View source for Astronomy:Andromeda–Milky Way collision

{{short description|Predicted galactic collision}}
<div class ="noresize">
[[File:Andromeda and Milky Way collision.ogv|thumb|upright=1.5|A [[Organization:NASA|NASA]] conception of the collision using [[Computer-generated imagery|computer-generated imagery]]]]
</div>

The '''Andromeda–Milky Way collision''' is a [[Astronomy:Interacting galaxy|galactic collision]] predicted to occur in about 4.5 billion years between the two largest galaxies in the [[Astronomy:Local Group|Local Group]]—the [[Astronomy:Milky Way|Milky Way]] (which contains the [[Astronomy:Solar System|Solar System]] and [[Astronomy:Earth|Earth]]) and the [[Astronomy:Andromeda Galaxy|Andromeda Galaxy]].<ref name=Sohn>{{cite journal|author1=Sangmo Tony Sohn|author2=Jay Anderson|author3=Roeland van der Marel|title=The M31 velocity vector. I. The Hubble Space Telescope proper-motion measurements|journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]]|volume=753|issue=1|pages=7|date=Jul 1, 2012|doi=10.1088/0004-637X/753/1/7|arxiv = 1205.6863 |bibcode = 2012ApJ...753....7S |s2cid=53071357 }}</ref><ref name="nature1">{{Cite journal |last=Cowen |first=Ron |doi=10.1038/nature.2012.10765 |title = Andromeda on collision course with Milky Way |journal=Nature |date=31 May 2012 |s2cid=124815138 |url=http://www.nature.com/news/andromeda-on-collision-course-with-the-milky-way-1.10765}}</ref><ref name="muir">{{cite web |first=Hazel |last=Muir |publisher=New Scientist |date=2007-05-14 |title=Galactic merger to 'evict' Sun and Earth |url=https://www.newscientist.com/article/dn11852-galactic-merger-to-evict-the-sun-and-the-earth.html#.VDVdDvl_uVB |access-date=2014-10-07 |archive-url=https://web.archive.org/web/20140420022909/http://www.newscientist.com/article/dn11852-galactic-merger-to-evict-sun-and-earth.html#.VDVeCfl_uVA |archive-date=20 April 2014 |url-status=live}}</ref><ref>{{cite magazine |last1=Cox|first1=T. J.|last2=Loeb|first2=Abraham |date=June 2008 |title=Our galaxy's collision with Andromeda |magazine=[[Astronomy:Astronomy (magazine)|Astronomy]] |issn=0091-6358 |page=28}}</ref><ref name="Universe Today">{{cite web |last1=Gough |first1=Evan |title=Universe Today |date=12 February 2019 |url=https://www.universetoday.com/141471/thanks-to-gaia-we-now-know-exactly-when-well-be-colliding-with-andromeda/ |publisher=The Astrophysical Journal |access-date=12 February 2019 |ref=utgaia}}</ref> The stars involved are sufficiently far apart that it is improbable that any of them will individually collide,<ref name="NASA">{{cite web |url=http://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html |author=NASA |title=NASA's Hubble Shows Milky Way is Destined for Head-On Collision |work=NASA |date=2012-05-31 |access-date=2012-10-13 |archive-url=https://web.archive.org/web/20140701085917/http://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html |archive-date=1 July 2014}}</ref> though some stars will be ejected.<ref>{{cite web|author=Jason Kottke|title=Behold Our Dazzling Night Sky When the Milky Way Collides with Andromeda in 4 Billion Years|url=https://kottke.org/19/10/behold-our-dazzling-night-sky-when-the-milky-way-collides-with-andromeda-in-4-billion-years}}</ref>{{sps|date=May 2022|certain=y}}

