Messier 81 and Messier 82

WISE image of the spiral galaxies Messier 82 (seen edge-on at the top of the image) and Messier 81 (see face-on at the bottom of the image). M81 and M82 swept by one another a few hundred million years ago--triggering a burst of star formation in both--and will likely pass near each other again multiple times until they eventually merge into a single galaxy.

Credit: NASA/JPL-Caltech/UCLA