/[[Infrared Processing and Analysis Center|IPAC]]
| accessdate = 2006-11-01
| accessdate = 2006-11-01
}}
</ref>
<ref name="nature">
{{cite web
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| title = Andromeda on collision course with the Milky Way
| url =http://www.nature.com/news/andromeda-on-collision-course-with-the-milky-way-1.10765#
| work = Nature International Weekly Journal of Science
| publisher = [[Nature (journal)]]
| accessdate = 2014-10-06
}}
}}
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| journal = [[Astrophysics (journal)|Astrophysics]]
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| bibcode = 2006Ap.....49....3K
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| title = Planetary camera observations of the double nucleus of M31
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| first15 = E. J., Jr.
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| last2 = Kormendy
| last2 = Kormendy
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| last9 = Joseph
| last9 = Joseph
| first9 = C. L.
| first9 = C. L.
| year = 2005
| date = 2005
| title = HST STIS Spectroscopy of the Triple Nucleus of M31: Two Nested Disks in Keplerian Rotation around a Supermassive Black Hole
| title = HST STIS Spectroscopy of the Triple Nucleus of M31: Two Nested Disks in Keplerian Rotation around a Supermassive Black Hole
| journal = [[Astrophysical Journal]]
| journal = [[Astrophysical Journal]]
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| last15 = Woodgate
| last15 = Woodgate
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| first = S.
| first = S.
| year = 1995
| date = 1995
| title = An Eccentric-Disk Model for the Nucleus of M31
| title = An Eccentric-Disk Model for the Nucleus of M31
| journal = [[Astronomical Journal]]
| journal = [[Astronomical Journal]]
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| last3 = Kato
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| year = 1969
| date = 1969
| title = Correlation between the Densities of X-Ray Sources and Interstellar Gas
| title = Correlation between the Densities of X-Ray Sources and Interstellar Gas
| journal = [[Astrophysics and Space Science]]
| journal = [[Astrophysics and Space Science]]
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| first = L. E.
| first = L. E.
| year = 1973
| date = 1973
| chapter = Hard Cosmic X-Ray Sources
| chapter = Hard Cosmic X-Ray Sources
| editor = Bradt, H.; Giacconi, R.
| editor = Bradt, H.; Giacconi, R.
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| last3 = Osborne
| last3 = Osborne
| first3 = J. P.
| first3 = J. P.
| year = 2005
| date = 2005
| title = Timing the bright X-ray population of the core of M31 with XMM-Newton
| title = Timing the bright X-ray population of the core of M31 with XMM-Newton
| class = astro-ph
| class = astro-ph
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| last2 = Huchra
| last2 = Huchra
| first2 = J. P.
| first2 = J. P.
| year = 2001
| date = 2001
| title = M31 Globular Clusters in the ''Hubble Space Telescope'' Archive. I. Cluster Detection and Completeness
| title = M31 Globular Clusters in the ''Hubble Space Telescope'' Archive. I. Cluster Detection and Completeness
| journal = [[Astronomical Journal]]
| journal = [[Astronomical Journal]]
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| last1 = Meylan
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| first1 = G.
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| last2 = Sarajedini
| last2 = Sarajedini
| first2 = A.
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| last6 = Rich
| last6 = Rich
| first6 = R. M.
| first6 = R. M.
| year = 2001
| date = 2001
| title = G1 in M31 – Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy?
| title = G1 in M31&nbsp;– Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy?
| journal = [[Astronomical Journal]]
| journal = [[Astronomical Journal]]
| volume = 122
| volume = 122
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| bibcode = 2001AJ....122..830M
| bibcode = 2001AJ....122..830M
| arxiv = astro-ph/0105013
| arxiv = astro-ph/0105013
| display-authors = 1
}}
}}
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</ref>
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| last1 = Huxor
| last1 = Huxor
| first1 = A. P.
| first1 = A. P.
| display-authors = 1
| last2 = Tanvir
| last2 = Tanvir
| first2 = N. R.
| first2 = N. R.
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| last8 = Lewis
| last8 = Lewis
| first8 = G. F.
| first8 = G. F.
| year = 2005
| date = 2005
| title = A new population of extended, luminous, star clusters in the halo of M31
| title = A new population of extended, luminous, star clusters in the halo of M31
| journal = [[Monthly Notices of the Royal Astronomical Society]]
| journal = [[Monthly Notices of the Royal Astronomical Society]]
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| bibcode = 2005MNRAS.360.1007H
| bibcode = 2005MNRAS.360.1007H
| doi = 10.1111/j.1365-2966.2005.09086.x
| doi = 10.1111/j.1365-2966.2005.09086.x
| display-authors = 1
}}
}}
</ref>
</ref>
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| last1 = Bekki
| last1 = Bekki
| first1 = K.
| first1 = K.
| display-authors = 1
| last2 = Couch
| last2 = Couch
| first2 = Warrick J.
| first2 = Warrick J.
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| last4 = Gregg
| last4 = Gregg
| first4 = Michael D.
| first4 = Michael D.
| year = 2001
| date = 2001
| title = A New Formation Model for M32: A Threshed Early-type Spiral?
| title = A New Formation Model for M32: A Threshed Early-type Spiral?
| journal = [[Astrophysical Journal Letters]]
| journal = [[Astrophysical Journal Letters]]
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| arxiv = astro-ph/0107117
| arxiv = astro-ph/0107117
| doi = 10.1086/323075
| doi = 10.1086/323075
| display-authors = 1
}}
}}
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| last = Ibata
| last = Ibata
| first = R.
| first = R.
| display-authors = 1
| last2 = Irwin
| last2 = Irwin
| first2 = Michael
| first2 = Michael
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| last5 = Tanvir
| last5 = Tanvir
| first5 = Nial
| first5 = Nial
| year = 2001
| date = 2001
| title = A giant stream of metal-rich stars in the halo of the galaxy M31
| title = A giant stream of metal-rich stars in the halo of the galaxy M31
| journal = [[Nature (journal)|Nature]]
| journal = [[Nature (journal)|Nature]]
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| doi = 10.1038/35083506
| doi = 10.1038/35083506
| pmid = 11452300
| pmid = 11452300
| display-authors = 1
}}
}}
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| last = Young
| last = Young
| first = L. M.
| first = L. M.
| year = 2000
| date = 2000
| title = Properties of the Molecular Clouds in NGC 205
| title = Properties of the Molecular Clouds in NGC 205
| journal = [[Astronomical Journal]]
| journal = [[Astronomical Journal]]
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| date = May 7, 2002
| date = May 7, 2002
| title = Crash Course: Simulating the Fate of Our Milky Way
| title = Crash Course: Simulating the Fate of Our Milky Way
| url = http://www.space.com/scienceastronomy/astronomy/galaxy_collides_020507-1.html
| url = http://www.go2data.com/CQS/AndromedaCollision/AndromedaCollision.htm
| publisher = [[Space.com]]
| publisher = [[Space.com]]
| accessdate = 2006-09-18
| accessdate = 2014-10-06
}} {{dead link| date = November 2011}}
| archiveurl = https://web.archive.org/web/20141008143950/http://www.go2data.com/CQS/AndromedaCollision/AndromedaCollision.htm
| archivedate = 8 October 2014
}}
</ref>-->
</ref>-->


