Quasars are the brightest and most powerful objects in our universe, and though they have shaped the cosmos, they might ultimately destroy everything that exists.
A quasar (also quasi-stellar object or QSO) is an active galactic nucleus of very high luminosity. A quasar consists of a super massive black hole surrounded by an orbiting accretion disk of gas.
As gas in the accretion disk falls toward the black hole, energy is released in the form of electromagnetic radiation. Quasars emit energy across the electromagnetic spectrum and can be observed at radio, infrared, visible, ultraviolet, and X-ray wavelengths. The most powerful quasars have luminosity exceeding 1041 W, thousands of times greater than the luminosity of a large galaxy such as the Milky Way.
The term originated as a contraction of “quasi-stellar radio source”, because quasars were first identified as sources of radio-wave emission, and in photographic images at visible wavelengths they resembled point-like stars. High-resolution images of quasars, particularly from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, and that some QSO host galaxies are strongly interacting or merging galaxies.
Quasars are found over a very broad range of distances (corresponding to red-shifts ranging from z < 0.1 for the nearest quasars to z > 7 for the most distant known quasars), and quasar discovery surveys have demonstrated that QSO activity was more common in the distant past. The peak epoch of quasar activity in the Universe corresponds to redshifts around 2, or approximately 10 billion years ago.