Quasars are luminous galaxies that emit radio waves in the spectrum between 10 MHz and 100 GHz, making them detectable as red flashes of light. They are thousands of light-years away from our own galaxy and form the core part of most distant galaxies.
The evidence is now overwhelming that quasars are the active cores of very distant galaxies. The central regions of some galaxies are unusually luminous they are very powerful emitters of all known electromagnetic radiation the mysterious quasar are apparently responsible. The observational evidence comes from the short pulses of red light recorded on distant galaxies.
Quasars are known to be objects of smaller dimensions relatives to other galactic bodies. It is the detection of these smaller objects that put quasars at the center of very distant galaxies. They are not visible by the naked eyes but can only be detected using a special machine.
Soon after quasars were discovered, astronomers detected fluctuations in brightness over time as short as a few hours. Those rapid fluctuations show quasars are small objects, not more than a few light-hours in diameter.