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The Andromeda–Milky Way collision is a galactic collision predicted to occur in about 4.5 billion years between the two largest galaxies in the Local Group—the Milky Way (which contains the Solar System and Earth) and the Andromeda Galaxy. The stars involved are sufficiently far apart that it is improbable that any of them will individually collide, though some stars will be ejected.

Certainty

The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second (68.4 mi/s) as indicated by blueshift. However, the lateral speed (measured as 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. Researchers then used the 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.

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, and several dwarf galaxies such as Sgr dSph are currently colliding with the Milky Way and being merged into it.

The studies also suggest that M33, the 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.

Stellar collisions

While the Andromeda Galaxy contains about 1 trillion (10) stars and the Milky Way contains about 300 billion (3×10), the chance of even two stars colliding is negligible because of the huge distances between the stars. For example, the nearest star to the Earth after the Sun is Proxima Centauri, about 4.2 light-years (4.0×10 km; 2.5×10 mi) or 30 million (3×10) solar diameters away.

To visualize that scale, if the Sun were a ping-pong ball, Proxima Centauri would be a pea about 1,100 km (680 mi) away, and the Milky Way would be about 30 million km (19 million mi) wide. Although stars are more common near the centers of each galaxy, the average distance between stars is still 160 billion (1.6×10) km (100 billion mi). That is analogous to one ping-pong ball every 3.2 km (2 mi). Thus, it is extremely unlikely that any two stars from the merging galaxies would collide.

Black hole collisions

The Milky Way and Andromeda galaxies each contain a central supermassive black hole (SMBH), these being Sagittarius A* (c. 3.6×10 M_☉) and an object within the P2 concentration of Andromeda's nucleus (1–2×10 M_☉). 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 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 "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 gravitational waves that will radiate further orbital energy until they merge completely. Gas taken up by the combined black hole could create a luminous quasar or an active galactic nucleus, releasing as much energy as 100 million supernova explosions. 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. 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.

Fate of the Solar System

Based on data available in 2007, two scientists with the Harvard–Smithsonian Center for Astrophysics 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. They also predict a 12% chance that the Solar System will be ejected from the new galaxy sometime during the collision. 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.

Excluding 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 runaway greenhouse effect on the planet by this time.

Possible triggered stellar events

When two spiral galaxies collide, the hydrogen present on their disks is compressed, producing strong star formation as can be seen on interacting systems like the 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 quasar.

Merger remnant

The galaxy product of the collision has been named Milkomeda or Milkdromeda. According to simulations, this object is likely to be a giant elliptical galaxy, but with a centre showing less stellar density than current elliptical galaxies. It is, however, possible the resulting object will be a large lenticular or super spiral galaxy, depending on the amount of remaining gas in the Milky Way and Andromeda.

Over the course of the next 150 billion years, the remaining galaxies of the Local Group will coalesce into this object, effectively completing its evolution.

