Quasars are distant astronomical objects that live only in galaxies with supermassive black holes that contain billions of times the mass of the sun.
The more massive the black hole, the more powerful the quasar can be.
Quasars, which have fascinated astronomers since their discovery, are considered to be some of the first light sources to have formed in the Universe.
Quasars are among the most distant and luminous objects known. The light astronomers receive has traveled an extremely long time to reach us. We see quasars as they were when the universe was many billions of years younger than it is today.
Most of the more than 2,000 known quasars existed in the early life of the galaxy.
A typical quasar is much more luminous than an entire galaxy and the brightest quasars outshine all the stars in the galaxies, in which they reside. This extraordinary feature makes them visible even at distances of billions of light-years.
However, quasars can vary in brightness significantly on timescales as short as a few days, meaning that the total size of the quasar cannot be more than a few light-days across.
Quasars emit tremendous amount of energies of millions, billions, or even trillions of electron volts.
Quasars are members of a class of objects known as active galactic nuclei (AGN) and there are many kinds of active galactic nuclei. Three important classes are: Seyfert galaxies, quasars and blazars.
Today astronomers know that the density of quasars is larger at great distances than it is in our part of the universe, which means that the quasars were much more numerous in the past than they are today.
The luminosity of distant quasars and located relatively near was also compared by astronomers. The studies suggest that quasars have grown approximately 100 times dimmer since they formed in the distant past.
Weak quasar-like activity in galaxies closer home may suggest that it is common for galaxies to have dormant quasars in their centers. Radio observations of AGN that quasars often produce huge jets, which are streams of particles coming from the central source with the speed of light.
Powerful high-energy quasars have been observed with the jet pointed towards us, which allows astronomers to see the resulting energetic radiation.