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 1041 W, thousands of times greater than the luminosity of a large galaxy such as the Milky Way.
More than 200,000 quasars are known, most from the Sloan Digital Sky Survey
The power of quasars originates from supermassive black holes that are believed to exist at the core of all galaxies. The Doppler shifts of stars near the cores of galaxies indicate that they are rotating around tremendous masses with very steep gravity gradients, suggesting black holes.
Because quasars are extremely distant, bright, and small in apparent size, they are useful reference points in establishing a measurement grid on the sky.[39] The International Celestial Reference System (ICRS) is based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky. Because they are so distant, they are apparently stationary to our current technology, yet their positions can be measured with the utmost accuracy by Very Long Baseline Interferometry (VLBI). The positions of most are known to 0.001 arcsecond or better, which is orders of magnitude more precise than the best optical measurements.
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