Quasars

Quasars are mysterious objects with high redshifts (i.e., traveling away from Earth at great speed) that appear almost like stars but are far more distant than any individual star we can now observe. These unusual objects were first discovered in the 1960s with radio telescopes, and they were called quasistellar radio sources, or quasars, for short. Quasars emit tremendous quantities of energy from very small volumes. These puzzling objects emit various portions of their energy in the form of radio waves, visible light, ultraviolet radiation, X-rays, and even gamma rays. A quasar that is relatively quiet in the radio frequency portion of the electromagnetic spectrum is often called a quasi-stellar object (QSO) or sometimes a quasi-stellar galaxy (QSG). Some of the most distant quasars yet observed are so far away that they are receding at more than 90 percent of the speed of light.

As bright, concentrated radiation sources, quasars are thought to be the nuclei of active galaxies. The optical brightness of some quasars has been observed to change by a factor

of two in about a week, with detectable changes occurring in just one day. Therefore, astrophysicists now speculate that such quasars cannot be much larger than about one light-day across.

The problem facing astrophysicists is to explain how a quasar can generate more energy than is possessed by an entire galaxy and generate this energy in so small a region of space. In fact, quasars are such peculiar astronomical objects they can radiate as much energy per second as is radiated by a thousand or more galaxies from a region that has a diameter about one-millionth of the host galaxy.

Quasars are high intensity sources of X-rays as well as visible light. They represent the most powerful type of X-ray source yet discovered in the universe. A pair of quasars called Q2345+007 A, B was imaged by the Chandra X-Ray Observatory in mid-2000. Close scrutiny of this X-ray image revealed that the supposed “twin quasars” were neither a mirage nor identical twins. After careful analysis scientists now regard this unusual quasar pair not as an illusion, but rather two different quasars created by merging galaxies. When galaxies collide, the flow of gas into the central supermassive black holes of each of the galaxies can be enhanced, resulting in two quasars. The X-ray “light” from this quasar pair started its journey toward Earth some 11 billion years ago. At that time galaxies were about three times closer together than they are now, so collisions between them were much more likely to happen.

Astrophysicists postulate that the power of a quasar depends on the mass of its central supermassive black hole and the rate at which it swallows matter. Quasars represent the extreme cases, where large quantities of gas pour into the black hole so rapidly that the energy output appears to be a thousand times greater than that of the galaxy itself.

X-rays from quasars and Active Galactic Nuclei (AGNs) are produced when in-falling matter is heated to temperatures of millions of degrees as it swirls toward the supermassive black hole at the galactic center. However, all the matter in the gravitational whirlpool is not doomed to fall into the black hole. In many quasars and AGNs a portion of the swirling gas escapes as a hot wind that is blown away from the accretion disk at speeds as high as a tenth of the speed of light.

Questions to Ponder

• How big are quasars compared to galaxies?
• How long does it take for a quasar to form?
• How do some Quasars become radio-quiet?