<ref name="The Sky At Night 2007-11-05">
<!-- <ref name="The Sky At Night 2007-11-05">
{{cite episode
{{cite episode
| title = The Grand Collision
| title = The Grand Collision
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| airdate = November 5, 2007
| airdate = November 5, 2007
}}
}}
</ref>
</ref> -->


<ref name="Cox & Loeb 2008">
<ref name="Cox & Loeb 2008">
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| last2 = Loeb
| last2 = Loeb
| first2 = A.
| first2 = A.
| year = 2008
| date = 2008
| title = The collision between the Milky Way and Andromeda
| title = The collision between the Milky Way and Andromeda
| journal = [[Monthly Notices of the Royal Astronomical Society]]
| journal = [[Monthly Notices of the Royal Astronomical Society]]
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| last = Cain
| last = Cain
| first = F.
| first = F.
| year = 2007
| date = 2007
| title = When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?
| title = When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?
| work = [[Universe Today]]
| work = [[Universe Today]]
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* [http://blackholes.stardate.org/directory/factsheet.php?p=M31 StarDate: M31 Fact Sheet]
* [http://blackholes.stardate.org/directory/factsheet.php?p=M31 StarDate: M31 Fact Sheet]
* [http://simbad.u-strasbg.fr/sim-id.pl?Ident=M+31 Simbad data on M31]
* [http://simbad.u-strasbg.fr/sim-id.pl?Ident=M+31 Simbad data on M31]
* [http://www.seds.org/messier/m/m031.html Messier 31, SEDS Messier pages]
* [http://messier.seds.org/m/m031.html Messier 31, SEDS Messier pages]
* [[Astronomy Picture of the Day]]
* [[Astronomy Picture of the Day]]
** [http://antwrp.gsfc.nasa.gov/apod/ap981017.html A Giant Globular Cluster in M31] 1998 October 17.
** [http://antwrp.gsfc.nasa.gov/apod/ap981017.html A Giant Globular Cluster in M31] 1998 October 17.
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* [http://herschel.esac.esa.int/Images/2011/M31_COMPO_A.jpg Multi-wavelength composite]
* [http://herschel.esac.esa.int/Images/2011/M31_COMPO_A.jpg Multi-wavelength composite]
* [http://www.andromedaproject.org Andromeda Project] (crowd-source)
* [http://www.andromedaproject.org Andromeda Project] (crowd-source)
* {{cite web|title=M31 – Andromeda Galaxy|url=http://www.deepskyvideos.com/videos/messier/M31_andromeda.html|work=Deep Sky Videos|publisher=[[Brady Haran]]|author=Gray, Meghan|coauthors=[[Nik Szymanek{{!}}Szymanek, Nik]]; Merrifield, Michael}}
* {{cite web|title=M31 – Andromeda Galaxy|url=http://www.deepskyvideos.com/videos/messier/M31_andromeda.html|work=Deep Sky Videos|publisher=[[Brady Haran]]|author=Gray, Meghan| author2=[[Nik Szymanek{{!}}Szymanek, Nik]]| author3=Merrifield, Michael}}
* [http://www.constellation-guide.com/andromeda-galaxy-messier-31-m31-ngc-224/ Andromeda Galaxy (M31) at Constellation Guide]
* [http://www.constellation-guide.com/andromeda-galaxy-messier-31-m31-ngc-224/ Andromeda Galaxy (M31) at Constellation Guide]
*[http://apod.nasa.gov/apod/ap130801.html APOD - 2013 August 1] (M31's angular size compared with full Moon)
*[http://apod.nasa.gov/apod/ap130801.html APOD - 2013 August 1] (M31's angular size compared with full Moon)
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{{Andromeda galaxy}}
{{Andromeda galaxy}}


