Accretion of material due to supermassive black holes in galactic cores leads to intense active galaxies luminosity.
The center of the galaxies has a high luminosity as compared to other parts postulated to be emitted by something other than the stars because of its intense luminosity. They are electromagnetic radiation source which is useful for discovering objects from outer space.
They can be observed by gamma rays, ultra-violet, radio, microwave, x-rays, infrared, and many more similar. Classification of an active galactic nucleus which is based on their observed characteristics has been made and the quasar is the powerful one.
