Twin quasar (Q0957+561), optical image. The quasar, lying in the constellation Ursa Major, is indicated by the label at centre right. Quasars are very distant, yet extremely luminous astronomical objects. They are thought to be the centres of active galaxies, emitting vast quantities of high-energy radiation as matter gravitates towards supermassive black holes. The Twin quasar is in fact a single quasar, but two images of it are seen as a result of gravitational lensing. A large mass concentration, such as a galaxy, between the quasar and Earth bends the light, causing a double image. This quasar, discovered in 1979, was the first known example of gravitational lensing. The bright object at upper left is the barred spiral galaxy NGC 3079.