This image of a pair of colliding galaxies called NGC 6240 shows them in a rare, short-lived phase of their evolution just before they merge into a single, larger galaxy. The prolonged, violent collision has drastically altered the appearance of both galaxies and created huge amounts of heat -- turning NGC 6240 into an "infrared luminous" active galaxy. Image credit: NASA/JPL-Caltech/STScI-ESAHat tip to Bad Astronomy (who explains the image in detail).