Wispy clouds of infrared emission first detected by the Infrared Astronomical Satellite and prominent at 60 and 100 μ‎m. Most infrared cirrus is believed to be emission from dust grains in hydrogen clouds, heated by ultraviolet radiation to 20–30 K. Some of the denser cirrus clouds have cooler cores. The clouds are brighter at 12 and 25 μ‎m than expected, possibly due either to a component of very small dust grains or to line emission from complex molecules.