High-resolution images of quasars, particularly from the Hubble Space Telescope, have shown that quasars occur in the centers of galaxies, and that some host galaxies are strongly interacting or merging galaxies. The term quasar originated as a contraction of 'quasi-stellar radio source'-because they were first identified during the 1950s as sources of radio-wave emission of unknown physical origin-and when identified in photographic images at visible wavelengths, they resembled faint, star-like points of light. The redshifts of quasars are of cosmological origin. Quasars are usually categorized as a subclass of the more general category of AGN. The radiant energy of quasars is enormous the most powerful quasars have luminosities thousands of times greater than that of a galaxy such as the Milky Way. Gas in the disc falling towards the black hole heats up and releases energy in the form of electromagnetic radiation. The emission from an AGN is powered by a supermassive black hole with a mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. It is sometimes known as a quasi-stellar object, abbreviated QSO. A quasar ( / ˈ k w eɪ z ɑːr/ KWAY-zar) is an extremely luminous active galactic nucleus (AGN).
