Quasars (QUAsi-Stellar Radio Source) is a very energetic and distant galaxy with an active galactic nucleus. They are the most luminous objects in the universe.
Quasars were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended sources similar to galaxies.
While there was initially some controversy over the nature of these objects - as recently as the early 1980s, there was no clear consensus as to their nature - there is now a scientific consensus that a quasar is a compact region in the centre of a massive galaxy surrounding its central supermassive black hole. Its size is 10-10,000 times the Schwarzschild radius of the black hole. The quasar is powered by an accretion disc around the black hole.
Above: A growing black hole can be seen at the centre of a faraway galaxy in this artist's concept. Astronomers using NASA's Spitzer and Chandra space telescopes discovered swarms of similar quasars hiding in dusty galaxies in the distant universe. The quasar is the orange object at the centre of the large, irregular-shaped galaxy. It consists of a dusty, doughnut-shaped cloud of gas and dust that feeds a central supermassive black hole. As the black hole feeds, the gas and dust heat up and spray out X-rays, as illustrated by the white rays. Beyond the quasar, stars can be seen forming in clumps throughout the galaxy. Other similar galaxies hosting quasars are visible in the background.
Quasars show a very high redshift, which is an effect of the expansion of the universe between the quasar and the Earth. They are the most luminous, powerful, and energetic objects known. When combined with Hubble's law, the implication of the redshift is that the quasars are very distant, thus from much earlier in the universe's history. The most luminous quasars radiate at a rate that can exceed the output of average galaxies, equivalent to one trillion (1012) suns.