|Quasars reside in a variety of galaxies. When seen through ground-based telescopes, these compact, enigmatic light sources resemble stars, yet they are billions of
parsecs away and several hundred billion times brighter than normal stars.
Only with the sharp imaging of the Hubble Space Telescope
could we clearly see the faint galaxies of small angular
size that they reside in. These HST images show examples of different home sites of quasars. All the sites must provide the fuel to power these unique light beacons. Astronomers believe that a quasar turns on when a massive black hole at the nucleus of a galaxy feeds on gas and stars.
most likely as a galaxy is forming, or when it is colliding
with another galaxy. Some host galaxies look fairly
undisturbed but they may have had such a collision in the
recent past. As the matter falls into the black hole, intense radiation is emitted. Eventually, the black hole will stop emitting radiation once it consumes all nearby matter. Then it needs debris from a collision of galaxies or another process to provide more fuel. The column of images on the left represents normal galaxies; the center, colliding galaxies; and the right, peculiar galaxies.
More on this photo here
We now know that quasars are at the centers of extremely distant galaxies and are often members of very distant galaxy clusters. They can outshine the combined light of all the stars in the galaxy surrounding it. But they are very far away from us. What this tells us (since we know that looking over a large distance is the same as looking back in time) is that quasars were much more plentiful in the early universe. But what happened to them? According to the now standard model for the quasar phenomenon, super-massive black holes at the centers of galaxies attract gas clouds from surrounding interstellar space. As these clouds accumulate around the black hole, they settle into a swirling disk that circulates gradually inward in an extremely violent death spiral. The same gravitational pull that prevents light from escaping a black hole accelerates anything in its vicinity to near-light speeds. When gas clouds traveling at such phenomenal speeds collide, they heat one another to temperatures measured in millions of degrees. The disk of gas orbiting a super-massive black hole therefore grows searingly hot and radiates enormous amounts of energy.
Above is an artist's impression of one of these cosmic engines. You are looking at the very center of a much larger galaxies. The swirling gas spirals in toward the black hole, which can't be seen because the intense light from the gas is overwhelming. Not all the gas goes into the black hole - some is ejected into powerful, narrow jets that can sometimes extend up to 1 Mpc from the host galaxy!
Eventually the black hole "eats" all the gas around it and we no longer see the intense glow of the quasar. This is one explanation for why there are no quasars in our own time (i.e. close to us).