Image to left: An artist's impression of the night sky, four billion years from now. The Andromeda spiral galaxy looms large in the sky as it begins its fiery merger with our own Milky Way galaxy. Credit: Space Telescope Science Institute

Four billion years in the future, a group of our descendents, (whatever they may look like!) stand with their teacher and look up at a night sky ablaze with thousands of dazzling white stars. All these new stars are being born, their teacher tells them, because of an immense cosmic event. 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.

Image to right: Some day the Andromeda galaxy and the Milky Way will meet like this. Credit: Frank Summers (Space Telescope Science Institute), Chris Mihos (Case Western Reserve University) and Lars Hernquist (Harvard University) + View video
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The 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 so far apart - grains of sand separated by the length of a football field - that 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, and it is the collision between clouds that has triggered a starry baby boom!

Image to left: The wispy tails of stars and gas thrown out during this collision between two galaxies has earned it the nickname of "the Mice." Credit: ACS Science & Engineering Team, NASA

Although our story describes an event in the distant future for the Milky Way, galaxy collisions are a common sight in the universe. Galaxies are cities of stars, each shining with the light of a hundred billion suns. When it was first realized that our own Milky Way was just one galaxy of billions that fill space, astronomers referred to them as "Island Universes." But galaxies are by no means isolated. Most live in groups or clusters with dozens or hundreds of members, and these cluster galaxies are all in constant motion, pulled and twisted by their neighbor's gravity, swarming like bees around a honey pot. In such an environment, collisions can and do happen, and the result is a cosmic firework display. The neat spiral pattern that a galaxy possessed is spun out into wisps and tails; and where immense gas clouds collide there is a glittering burst of newborn stars.

Image to right: 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)

At present 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?

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