== Certainty ==
[[File:Andromeda and Milky Way collision sequence.jpg|thumb|303x303px|This series of photo illustrations shows the predicted merger between the [[Astronomy:Milky Way|Milky Way galaxy]] and the neighboring Andromeda Galaxy.]]
The Andromeda Galaxy is approaching the Milky Way at about 300 km/s (200 miles per second)<ref name="nature1"/><ref>{{Cite web|url=https://science.nasa.gov/science-news/science-at-nasa/2012/31may_andromeda|title=Astronomers Predict Titanic Collision: Milky Way vs. Andromeda {{!}} Science Mission Directorate|website=science.nasa.gov|language=en|access-date=2018-10-13}}</ref> as indicated by [[Astronomy:Blueshift|blueshift]]. However, the lateral speed (measured as [[Astronomy:Proper motion|proper motion]]) is very difficult to measure with sufficient precision to draw reasonable conclusions. Until 2012, it was not known whether the possible collision was definitely going to happen or not.<ref name=vanderMarel2012/> Researchers then used the [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]] to measure the positions of stars in Andromeda in 2002 and 2010, relative to hundreds of distant background galaxies. By averaging over thousands of stars, they were able to obtain the average proper motion with sub-pixel accuracy. The conclusion was that Andromeda is moving southeast in the sky at less than 0.1 milliarc-seconds per year, corresponding to a speed relative to the Sun of less than 200 km/s towards the south and towards the east. Taking also into account the Sun's motion, Andromeda's tangential or sideways velocity with respect to the Milky Way was found to be much smaller than the speed of approach (consistent with zero given the uncertainty) and therefore it will eventually merge with the Milky Way in around 5 billion years.<ref name=Sohn/><ref>{{cite journal |display-authors=0|last1=Roeland P. van der Marel |title=The M31 velocity vector. II. Radial orbit toward the Milky Way and implied Local Group mass |journal=The Astrophysical Journal |date=Jun 8, 2012 |volume=753 |issue=1 |page=8 |doi=10.1088/0004-637X/753/1/8 |arxiv=1205.6864 |bibcode=2012ApJ...753....8V |s2cid=26797522 |url=https://iopscience.iop.org/article/10.1088/0004-637X/753/1/8/pdf}}</ref><ref name="nature1"/>

Such collisions are relatively common, considering galaxies' long lifespans. Andromeda, for example, is believed to have collided with at least one other galaxy in the past,<ref>
{{cite web |publisher=MSNBC |date=2007-01-29 |title=Andromeda involved in galactic collision |url=http://www.msnbc.msn.com/id/16872449/ |access-date=2014-10-07 |archive-url=https://web.archive.org/web/20140831160840/http://www.sciencedaily.com/releases/2009/12/091203101426.htm |archive-date=31 August 2014 |url-status=dead}}</ref> and several dwarf galaxies such as [[Astronomy:Sagittarius Dwarf Spheroidal Galaxy|Sgr dSph]] are currently colliding with the Milky Way and being merged into it.

The studies also suggest that M33, the [[Astronomy:Triangulum Galaxy|Triangulum Galaxy]]—the third-largest and third-brightest galaxy of the Local Group—will participate in the collision event, too. Its most likely fate is to end up orbiting the merger remnant of the Milky Way and Andromeda galaxies and finally to merge with it in an even more distant future. However, a collision with the Milky Way, before it collides with the Andromeda Galaxy, or an ejection from the Local Group cannot be ruled out.<ref name=vanderMarel2012>{{cite journal |first1=Roeland P. |last1=van der Marel |first2=Gurtina |last2=Besla |first3=T.J. |last3=Cox |first4=Sangmo Tony |last4=Sohn |first5=Jay |last5=Anderson |title=The M31 Velocity Vector. III. Future Milky Way-M31-M33 Orbital Evolution, Merging, and Fate of the Sun |journal=The Astrophysical Journal |date=1 July 2012 |volume=753 |issue=9 |pages=9 |doi=10.1088/0004-637X/753/1/9 |arxiv=1205.6865 |bibcode=2012ApJ...753....9V |s2cid=53071454 }}</ref>

== Stellar collisions ==
While the Andromeda Galaxy contains about 1 trillion ({{10^|12}}) [[Astronomy:Star|star]]s and the Milky Way contains about 300 billion (3{{e|11}}), the chance of even two stars colliding is negligible because of the huge distances between the stars. For example, the nearest star to the [[Astronomy:Earth|Earth]] after the [[Astronomy:Sun|Sun]] is [[Astronomy:Proxima Centauri|Proxima Centauri]], about {{convert|4.2|ly|km mi}} or 30 million (3{{e|7}}) solar diameters away. 

To visualize that scale, if the Sun were a ping-pong ball, Proxima Centauri would be a pea about {{cvt|1100|km}} away, and the Milky Way would be about {{Convert|30|e6km|e6mi|abbr=unit}} wide. Although stars are more common near the centers of each galaxy, the average distance between stars is still 160 billion (1.6{{e|11}}) km (100 billion mi). That is analogous to one ping-pong ball every {{cvt|3.2|km|0}}. Thus, it is extremely unlikely that any two stars from the merging galaxies would collide.<ref name="NASA"/>

