A luminous galaxy which emits most of its radiation in the far-infrared. ULIRGs were discovered by the Infrared Astronomical Satellite in the 1980s. The energy source that is heating the dust cannot generally be seen with optical telescopes because the dust hides it, but observations in other wavebands generally show that the dust is being heated either by an active galactic nucleus or by a starburst, a large number of recently formed stars.