Collision of Andromeda and the Milky Way

Almost all galaxies are moving away from us because of the Hubble expansion of the Universe. However, some nearby galaxies can actually move toward us because of local motion over and above that associated with the expansion of the Universe (Recall that this is called peculiar motion).

A Big Sb Spiral in our Future

One example of peculiar motion is that of the Local Group Sb spiral galaxy Andromeda, shown in the top right image. Andromeda is presently about 780 kpc away, but is approaching us with a radial velocity of -300 km/s (the negative sign indicating radial motion toward us). Based on this motion, it appears that Andromeda and the Milky Way will collide in about 2-3 billion years because of their mutual gravitation interaction (the interaction of Andromeda and the Milky Way dominates the structure of the Local Group of galaxies, since they are by far the most massive galaxies in it).

Simulation of the Collision

Here is a computer simulation of the predicted collision between Andromeda and the Milky Way. Notice in this simulation that the two galaxies pass through each other and fling out great streamers of matter before colliding again and beginning to merge into a single galaxy. As we shall discuss later in conjunction with the evolution of galaxies, in a collision between galaxies the individual stars are very unlikely to collide with each other directly. The interaction is mostly gravitational between the stars, but the compression of gas and dust as the galaxies pass through each other will likely trigger strong star formation.