== Black hole collisions ==
The Milky Way and Andromeda galaxies each contain a central [[Astronomy:Supermassive black hole|supermassive black hole]] (SMBH), these being [[Astronomy:Sagittarius A*|Sagittarius A*]] (c. {{Val|3.6|e=6|ul=solar mass}}) and an object within the [[Astronomy:Andromeda Galaxy#Nucleus|P2 concentration of Andromeda's nucleus]] ({{Val|2|p=1–|e=8|u=solar mass}}). These black holes will converge near the centre of the newly formed galaxy over a period that may take millions of years, due to a process known as [[Astronomy:Dynamical friction|dynamical friction]]: as the SMBHs move relative to the surrounding cloud of much less massive stars, gravitational interactions lead to a net transfer of orbital energy from the SMBHs to the stars, causing the stars to be [[Astronomy:Gravity assist|"slingshotted"]] into higher-radius orbits, and the SMBHs to "sink" toward the galactic core. When the SMBHs come within one light-year of one another, they will begin to strongly emit [[Astronomy:Gravitational wave|gravitational wave]]s that will radiate further orbital energy until they merge completely. Gas taken up by the combined black hole could create a luminous [[Astronomy:Quasar|quasar]] or an [[Active galactic nucleus|active galactic nucleus]], releasing as much energy as 100 million [[Astronomy:Supernova|supernova]] explosions.<ref>{{cite news|first=Denis |last=Overbye | title=More Evidence for Coming Black Hole Collision |work=The New York Times |date=September 16, 2015 |url=https://www.nytimes.com/2015/09/22/science/space/more-evidence-for-coming-black-hole-collision.html}}</ref><ref>{{cite press release |author=<!--Not stated--> |title=Astronomers solve the 60-year mystery of quasars – the most powerful objects in the Universe |url=https://www.sheffield.ac.uk/news/astronomers-solve-60-year-mystery-quasars-most-powerful-objects-universe |publisher=University of Sheffield |date=2023-04-26 |access-date=2023-09-10 }}</ref> As of 2006, simulations indicated that the Sun might be brought near the centre of the combined galaxy, potentially coming near one of the black holes before being ejected entirely out of the galaxy.<ref>{{cite journal |url=http://acme.highpoint.edu/~afuller/PHY-1050/resources/GreatMilkyWayAndromedaCollision.pdf |title=The Great Milky Way-Andromeda Collision |first=John |last=Dubinski |journal=Sky & Telescope |date=October 2006 |volume=112 |issue=4 |pages=30–36 |bibcode=2006S&T...112d..30D |access-date=2014-10-06 |archive-url=https://web.archive.org/web/20150824174913/http://acme.highpoint.edu/~afuller/PHY-1050/resources/GreatMilkyWayAndromedaCollision.pdf |archive-date=2015-08-24 |url-status=dead }}</ref> Alternatively, the Sun might approach one of the black holes a bit closer and be torn apart by its gravity. Parts of the former Sun would be pulled into the black hole.<ref>{{cite news |first=Peter |last=Jonkler |title=Unique SOS Signal from Torn Apart Star Points to Medium-Size Black Hole |work=Astrophysics |date=November 6, 2013 |publisher=Radboud University |url=http://www.ru.nl/astrophysics/news-agenda/news/@924091/unique-sos-signal/ |access-date=August 13, 2017 |archive-url=https://web.archive.org/web/20180809112034/https://www.ru.nl/astrophysics/news-agenda/news/@924091/unique-sos-signal/ |archive-date=August 9, 2018 |url-status=dead }}</ref>

== Fate of the Solar System ==

Two scientists with the [[Organization:Harvard–Smithsonian Center for Astrophysics|Harvard–Smithsonian Center for Astrophysics]] stated that when, and even whether, the two galaxies collide will depend on Andromeda's transverse velocity.<ref name="muir"/> Based on current calculations they predict a 50% chance that in a merged galaxy, the Solar System will be swept out three times farther from the galactic core than its current distance.<ref name="muir"/> They also predict a 12% chance that the Solar System will be ejected from the new galaxy sometime during the collision.<ref name=Cain>{{cite web|title=When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?|author=Cain, Fraser | work=Universe Today|url=http://www.universetoday.com/2007/05/10/when-our-galaxy-smashes-into-andromeda-what-happens-to-the-sun/|date=2007|access-date=2007-05-16| archive-url= https://web.archive.org/web/20070517021426/http://www.universetoday.com/2007/05/10/when-our-galaxy-smashes-into-andromeda-what-happens-to-the-sun/| archive-date= 17 May 2007 | url-status= live}}</ref><ref name=Cox2008>{{cite journal|title=The Collision Between The Milky Way And Andromeda | author=Cox, T. J. | author2=Loeb, Abraham | journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] | arxiv=0705.1170 | date=2008 | doi=10.1111/j.1365-2966.2008.13048.x|volume=386|issue=1 | pages=461–474 | bibcode=2008MNRAS.386..461C| s2cid=14964036 }}</ref> Such an event would have no adverse effect on the system and the chances of any sort of disturbance to the Sun or planets themselves may be remote.<ref name=Cain/><ref name=Cox2008/>

