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Quasar in astronomy, extragalactic object stellar appearance, extremely distant and bright. The quasar name was initially attributed to intense radio sources who had seen in a stellar appearance. Later, however, they are spotted other bodies, much more numerous, that were not strong emitters of radio waves, but they had the same optical properties of the quasar. To them, it was given the name of ‘quasi-stellar objects’ or, more briefly, QSO (quasi-stellar objects). Quasars and the QSOs are considered members of a class and the term quasar it is also used extensively to indicate objects with weak radio emission. Quasars emit radiation in almost all regions of the electromagnetic spectrum, from radio waves to X-rays and gamma rays. It is observed, in particular, a lack of ultraviolet emission, which is often used to identify these objects as distinguished from the stars. The dimensions can be inferred from the variability of their brightness. In fact, according to the theory of relativity, the brightness of a source can not vary over shorter time periods than those employed by a light signal to cross the emission region. It is possible, therefore, to fix an upper limit to the size of the source, by multiplying the speed of light for the time on which its brightness undergoes substantial variations. The most recent observations have revealed the presence of halos around nearly all quasars. These zones are interpreted as galaxies (the so-called host galaxies), which constitute the core of quasars. The theory of general relativity predicts that a massive body, located on the line of sight of a source, they deflect the light rays, producing, in certain circumstances, a double image. The phenomenon, known as gravitational lensing or gravitational mirage, was suggested by A. Einstein in 1936. The first experimental confirmation of the effect was provided in 1980 by the quasar 0957 + 561, which turned out to be two identical images at a distance 6 “angle. Since then there have been several other discoveries ghost pictures of quasars.
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