Andromeda–Milky Way collision

The Andromeda–Milky Way collision is a predicted galaxy collision that will take place in approximately 4 billion years' time between the two largest galaxies in the Local Group—the Andromeda Galaxy and the Milky Way, which contains the Solar System and Earth.^[1][2][3]

Stellar collisions

While the Andromeda Galaxy contains about one trillion (10¹²) stars and the Milky Way contains about three hundred billion (3x10¹¹); the chance of even two stars colliding is negligible because of the huge distances between each star. For example, the nearest star to the Sun is Proxima Centauri, about 3x10⁷ solar diameters (4x10¹³ km or 4.2 ly) away. If the Sun were a ping-pong ball in Paris, the equivalent Proxima Centauri would be a pea-sized ball in Berlin (and our galaxy would be about 1.9x10⁷ km wide, about a third of the distance to Mars).

Stars are much denser near the centres of each galaxy, the average separation is only 1.6x10¹¹ km. But that is still a density which represents one ping-pong ball every 3.2 km. Thus, it is extremely unlikely that any two stars may 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.^[4] 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.^[5]

Such collisions are relatively common, however. Andromeda, for example, is believed to have collided with at least one other galaxy in the past,^[6] and several dwarf galaxies such as SagDEG are currently colliding with the Milky Way and being merged into it.

In 2012 researchers came to conclusion that it is a definite hit after using the Hubble Space Telescope between 2002 and 2010 to painstakingly track the motion of Andromeda.^[7]

The fate of the Solar System

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.^[8] 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.^[8][9]

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.^[10][11]

Possible quasars

It is believed that the collision between the Milky Way and Andromeda Galaxies could create a quasar.

The quasar, if it were to be created at the center of the Andromeda Galaxy, would be visible from Earth, and would be as bright as the Full Moon despite being 10,000 light years away, but we would not be able to see its accretion disk and it would only appear as a blinking star due to atmospheric fluctuations.

However, if a quasar were to be created at the center of our own Milky Way, we would not be able to see it due to all the dust between our planet and the galactic center. A quasar at the center of the Andromeda Galaxy would be brighter than one at the center of the Milky Way since the black hole at the center of Andromeda is larger than our own galactic center black hole. Of course, as mentioned before, all life on Earth would have been wiped out by this point even if such phenomena were to happen.^{[12][13][14][15]}

See also

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

References