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Definition : a small 'quasi-stellar' object (which on contraction produces the name quasar) that emits an enormous amount of energy, mainly as infrared radiation. They can also emit X-rays and about one tenth are radio sources.
Quasars are compact sources and have the largest REDSHIFTS found, a feature which has made them important in astronomy, as they are the most distant observable objects at up to 10^9 light years away.
An apparent paradox is that although quasras generate enormous quantities of energy, the light-producing area is only one light-day, or less, across. The explanation is that the energy source is the falling of matter into a vast black hole, at the center of the quasar.
By the end of the 1980s, several thousand quasars had been identified and the red shifts of a few hundred determined; in a small number of these, the shift factor is greater than 4. If the red shift is assumed to be cosmological, these quasars would have velocities greater than 93 percent of that of light. According to Hubble's law, their distances would thus be greater than 10 billion light-years, and their observed light would have been traveling practically as long as the age of the universe.
Radio measurements, however, combined with the fact that electromagnetic waves emitted by some quasars vary strongly over a period of a few months, indicate that quasars must be much smaller than ordinary galaxies. Because the size of a fluctuating radiation source cannot be much larger than the distance light would travel from one end of the object to the other during one fluctuation period, astronomers estimate that the variable quasars cannot be larger than one light-year across, which is 100,000 times smaller than the Milky Way.
No satisfying explanation exists for a mechanism that could produce such amounts of energy in a relatively small volume. One theory gaining wide acceptance is that quasars are the superluminous cores of galaxies and that they and radio galaxies may actually be equivalent objects seen from different angles.
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