[[Category:Spiral galaxies]]
[[Category:Unbarred spiral galaxies]]
[[Category:Unbarred spiral galaxies]]
[[Category:Local Group]]
[[Category:Local Group]]
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[[Category:Messier objects]]
[[Category:Messier objects]]
[[Category:NGC objects]]
[[Category:NGC objects]]
[[Category:PGC objects|02557]]
[[Category:PGC objects|002557]]
[[Category:UGC objects|00454]]
[[Category:UGC objects|00454]]
[[Category:LEDA objects|2557]]
[[Category:Objects within 10 Mly of Earth]]
[[Category:Objects within 10 Mly of Earth]]
 
[[Category:Astronomical objects known since antiquity]]
{{Link FA|pl}}
[[Category:MCG objects|+07-02-016]]
{{Link FA|af}}
[[Category:IRAS objects|00400+4059]]

Latest revision as of 13:56, 11 January 2015

{{#invoke:Infobox|infobox}}

The Andromeda Galaxy Template:IPAc-en is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years; 2.4Template:E km) from Earth[1] 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 44 other smaller galaxies.

The Andromeda Galaxy is probably the most massive galaxy in the Local Group as well,[2] despite earlier findings that suggested that the Milky Way contains more dark matter and could be the most massive in the grouping.[3] The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars:[4] at least twice the number of stars in the Milky Way galaxy, which is estimated to be 200–400 billion.[5]

The Andromeda Galaxy is estimated to be 1.5Template:E solar masses,[2] while the mass of the Milky Way is estimated to be 8.5Template:E solar masses. In comparison a 2009 study estimated that the Milky Way and M31 are about equal in mass,[6] 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 [7] or perhaps a large disk galaxy.[8]

At 3.4, the apparent magnitude of the Andromeda Galaxy is one of the brightest of any Messier objects,[9] 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

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".[10][11] 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 German astronomer Simon Marius on December 15, 1612.[12] 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.[13]

William Huggins in 1864 observed the spectrum of M31 and noted that it differed from a gaseous nebula.[14] 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".[15]

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.[17] 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.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[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 Template:Convert, moving in the direction of the Sun.[18]

Island universe

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 Template:Convert. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.[19]

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 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 method to estimate the distance of M31 using the measured velocities of its stars. His result put the Andromeda Nebula far outside our Galaxy at a distance of about Template:Convert.[20] 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.[21]

Stars in the Andromeda Galaxy's disc[22]

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.)[23] 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.[24]

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[25][26] (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.[27] 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.[28]

General

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.[29][30]

Formation and history

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.[31]

A paper published in 2012[32] 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

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 Template:Convert 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.[33][34]

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,Template:Efn 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 Template:Convert and the whole Andromeda Galaxy at about Template:Convert.[1] 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 Template:Convert.[35]

Averaged together, all these distance measurements give a combined distance estimate of Template:Convert.Template:Efn Based upon the above distance, the diameter of M31 at the widest point is estimated to be Template:Convert. Applying trigonometry (arctangent), that figures to extending at an apparent 3.18° angle in the sky.