Excluding [[Engineering:Planetary engineering|planetary engineering]], by the time the two galaxies collide, the surface of the Earth will have already become far too hot for liquid water to exist, ending all terrestrial life; that is currently estimated to occur in about 0.5 to 1.5 billion years due to gradually increasing luminosity of the Sun; by the time of the collision, the Sun's luminosity will have risen by 35-40%, likely initiating a [[Earth:Runaway greenhouse effect|runaway greenhouse effect]] on the planet by this time.<ref name=Schroeder>{{cite journal |last1=Schröder |first1=K.-P. |last2=Smith |first2=R. C. |title=Distant future of the Sun and Earth revisited |journal=Monthly Notices of the Royal Astronomical Society |volume=386 |issue=1 |page=155 |date=2008 |doi=10.1111/j.1365-2966.2008.13022.x |arxiv=0801.4031 |bibcode=2008MNRAS.386..155S|s2cid=10073988 }}</ref><ref>{{cite news |first=D. |last=Carrington |title=Date set for desert Earth |url=http://news.bbc.co.uk/1/hi/sci/tech/specials/washington_2000/649913.stm |work=BBC News |access-date=2007-03-31 | date=2000-02-21 |archive-url=https://web.archive.org/web/20140712074821/http://news.bbc.co.uk/2/hi/sci/tech/specials/washington_2000/649913.stm |archive-date=12 July 2014 }}</ref>

== Possible triggered stellar events ==
When two spiral galaxies collide, the [[Chemistry:Hydrogen|hydrogen]] present on their disks is compressed, producing [[Astronomy:Starburst galaxy|strong star formation]] as can be seen on interacting systems like the [[Astronomy:Antennae Galaxies|Antennae Galaxies]]. In the case of the Andromeda–Milky Way collision, it is believed that there will be little gas remaining in the disks of both galaxies, so the mentioned starburst will be relatively weak, though it still may be enough to form a [[Astronomy:Quasar|quasar]].<ref name=Cox2008/>

== Merger remnant ==
The galaxy product of the collision has been named ''Milkomeda'' or ''Milkdromeda''.<ref>{{Cite web|title=Milkomeda, Our Future Home |url=https://www.cfa.harvard.edu/news/su200822 |publisher=[[Organization:Harvard–Smithsonian Center for Astrophysics|Harvard–Smithsonian Center for Astrophysics]] |date=2008-05-27 |access-date=2015-09-27 |url-status=dead |archive-url=https://web.archive.org/web/20150928082648/https://www.cfa.harvard.edu/news/su200822 |archive-date=2015-09-28}}</ref> According to simulations, this object is likely to be a giant [[Astronomy:Elliptical galaxy|elliptical galaxy]], but with a centre showing less stellar density than current elliptical galaxies.<ref name=Cox2008/> It is, however, possible the resulting object will be a large [[Astronomy:Lenticular galaxy|lenticular]] or [https://www.nasa.gov/feature/jpl/astronomers-discover-colossal-super-spiral-galaxies super spiral galaxy], depending on the amount of remaining gas in the Milky Way and Andromeda.<ref name="Ueda2014">{{cite journal
 | author=Junko Ueda
 | title=Cold molecular gas in merger remnants. I. Formation of molecular gas disks
 | journal=The Astrophysical Journal Supplement Series 
 | volume=214
 | issue=1
 | pages=1
 | bibcode= 2014ApJS..214....1U
 | doi=10.1088/0067-0049/214/1/1|display-authors=0|arxiv = 1407.6873 | year=2014
 | s2cid=716993
 }}</ref>

[[Unsolved:Timeline of the far future|Over the course of the next 150 billion years]], the remaining galaxies of the [[Astronomy:Local Group|Local Group]] will coalesce into this object, effectively completing its evolution.<ref>{{cite journal
 | author= Fred C. Adams
 | author2= Gregory Laughlin
 | title=A dying universe: the long-term fate and evolution of astrophysical objects
 | journal=Reviews of Modern Physics
 | date=1997
 | volume=69
 | issue=2
 | pages=337–372
 | bibcode=1997RvMP...69..337A
 | doi=10.1103/RevModPhys.69.337
 | arxiv=astro-ph/9701131 | s2cid= 12173790
 }}</ref>

== See also ==
*[[Astronomy:NGC 2207 and IC 2163|NGC 2207 and IC 2163]]
*[[Astronomy:Mayall's Object|Mayall's Object]]

==References==
{{reflist|30em}}

==External links==
* {{cite web|last=Merrifield|first=Michael|title=Milky Way vs Andromeda: The ultimate head-on crash|url=http://www.deepskyvideos.com/videos/other/andromeda_collision.html|work=Deep Space Videos|publisher=[[Biography:Brady Haran|Brady Haran]]}}

{{Andromeda galaxy}}
{{Milky Way}}
{{DEFAULTSORT:Andromeda-Milky Way collision}}
[[Category:Interacting galaxies]]
[[Category:Milky Way]]
[[Category:Andromeda Galaxy]]
[[Category:Local Group]]
[[Category:Hypothetical impact events]]
[[Category:Astronomical events]]

{{Sourceattribution|Andromeda–Milky Way collision}}