See also

• NGC 2207 and IC 2163
• Mayall's Object
• Ultimate fate of the universe

References

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2. ^ Cowen, Ron (31 May 2012). "Andromeda on collision course with Milky Way". Nature. doi:10.1038/nature.2012.10765. S2CID 124815138.
3. ^ Muir, Hazel (2007-05-14). "Galactic merger to 'evict' Sun and Earth". New Scientist. Archived from the original on 20 April 2014. Retrieved 2014-10-07.
4. Cox, T. J.; Loeb, Abraham (June 2008). "Our galaxy's collision with Andromeda". Astronomy. p. 28. ISSN 0091-6358.
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7. Jason Kottke (21 October 2019). "Behold Our Dazzling Night Sky When the Milky Way Collides with Andromeda in 4 Billion Years".
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9. ^ van der Marel, Roeland P.; Besla, Gurtina; Cox, T.J.; Sohn, Sangmo Tony; Anderson, Jay (1 July 2012). "The M31 Velocity Vector. III. Future Milky Way-M31-M33 Orbital Evolution, Merging, and Fate of the Sun". The Astrophysical Journal. 753 (9): 9. arXiv:1205.6865. Bibcode:2012ApJ...753....9V. doi:10.1088/0004-637X/753/1/9. S2CID 53071454.
10. van der Marel, Roeland P.; Fardal, Mark; Besla, Gurtina; Beaton, Rachael L.; Sohn, Sangmo Tony; Anderson, Jay; Brown, Tom; Guhathakurta, Puragra; et al. (July 2012). "The M31 velocity vector. II. Radial orbit toward the Milky Way and implied Local Group mass". The Astrophysical Journal. 753 (1): 8. arXiv:1205.6864. Bibcode:2012ApJ...753....8V. doi:10.1088/0004-637X/753/1/8. ISSN 0004-637X. S2CID 26797522.
11. "Andromeda involved in galactic collision". MSNBC. 2007-01-29. Archived from the original on 31 August 2014. Retrieved 2014-10-07.
12. Overbye, Denis (September 16, 2015). "More Evidence for Coming Black Hole Collision". The New York Times.
13. "Astronomers solve the 60-year mystery of quasars – the most powerful objects in the Universe" (Press release). University of Sheffield. 2023-04-26. Retrieved 2023-09-10.
14. Dubinski, John (October 2006). "The Great Milky Way-Andromeda Collision" (PDF). Sky & Telescope. 112 (4): 30–36. Bibcode:2006S&T...112d..30D. Archived from the original (PDF) on 2015-08-24. Retrieved 2014-10-06.
15. Jonkler, Peter (November 6, 2013). "Unique SOS Signal from Torn Apart Star Points to Medium-Size Black Hole". Astrophysics. Radboud University. Archived from the original on August 9, 2018. Retrieved August 13, 2017.
16. ^ Cain, Fraser (2007). "When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?". Universe Today. Archived from the original on 17 May 2007. Retrieved 2007-05-16.
17. ^ Cox, T. J.; Loeb, Abraham (2008). "The Collision Between The Milky Way And Andromeda". Monthly Notices of the Royal Astronomical Society. 386 (1): 461–474. arXiv:0705.1170. Bibcode:2008MNRAS.386..461C. doi:10.1111/j.1365-2966.2008.13048.x. S2CID 14964036.
18. Schröder, K.-P.; Smith, R. C. (2008). "Distant future of the Sun and Earth revisited". Monthly Notices of the Royal Astronomical Society. 386 (1): 155. arXiv:0801.4031. Bibcode:2008MNRAS.386..155S. doi:10.1111/j.1365-2966.2008.13022.x. S2CID 10073988.
19. Carrington, D. (2000-02-21). "Date set for desert Earth". BBC News. Archived from the original on 12 July 2014. Retrieved 2007-03-31.
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21. Junko Ueda; et al. (2014). "Cold molecular gas in merger remnants. I. Formation of molecular gas disks". The Astrophysical Journal Supplement Series. 214 (1): 1. arXiv:1407.6873. Bibcode:2014ApJS..214....1U. doi:10.1088/0067-0049/214/1/1. S2CID 716993.
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External links

• Merrifield, Michael. "Milky Way vs Andromeda: The ultimate head-on crash". Deep Space Videos. Brady Haran.

v t e Andromeda Galaxy
Location	Andromeda Galaxy → Andromeda subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each  → may be read as "within" or "part of".
Satellite galaxies	M32 M110 NGC 185 NGC 147 Andromeda I Andromeda II Andromeda III Andromeda IV* Andromeda V Andromeda VI Andromeda VII Andromeda VIII Andromeda IX Andromeda X Andromeda XI Andromeda XII Andromeda XIII Andromeda XIV Andromeda XV Andromeda XVI Andromeda XVII Andromeda XVIII Andromeda XIX Andromeda XX Andromeda XXI Andromeda XXII Andromeda XXIII Andromeda XXIV Andromeda XXV Andromeda XXVI Andromeda XXVII Andromeda XXVIII Andromeda XXIX Triangulum subgroup* *It is uncertain whether these are companion galaxies of the Andromeda Galaxy
Catalogued stars	AE Andromedae AF Andromedae M31-RV
Other	Andromeda–Milky Way collision S Andromedae Mayall II M32p NGC 206 3XMM J004232.1+411314
Category

v t e Milky Way
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Related	Alternate names Andromeda–Milky Way collision Baade's Window In mythology Zone of Avoidance
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