Short for QUAsi-StellAr Radio source, a quasar is an extremely powerful and distant active galactic nucleus (a compact region at the centre of a galaxy which has a much higher than normal luminosity). It derives most of its energy from very hot matterswirling into a central supermassive black hole, and can generate as much light as a hundred normal galaxies from a much smaller volume. It is one of the most powerful objects in the universe, and among the most distant things ever seen in space.