Introductory Astronomy: Quasars

Approximately 5000 quasars have been found to date. Quasars are small, powerful, extremely remote objects. They are thought to be very distant, very powerful active galactic nuclei. They have rapid brightness fluctuations which indicates that they are no more than a few light-days or light-weeks (the distance light can travel in a day or week) in diameter. They are therefore very compact objects. At the same time, they are superluminous, approximately 10 - 1000 times more luminous than a normal galaxy.

Compact, superluminous objects like quasars can be understood in terms of the unified model. In this model, the quasar would be powered by a supermassive black hole (about 10^7 to 10^9 solar masses) at its center surrounded by a rotating accretion disk. If about one solar mass of material per year flowed into the black hole, the energy released could power a quasar. Most of the light we see may come from the hot gas in the inner parts of the accretion disk. Because this is a very small region, rapid fluctuations could occur due to random fluctuations in the flow of matter into the black hole.

This picture of quasars is supported by the fact that we see jets near some quasars (these jets could be powered by the black hole) and by the presence of double radio lobes near some quasars. In addition, we see different kinds of quasars, some with jets, some without, etc. These variations can be explained easily by the unified model. Different orientations of the accretion disk to our line of sight produce different types of quasars. When we view the quasar through its accretion disk, we see a different type of quasar than when we view it along the jet, exactly the same as for the closer, more familiar active galaxies discussed in this section.

Quasars are very distant objects. The light we see from quasars has had to travel so far to reach us that it must have been emitted very soon after the beginning of the universe in order to arrive here today. This means that quasars give us a glimpse into the past; their light provides a clue to what the universe was like very soon after it formed.