A Seyfert galaxy is any of a class of galaxies known to have active nuclei. Such galaxies were named for the American astronomer Carl K. Seyfert, who first called attention to them in 1944. The nuclear spectra of Seyfert galaxies show broad emission lines, which are indicative of a central concentration of hot gas that is expanding at speeds of up to thousands of kilometres per second. Galaxies of this type appear normal in ordinary photographs but are extremely strong sources of infrared radiation. Moreover, many are powerful sources of radio energy and X rays as well. About 1 percent of all spiral galaxies are thought to exhibit Seyfert properties; or perhaps all spiral galaxies are Seyferts 1 percent of the time
Excerpt from the Encyclopedia Britannica without permission.