Mass and luminosity estimates

Mass

Mass estimates for the Andromeda Galaxy's halo (including dark matter) give a value of approximately Template:Solar mass[2] (or 1.5 trillion solar masses) compared to Template:Solar mass 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[36] and its galactic stellar disk is about twice the size of that of the Milky Way.[37]

Luminosity

M31 appears to have significantly more common stars than the Milky Way, and the estimated luminosity of M31, Template:Solar luminosity, is about 25% higher than that of our own galaxy.[38] 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[39]) , 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,Template:Efn compared to −20.9 for the Milky Way), and a total luminosity in that wavelength of Template:Solar luminosity[40]

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.[41] 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.[38] 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.[42]

Structure

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)

File:A Swift Tour of M31.OGG

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.[43] 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.[44]

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.[37] 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 Template:Convert in diameter. Previously, estimates of the Andromeda Galaxy's size ranged from Template:Convert 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.[45] 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 Template:Convert at a radius of Template:Convert, then descends to a minimum at Template:Convert where the rotation velocity may be as low as Template:Convert. Thereafter the velocity steadily climbs again out to a radius of Template:Convert, where it reaches a peak of Template:Convert. The velocities slowly decline beyond that distance, dropping to around Template:Convert at Template:Convert. These velocity measurements imply a concentrated mass of about Template:Solar mass in the nucleus. The total mass of the galaxy increases linearly out to Template:Convert, then more slowly beyond that radius.[46]

The spiral arms of M31 are outlined by a series of H II regions, first studied in great detail by Walter Baade and described by him as resembling "beads on a string". his studies show two spiral arms that appear to be tightly wound, although they are more widely spaced than in our galaxy.[47] His descriptions of the spiral structure, as each arm crosses the major axis of M31, are as follows[48]§pp1062[49]§pp92:

Baade's spiral arms of M31
Arms (N=cross M31's major axis at north, S=cross M31's major axis at south) Distance from center (arcminutes) (N*/S*) Distance from center (kpc) (N*/S*) Notes
N1/S1 3.4/1.7 0.7/0.4 Dust arms with no OB associations of HII regions.
N2/S2 8.0/10.0 1.7/2.1 Dust arms with some OB associations.
N3/S3 25/30 5.3/6.3 As per N2/S2, but with some HII regions too.
N4/S4 50/47 11/9.9 Large numbers of OB associations, HII regions, and little dust.
N5/S5 70/66 15/14 As per N4/S4 but much fainter.
N6/S6 91/95 19/20 Loose OB associations. No dust visible.
N7/S7 110/116 23/24 As per N6/S6 but fainter and inconspicuous.

Since the Andromeda Galaxy is seen close to edge-on, however, the studies of its spiral structure are difficult. While as stated above 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 Template:Convert and that can be followed outward from a distance of roughly Template:Convert from the core, other alternative spiral structures have been proposed such as a single spiral arm[50] or a flocculent[51] pattern of long, filamentary, and thick spiral arms.[43][52]

The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellites M32 and M110.[53] This can be seen by the displacement of the neutral hydrogen clouds from the stars.[54]

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 Template:Convert from the core.[55] 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.[53]

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.[56] 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.[57]

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.[36] 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.[58] The stars in the extended halos of M31 and the Milky Way may extend nearly one-third the distance separating the two galaxies.

Nucleus

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.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[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 Template:Convert. 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 Template:Solar mass in 1993,[59] and at 1.1–2.3 × 108 Template:Solar mass in 2005.[60] The velocity dispersion of material around it is measured to be ≈ 160 km/s.[61]

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.[62] 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.[63]

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,[64] 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.[62]

Discrete sources

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.[65] A balloon flight on October 20, 1970, set an upper limit for detectable hard X-rays from M31.[66]

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.[67]

There are approximately 460 globular clusters associated with the Andromeda Galaxy.[68] 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.[69] 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.[70] The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half.[11] 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;[71] however other studies have shown is actually similar in properties to G1.[72]

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[73]

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.[74]

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 . 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.[75]

Satellites

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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.[76]

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.[77] M110 does contain a dusty lane, which may indicate recent or ongoing star formation.[78]

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.[79]

Future collision with the Milky Way

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The Andromeda Galaxy is approaching the Milky Way at about Template:Convert.[80] It has been measured approaching relative to our sun at around Template:Convert[43] as the sun orbits around the center of our galaxy at a speed of approximately Template:Convert. 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[81] or perhaps even a large disk galaxy.[8] 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.[82]

See also

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File:DPAG 1999 2077 Andromeda-Galaxie.jpg
The Andromeda Galaxy on a German postage stamp of 1999

Notes

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References

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External links

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