Andromeda–Milky Way collision

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A NASA conception of the collision using computer-generated imagery.

The Andromeda–Milky Way collision is a galactic collision predicted to occur in about 4 billion years between two galaxies in the Local Group—the Milky Way (which contains the Solar System and Earth) and the Andromeda Galaxy.[1][2][3][4] The stars involved are sufficiently far apart that it is improbable that any of them will individually collide.[5] Some stars will be ejected from the resulting galaxy, nicknamed Milkomeda or Milkdromeda.


Based on data from the Hubble Space Telescope, the Milky Way galaxy (pictured right-center) and Andromeda galaxy (left-center) are predicted to distort each other with tidal pull in 3.75 billion years, as shown in this illustration.

The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second (68 mi/s)[2][6] as indicated by blueshift. However, the lateral velocity is very difficult to measure with a precision to draw reasonable conclusions: a lateral speed of only 7.7 km/s would mean that the Andromeda Galaxy is moving toward a point 177,800 light-years to the side of the Milky Way ((7.7 km/s) / (110 km/s) × (2 540 000 ly)), and such a speed over an eight-year time frame amounts to only 1/3000 of a Hubble Space Telescope pixel (Hubble's resolution≈0.05 arcsec: (7.7 km/s)/(300 000 km/s)×(8 y)/(2 540 000 ly)×180°/π×3600 = 0.000 017 arcsec). Until 2012, it was not known whether the possible collision was definitely going to happen or not.[7] In 2012, researchers concluded that the collision is sure using Hubble to track the motion of stars in Andromeda between 2002 and 2010 with sub-pixel accuracy.[1][2] Andromeda's tangential or sideways velocity with respect to the Milky Way was found to be much smaller than the speed of approach and therefore it is expected that it will directly collide with the Milky Way in around four 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,[8] 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.[7]

Stellar collisions[edit]


Black hole collisions[edit]

The Milky Way and Andromeda galaxies each contain a central supermassive black hole (SMBH), these being Sagittarius A* (ca. 3.6×106 M) and an object within the P2 concentration of Andromeda's nucleus (1–2×108 M). These black holes will converge near the center 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.[9] As of 2006, simulations indicated that the Sun might be brought near the center of the combined galaxy, potentially coming near one of the black holes before being ejected entirely out of the galaxy.[10] 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.[11]

Fate of the Solar System[edit]

Two scientists with the Harvard–Smithsonian Center for Astrophysics stated that when, and even whether, the two galaxies collide will depend on Andromeda's transverse velocity.[3] 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.[3] They also predict a 12% chance that the Solar System will be ejected from the new galaxy sometime during the collision.[12][13] 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.[12][13]


Possible triggered stellar events[edit]


Merger remnant[edit]

The galaxy product of the collision has been nicknamed Milkomeda or Milkdromeda.[16] According to simulations, this object will look like a giant elliptical galaxy, but with a center showing less stellar density than current elliptical galaxies.[13] It is, however, possible the resulting object will be a large disk galaxy, depending on the amount of remaining gas in the Milky Way and Andromeda.[17]

In the far future, roughly 150 billion years from now, the remaining galaxies of the Local Group will coalesce into this object, that being the next evolutionary stage of the local group of galaxies.[18]

See also[edit]


  1. ^ a b Sangmo Tony Sohn; Jay Anderson; Roeland van der Marel (Jul 1, 2012). "The M31 velocity vector. I. Hubble Space Telescope proper-motion measurements". The Astrophysical Journal. arXiv:1205.6863Freely accessible. Bibcode:2012ApJ...753....7S. doi:10.1088/0004-637X/753/1/7. 
  2. ^ a b c Cowen, Ron (31 May 2012). "Andromeda on collision course with the Milky Way". Nature. doi:10.1038/nature.2012.10765. 
  3. ^ a b c 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. 
  6. ^
  7. ^ a b 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): 1–21. arXiv:1205.6865Freely accessible. Bibcode:2012ApJ...753....9V. doi:10.1088/0004-637X/753/1/9. 
  9. ^ Denis Overbye, “More Evidence for Coming Black Hole Collision,” New York Times, (September 16, 2015),
  10. ^ Dubinski, John (October 2006). "The Great Milky Way-Andromeda Collision" (PDF). Sky & Telescope. pp. 30–36. 
  11. ^ Peter Jonkler, “Unique SOS Signal from Torn Apart Star Points to Medium-Size Black Hole,” Astrophysics, (November 6, 2013, Radboud University),
  13. ^ a b c d 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:0705.1170Freely accessible. Bibcode:2008MNRAS.tmp..333C. doi:10.1111/j.1365-2966.2008.13048.x. 
  14. ^ 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:0801.4031Freely accessible. Bibcode:2008MNRAS.386..155S. doi:10.1111/j.1365-2966.2008.13022.x. 
  16. ^ "Milkomeda, Our Future Home". Retrieved 2015-09-27. 
  17. ^ Junko Ueda; et al. "Cold molecular gas in merger remnants. I. Formation of molecular gas disks". The Astrophysical Journal Supplement Series. 214 (1). arXiv:1407.6873Freely accessible. Bibcode:2014ApJS..214....1U. doi:10.1088/0067-0049/214/1/1. 

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