Andromeda–Milky Way collision

The Andromeda–Milky Way collision is a predicted galaxy collision that is expected to 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.

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.

Up to 2012, there was no way to know whether the possible collision was definitely going to happen or not. Frank Summers of the Space Telescope Science Institute 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.

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

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. 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. They also predict a 12% chance that the Solar System will be ejected from the new galaxy some time during the collision. 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.

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.

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.