More than 30 years ago the Infrared Astronomy Satellite discovered that the universe contained many almost invisible yet enormously luminous galaxies—some of them more than 1,000 times brighter than our own Milky Way. These galaxies shine primarily in the infrared, powered by dramatic bursts of star formation that are buried deep within clouds of dust and molecular gas. The dust absorbs the ultraviolet light emitted by the hot young stars, obscures the optical light, and re-radiates the energy at far infrared wavelengths. These wavelengths are about one hundred times longer than optical wavelengths and characterized by cool temperatures, roughly 40 degrees kelvin. Astronomers argue that, at least in some cases, the hyperactivity is triggered by collisions between galaxies that induce the collapse of their clouds of gas into new stars.