NASA - National Aeronautics and Space Administration

Let's look four billion years into the future. A group of our descendents stands with their teacher and looks up at a night sky ablaze with thousands of dazzling white stars. All of these new stars are being born, their teacher tells them, because of an immense cosmic event.

Image to left: Many years from now, our descendants may see the mixing of the galaxies. Credit: NASA

Our galaxy, the Milky Way, has collided with another galaxy, called Andromeda. Although the two galaxies are passing through each other at a million miles an hour, the whole process will take many millions of years to complete. And when everything settles down, the two galaxies will have merged into one.

The students fear that this may be the end of life, as they know it. But, their teacher reassures her class that there is very little chance of stars from the Andromeda galaxy hitting the Sun or the Earth. Even though the galaxies pass clear through each other, she says, stars in a galaxy are spaced very far apart.

Image to right: An artist's impression of the night sky, four billion years from now. Credit: Space Telescope Science Institute

They are like grains of sand separated by the length of a football field. The Andromeda stars simply pass by. But galaxies are more than just stars. They contain giant clouds of gas and dust. And, when galaxies collide, these clouds smash into one another. The clouds contain the raw materials needed to make new stars. It is the collision between clouds that has triggered a starry baby boom!
 

Image to left: This movie is a computer animation showing the collision between two similar sized spiral galaxies. Credit: Frank Summers (Space Telescope Science Institute), Chris Mihos (Case Western Reserve University) and Lars Hernquist (Harvard University)
+ View video
+ Download viewer

Our story describes an event in the distant future for the Milky Way. But, galaxy collisions are common in the universe.

But don't worry about a collision during your lifetime. Right now, the Milky Way and Andromeda are so far apart that even light takes two million years to journey between them. But on the scale of galaxies, they are quite close together. Imagine the Milky Way galaxy as a music CD (the thickness compared to its diameter is about right).

Andromeda is a spiral galaxy of similar size, so we can think of it as a second CD. Now hold these CDs about eight feet apart. The gap is closing such that, in about four billion years, the CDs will touch. In that far-off time, the Sun will still be shining and the Earth may still be a planet teeming with life. What will our distant descendants make of the night sky?

Image to right: The beginnings of a merger. Two galaxies draw together, probably orbiting each other several times before the larger of the two (on the left) consumes the smaller. Credit: NASA and Hubble Heritage Team (STScI)