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The Power Source of Quasars

Quasars are by far the brightest objects in the universe. When astronomers first calculated the energy output of quasars, many of them didn't believe anything could emit that much energy. Some astronomers started looking for other explanations. Many people proposed that the redshifts did not indicate distance and were due to other causes. But further research eventually ruled out the alternative theories, leading most astronomers to conclude that quasars really are the most distant and luminous objects in the universe.

There are very few energy sources that produce enough energy to power a quasar. The possible source that best fits the observed properties of quasars is a supermassive black hole.

A black hole is a region of space from which nothing can escape, not even light. Small black holes result from the deaths of very massive stars. The black holes at the centers of quasars have masses of millions or even billions times our Sun's. Although the mass of a typical quasar black hole is very large, its radius is only about as large as our solar system. No one knows how these supermassive black holes come about; their origin is the subject of intense research.

A dust disk believed to circle a black hole in the galaxy NGC 7052
Image courtesy the Space Telescope Science Institute

At the center of a quasar, the black hole is surrounded by a large, rotating cloud of gas. As the gas falls into the black hole, it is heated up to millions of degrees. The gas emits thermal radiation due to its enormous heat. This thermal radiation spans the spectrum, making the quasar bright in the visible spectrum as well as x-rays.

There is a limit as to how bright a quasar can be, called the Eddington limit, which depends on the mass of the black hole. If too much gas falls into the black hole at once, the gas heats up and creates pressure. This pressure slows down the flow of gas, keeping the luminosity of the quasar below the Eddington limit.

One of the most important facts about quasars is that they are all very distant from us. The closest quasar is about 800 million light years away. Therefore, we can conclude that there are no quasars in the universe today and the last quasar disappeared about 800 million years ago.

Where did the quasars go? No one can say for sure. Given their power source, however, it is most likely that they simply ran out of fuel. The black holes eventually consumed all the gas and dust in the disk surrounding them, so the quasars ceased to shine.

Now, let's analyze some of the quasars found by the SDSS.