A quasar (/ˈkwɑːzɑːr/) is an extremely luminous active galactic nucleus (AGN). Quasars are the most powerful and energetic class of AGN, with luminosities exceeding 1041 W at their peaks. They were first observed as radio sources in the 1960s. Their radio emission is thought to be powered by accretion of material onto a supermassive black hole, making them some of the most luminous objects in the known Universe.
As gas falls toward a black hole, it forms an accretion disk around it. This disk can become so hot that it emits large amounts of radiation across the electromagnetic spectrum, from X-rays to ultraviolet light to visible light. The intense radiation from quasars can even ionize surrounding gas clouds, making them glow. All these different types of emission make quasars among the brightest objects in the sky.
The name “quasar” comes from a combination of two words: “quasi-stellar” and “radio source”. Quasi-stellar means that they look like stars when viewed through a telescope, but they are actually much more distant galaxies with very bright centers powered by black holes. Radio source refers to their discovery as strong radio emitters using telescopes designed to detect radio waves from space.
Today we know that there are three main types of quasars: blazars, radio galaxies, and Seyfert galaxies. Blazars are a type of AGN where the central black hole is pointing directly at us so we see a lot of Doppler boosted radiation coming our way. Radio galaxies are similar to blazars except that their jets aren’t pointing directly towards us, so we don’t see as much boosted radiation coming our way but we do see extended structures in radio maps due to their lobe emission.. Seyfert galaxies have weaker jets than either blazars or radio galaxies and tend not to show any extended structure in radio maps; however they do often show broad emission lines in their optical spectra which provides clues about fast moving gas near their central black holes..