Any cosmic source of infrared radiation. All stars emit infrared radiation, and the cooler they are the more energy they emit in the infrared. Warm dust is also a major infrared source. Prominent infrared sources include red giants and supergiants with dust shells, H II regions, the galactic centre, star-forming regions, and active galaxies. Many active galaxies (especially starburst galaxies) emit the bulk of their energy in the infrared.