The fact that the Andromeda Galaxy is heading towards us has been known since 1912, when American astronomer Vesto Slipher noticed that the light from the galaxy was shifted to shorter wavelengths due to the Doppler effect, much like the siren of an oncoming ambulance at a higher altitude. However, at that time, there was still the idea that galaxies were “nebulae” near the Milky Way. It was only much later that the Andromeda Galaxy was revealed to be a cousin of the Milky Way, heading almost directly towards us at a distance of more than 2.5 million light-years and at a speed of 110 kilometers per second.
A 2008 study suggested that Andromeda and the Milky Way will inevitably collide in about five billion years. But such predictions are highly unreliable, because the motions and masses of galaxies are difficult to determine precisely. An additional complicating factor is the gravitational interactions between Andromeda, the Milky Way, and smaller galaxies in the so-called Local Group, a collection of about a hundred galaxies, the largest of which are Andromeda and the Milky Way.
No way out
In the new study, Till Saawala of the University of Helsinki and colleagues used the latest and most accurate motions and masses of the four largest galaxies in the Local Group. They first created a computer simulation using just the Milky Way and Andromeda. They found that the chance of a collision was just under 50 percent. But after they also included the gravitational influence of the Triangulum Galaxy — the third-largest galaxy in the Local Group — the probability of a collision rose to about 67 percent. However, after adding the Large Magellanic Cloud — the fourth-largest member of the Local Group — the probability of a collision dropped again to 50 percent. If it did happen, it wouldn’t happen for another eight billion years.
Moreover, the galaxies of the Local Group have such a tight grip on each other that in tens of billions of years they will merge anyway to form one large elliptical galaxy: no way out.
