The large redshifts of quasars are a result of the expansion of the universe. Quasars are very distant, so the light we are now receiving has traveled a long time to reach us. We see quasars as they were when the universe was many billions of years younger than it is today.
A typical quasar is much more luminous than an entire galaxy. Most of the radiant output of a quasar is continuous radiation that is a combination of synchrotron radiation and thermal emission from gas and dust.
Ionization of gas near the center of a quasar by ultraviolet radiation and X rays produces emission lines. The broad and narrow emission lines are produced in different regions of the quasar.
Quasars can vary in brightness in as little as half a day. Variations this fast show that the radiation the quasars emit must be produced in a region as small as or smaller than our solar system.
In many cases, radio maps of quasars show components that seem to be separating faster than the speed of light. This superluminal motion is probably due to an illusion that occurs when light is emitted by something moving almost directly at us at a speed only slightly less than that of light.
Radio galaxies have huge lobes of radio emission fed by high-speed jets of matter from the heart of the central galaxy. In some cases, radio galaxies have two lobes, but only a single jet can be seen. This occurs because a jet pointed at us looks brighter and a jet pointed away looks dimmer than either would if they were viewed from the side.
There are many kinds of active galactic nuclei, including Seyfert galaxies, radio galaxies, blazars, and starburst galaxies. All of these share some properties with quasars but are different in other important respects.
Quasars probably produce so much energy in such small regions because they contain massive black holes that accrete surrounding gas. Black holes produce energy very efficiently from the matter that falls into them.
The intense radiation in quasars would blow away infalling gas if the gravity in quasars weren't large enough to overcome the outward force of radiation. As a result, the cores of quasars must contain 100 million solar mass or more of matter.
Because it has angular momentum, matter approaching a black hole forms an accretion disk. Friction in the accretion disk releases considerable energy, which flows to the surface of the disk and is then radiated away. The surface of the accretion disk is about as hot as a star.
It may be possible to distinguish the different kinds of active galactic nuclei on the basis of luminosity and the angle from which we view them. Viewing angle determines whether the center of the active nucleus is hidden by an obscuring torus and whether jets of outflowing matter point nearly at us. If they do, they appear increased in brightness.
The host galaxies of many active galactic nuclei, including the nearer quasars, can be detected. The host galaxies are often distorted because of interactions with other galaxies. The host galaxies of distant quasars can't be seen because they appear too small and faint.
The density of quasars is larger at great distances than it is in our part of the universe. This shows that quasars were more numerous in the past than they are today.
A luminosity function gives the density of quasars of different brightness. Comparing luminosity functions for distant and relatively near quasars suggests that quasars have grown about one hundred times dimmer since they formed. Weak quasarlike activity in nearby galaxies may indicate that it is common for galaxies to have dormant quasars in their centers.
Absorption lines in the spectra of quasars are produced when light from the quasar passes through clouds of gas between the quasar and us. Some of the absorbing clouds appear to be intergalactic clouds, which were more numerous in the time of quasars than they are today.
The images of distant quasars and galaxies are sometimes distorted by gravitational lensing caused by intervening galaxies. The lens can produce multiple images of the background galaxy or quasar or arcs or rings of light around the massive intervening galaxy.
Gamma ray bursts are brief blasts of gamma rays that originate in distant galaxies. Although the mechanism that produces gamma ray bursts is not yet understood, neutron stars or black holes may be involved.
