A hot white-dwarf star in the constellation Coma Berenices with a surface temperature of approximately 50 000 K. The SAS-3 satellite discovered that it was a strong emitter of soft X-rays, and it was also the first cosmic source of extreme ultraviolet (EUV) radiation to be discovered. Because of its simple, well-known spectrum, it is used as a standard for calibrating EUV astronomy missions.