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infrared, ultraviolet, x-ray, and gamma-ray wavelengths), and some are highly variable, changing in brightness considerably over a short periods of time. The current model of active galaxies suggests that such a galaxy may be powered by a supermassive black hole at its center, from which pour forth jets of hot gas in opposite directions at nearly the speed of light. The black hole is surrounded by a disk of gas that is spiraling into it. As it does so, the gas gives off huge amounts of radiation before disappearing inside the black hole. Scientists currently recognize many phenomena associated with active galaxies, including quasars and blazars. Quasars are extremely distant objects, some as far away as 12 billion light-years. A quasar is also extremely luminous, perhaps a hundred or even a thousand times brighter than a normal galaxy. Some scientists have suggested that quasars may be highly active galactic nuclei, burning so brightly that they blot out all the light from the galaxies’ stars. A blazar is believed to be an active galaxy that has one of its jets pointed toward Earth, so that observers here on Earth are looking directly into the jet of escaping energy. Lower-luminosity active galaxies also exist. Astronomers suspect they may be weaker versions of quasars or may be powered by star formation rather than supermassive black holes. Active galaxies are the subject of much debate among astronomers. The study of these distant objects is one of astronomy’s greatest challenges, and astronomers are continually making new discoveries about them, and devising new theories to explain their characteristics. CLASSZONE.COM Observe some regular, irregular, and very peculiar galaxies. Keycode: ES2809 ACTIVE GALAXY In this artist’s rendering, an active galaxy’s jets of gas are clearly seen pouring from its nucleus. Chapter 28 Stars and Galaxies 633