Today, the Milky Way is 2.5 million light years away from its closest neighbor – Andromeda.  But the distant galaxy is far from stagnant.  Pulled by gravity, Andromeda is on a head-on collision course – with us, moving at a speed of 402,000 kilometers per hour.  The two galaxies will collide in 4 billion years.  Despite the vast distance from Earth, astronomers are able to measure the speed of galaxies by observing their radial velocity.  The Doppler effect reveals that objects moving toward us are blue-shifted.  Thus, when Vesto Slipher observed the light of Andromeda being slightly compressed, in the early 1900s, he concluded that the galaxy, thought to be a nebula at the time, was heading towards us.

"After nearly a century of speculation about the future destiny of Andromeda and our Milky Way, we at last have a clear picture of how events will unfold over the coming billions of years."

-Sangmo Tony Sohn of STScI.

Using years of Hubble Space Telescope observations, Harvard astrophysicists modeled the proper motion (the motion side to side) of Andromeda and found that a galactic collision is inevitable.  Unlike the radial velocity, the proper motion must be calculated by comparing images taken at different times by Hubble to measure how far Andromeda has moved relative to background galaxies.  They also note that the Solar System would likely be located on the outskirts of the new galaxy.

Collisions between galaxies are not uncommon.  One example is presented by the Mice Galaxies, photographed in 2002 by Hubble.  These two spiral galaxies began merging about 290 million years ago and will eventually form a single galaxy.

Although the thought of our galaxy smashing into another seems forbidding, our Solar System will almost definitely survive.  Within galaxies, the stars are so far apart from one another that collisions between them are improbable.  The spiral of the Milky Way and the ellipses of Andromeda will combine to form a giant elliptical galaxy astronomers call ‘Milkomeda.’  The formation will be far from instantaneous.  As they get closer, tidal pull will begin to distort the galaxies.  Within dense gas clouds, stars will form as the two galaxies simultaneously come together and fly apart as they settle into a single large galaxy.  Though humans will not likely be around to witness the merger in 4 billion years, the cosmic light show that accompanies the birth of our new galaxy will be spectacular.