A quasar is an extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surrounded by a gaseous accretion disk. As gas in the disk falls towards the black hole, energy is released in the form of electromagnetic radiation, which can be observed across the electromagnetic spectrum. The power radiated by quasars is enormous; the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way. Usually, quasars are categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin. ( Learn more  )

Quasars are the brightest phenomenon in the entire universe, able to outshine any type of star, and even entire galaxies containing hundreds of billions of them. Quasars are the product of monster black holes at the centres of large galaxies, and so when we find multi-billion light year clusters of quasars dotted around the early universe, we are forced to reconsider some of our fundamental assumptions of cosmology.