The Milky Way and Andromeda collision is a predicted galaxy collision that is due to take place in approximately 3 billion years' time between the two largest galaxies in the Local Group – the Andromeda Galaxy and the Milky Way which contains the Earth.

Illustration

It is often used as an example of the kind of phenomena associated with such collisions in simulations. As with all such collisions, it is unlikely that objects such as stars contained within each galaxy will actually collide, as galaxies are in fact very diffuse—the nearest star to the Sun is in fact almost thirty million solar diameters away from the Earth. (If the sun were scaled to the size of an American quarter, 24.26 mm (0.955 in), the next closest quarter/star would be 700 km (475 miles) away.) If the theory is correct, the stars and gas contained in Andromeda will be visible to a naked-eye viewer in approximately 2 billion years. If the collision occurs, the galaxies will likely merge into one larger galaxy. Various names have been proposed for the resulting merged galaxy, the most dominant being Milkomeda.

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 (or "proper motion") 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. This suggests that the dark matter halos, although possibly not the actual disks, of the galaxies will collide. A future European Space Agency spacecraft, the Gaia mission, expected to launch around 2011, is intended to measure the positions of stars in the Andromeda galaxy with sufficient precision to pin down the transverse velocity.

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.

Such collisions are relatively common however. Andromeda, for example, is believed to have collided with at least one other galaxy in the past.

The fate of the Solar System

It is possible, but not certain, that our Solar System may 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.

If the Solar System moves towards the centre of the collision life on Earth could end catastrophically. Though the same author suggests a more peaceful merger of the two galaxies in a scientific paper.