Quasi-stellar object

A quasar (/ˈkweɪzɑːr/) (also quasi-stellar object or QSO) is an active galactic nucleus of very high luminosity. A quasar consists of a supermassive 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 luminosities exceeding 10⁴¹W, thousands of times greater than the luminosity of a large galaxy such as the Milky Way.

The term "quasar" 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 quasar host galaxies are strongly interacting or merging galaxies.

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