Andromeda–Milky Way collision

The Andromeda–Milky Way collision is a predicted galaxy collision that could possibly take place in approximately 3 to 5 billion years' time between the two largest galaxies in the Local Group—the Andromeda Galaxy and the Milky Way, which contains Earth.^[1][2]

Stellar Collisions

Each galaxy contains over a hundred billion stars, which is 100 million trillion possible collisions. Nevertheless, the chance of even two stars colliding is negligible because of the huge distances between them. For example, the nearest star to the Sun is Proxima Centauri, about thirty million solar diameters away. If the sun were a ping-pong ball in Paris, the equivalent Proxima would be a pea-sized ball in Berlin (and our galaxy would be 12 million miles wide—about a third of the distance to Mars).

Stars are much denser near the galactic centers—the average separation is only 100 billion miles. But that is still a density which is about equal to one ping-pong ball every two miles. Thus, it is very, very unlikely that any two stars will collide.

Uncertainty

There is, as yet, no way to know whether the possible collision is definitely going to happen or not. The radial velocity of the Andromeda galaxy with respect to the Milky Way can be measured by examining the Doppler shift of spectral lines from stars in the galaxy, but the transverse velocity cannot be directly measured. Thus, while it is known that the Andromeda galaxy is getting closer to the Milky Way by about 120 km/s, there is no way to tell whether it is going to collide or miss. The best indirect estimates of the transverse velocity indicate that it is less than 100 km/s.^[3] This suggests that the dark matter halos, although possibly not the actual disks, of the galaxies will collide.

Frank Summers of the Space Telescope Science Institute has created a CGI visualization of the predicted event, based on research by Professors Chris Mihos of Case Western Reserve University and Lars Hernquist of Harvard University.^[4]

Such collisions are relatively common, however. Andromeda, for example, is believed to have collided with at least one other galaxy in the past,^[5] and several dwarf galaxies such as SagDEG are currently colliding with the Milky Way and being merged into it.

The fate of the Solar System

See also: Formation and evolution of the Solar System#Galactic collision and planetary disruption

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.^[1] 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 it is currently located.^[1] They also predict a 12% chance that the Solar System will be ejected from the new galaxy some time during the collision.^[6] Such an event would have no adverse effect on the system and chances of any sort of disturbance to the Sun or planets themselves may be remote.^[6][7]

Without intervention, by the time that 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, which is currently estimated to occur in about 1.4 billion years due to gradually increasing luminosity of the Sun.^[8][9]

See also

• Antennae Galaxies – A later stage in collision
• NGC 2207 and IC 2163 – An example of two colliding galaxies

References

1. ^ ^{a b c} Hazel Muir, "Galactic merger to 'evict' Sun and Earth," New Scientist 4 May 2007
2. ^ Astronomy, June 2008, page 28, by Abraham Loeb and T. J. Cox
3. ^ Abraham Loeb, Mark J. Reid, Andreas Brunthaler, and Heino Falcke The Astrophysical Journal, 633:894–898, November 10, 2005
4. ^ Hubblesite Newscenter: Hubble Astronomer Creates Spectacular Galaxy Collision Visualization for the National Air and Space Museum
5. ^ "Andromeda involved in galactic collision" MSNBC 10:38 a.m. PT 29 January 2007
6. ^ ^{a b} Cain, Fraser (2007). "When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?". Universe Today. http://www.universetoday.com/2007/05/10/when-our-galaxy-smashes-into-andromeda-what-happens-to-the-sun/. Retrieved 2007-05-16. 
7. ^ Cox, T. J.; Loeb, Abraham (2007). "The Collision Between The Milky Way And Andromeda". Monthly Notices of the Royal Astronomical Society 386: 461. arXiv:0705.1170. Bibcode 2008MNRAS.tmp..333C. doi:10.1111/j.1365-2966.2008.13048.x. 
8. ^ 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. 
9. ^ Carrington, D. (2000-02-21). "Date set for desert Earth". BBC News. http://news.bbc.co.uk/1/hi/sci/tech/specials/washington_2000/649913.stm. Retrieved 2007-03-31.