Background image: Artist’s impression of a quasar, with a central massive black hole driving the X-ray emission from hot, accreting gas.
Discovered in 1963 by Matthews and Sandage, quasars (short for quasi-stellar radio sources) were first identified as distant, point-like radio sources. They were subsequently discovered to be distant, very early, and highly energetic active galactic nuclei (AGNs), emitting radiation across the electromagnetic spectrum, from radio to gamma-ray, but predominantly at UV/optical frequencies, subsequently red-shifted to radio waves.
It is thought the radiation is emitted in directed beams from the accretion disc surrounding a supermassive black hole, and therefore that quasars are black holes. They are the most luminous known objects in the Universe.