Galaxy collisions
Collisions between galaxies are common. Indeed, most galaxies have probably been involved in one or more encounters during their lifetimes. One example is our own Milky Way, which is bound by gravity to the Andromeda galaxy, our neighbor, and towards which we are approaching at a speed of about 50 kilometers per second, perhaps to meet in another billion years or so. Galaxy-galaxy interactions are thought to stimulate vigorous star formation because the encounters somehow induce the interstellar gas to condense into stars. These stimulated starbursts in turn light up the galaxies, especially at infrared wavelengths, making some systems hundreds or even thousands of times brighter than the Milky Way while they are active. Many of the massive stars that are produced become supernovae whose explosive deaths enrich the environment with carbon, oxygen, and all the other elements that are essential for life. Interacting galaxies are important not only in...