Quasi-stellar Objects

Type of physical science: Astronomy; Astrophysics

Field of study: Galaxies

A quasi-stellar object is the bright, active core of a distant galaxy. These objects include the brightest and most distant objects in the universe.

Overview

Quasi-stellar objects, commonly referred to as quasars, are the brightest and most distant objects in the universe. The name "quasi-stellar" means "like a star," which describes the appearance of a quasar. All quasars are much too faint to be seen without the aid of a telescope, through which the objects appear as mere pinpoints of light, similar to the typical star. The quasi-stellar objects, however, are much farther from Earth than the stars. The fact that they are visible at all from these distances suggests that they must be extremely bright, more than ten times brighter than even the brightest galaxies.

A quasar is the bright core of a certain type of spiral galaxy. Because these spiral galaxies are so far away, the spiral arms that are generally seen in nearby galaxies have faded away so that only the bright central core is visible. Some of the closer quasars are surrounded by a faint spiral structure that is barely detectable.

The bulk of the energy in a quasar comes from a relatively small region in the core, smaller than the size of the solar system. This region is much too small to contain the huge number of stars that would be required to radiate all the energy if all the energy were from stars, as is the case in most normal galaxies. The only possible object that can provide all this energy in such a small volume is a black hole in the center of every quasar. A black hole is an object that is so dense that nothing can escape from it, including light. Because black holes have such a strong gravitational force, they collapse to a single point. The black holes in the centers of quasars have masses that range from about a million to a billion times that of the sun.

Because of its strong gravitational pull, a massive black hole in the center of a quasar will attract all the surrounding matter, which consists primarily of gas (mostly hydrogen) and dust. As the matter spirals in toward the black hole, it forms a flat disk known as an accretion disk. As the matter gets closer and closer to the black hole, it speeds up. Some of the energy contained in the fast-moving particles is converted into heat energy, causing the accretion disk to reach temperatures of up to 108 Kelvins. This hot matter emits electromagnetic radiation in the form of ultraviolet radiation, blue light, and X rays.

Once the matter gets very close to the black hole, no signal (including light) can escape. It is the matter spiraling in the accretion disk outside the black hole, therefore, that provides the energy for the entire quasar. Every year, the black hole in the core of a quasar attracts and consumes a mass equal to about ten times that of the sun.

In addition to the X-ray, ultraviolet, and blue radiation that comes from the hot accretion disk, quasars also emit radio waves. These waves are caused by electrons moving at high speeds that lose energy as they move through magnetic fields. In order to give off the radio emission, the electrons must be accelerated to velocities approaching the speed of light. Although it is believed that all quasars emit some radio emission, about 1 percent of all quasars have extraordinarily high radio emission. The difference between these "radio loud" quasars and the majority of "radio quiet" quasars is not understood clearly.

Quasars also emit infrared radiation, which comes from cool dust that surrounds the active core. The dust absorbs some of the ultraviolet radiation from the core and re-radiates it as infrared radiation.

Observationally, a quasar appears similar to another type of galaxy known as a Seyfert galaxy. The Seyfert galaxies are much closer than the quasars and are, therefore, easier to study.

Seyferts have bright, active cores that are also fueled by massive black holes. The primary difference between quasars and Seyfert galaxies is that the average quasar is about one hundred times brighter than the average Seyfert galaxy.

Observations of quasars show that their light does not appear as it is emitted, but rather is shifted toward the red end of the electromagnetic spectrum. This shift is caused by the Doppler effect, which predicts that an object that recedes from an observer at a high speed will appear to be redder than it really is. The redshift is defined as the change in the wavelength of light divided by the wavelength at which the light is emitted. Objects with higher redshifts are moving away at faster speeds. Since quasars are the most redshifted objects in the universe, they are all receding from Earth faster than any other type of object. Their redshifts can typically reach values over one and are, in some cases, as high as four. In other words, their recessional speeds are generally more than 90 percent of the speed of light, making them, by far, the fastest moving objects in the universe.

The recessional velocity of a distant astronomical object is correlated directly with the distance to the object. The big bang theory predicts that all matter in the universe began at a single point. After an initial explosion, everything moved away from everything else. The objects that were moving the fastest were able to move the farthest away. Therefore, there is a relationship between the distance that objects are from Earth and the recessional speeds of the objects. Measuring the recessional speeds by the redshifts provides a method for measuring the distances to quasars. There are some quasars that are more than 10 billion light-years away. (A light-year is defined as the distance that light travels in one year, which is about 1 x 10¹⁶ meters.) These are the most distant objects known to date, which are located near the limit of the observable universe.

Because light travels at the finite speed of 3 x 10⁸ meters per second, it takes light a long time to get from the quasars to Earth. For some of the most distant quasars, this time--which is known as the "look back" time--can be nearly 10 billion years. Thus, quasars are being observed not as they are now, but as they were billions of years ago. The fact that there are no nearby quasars indicates that quasars do not exist today, but are remnants from the past. Over time, something has fundamentally changed about the nature of the universe.

It is not known how quasars formed. One theory is that they formed from other spiral galaxies. Some spiral galaxies have rapid bursts of star formation that take place in their cores; they are known as starburst galaxies. The massive stars that form in the cores of these galaxies have relatively short lifetimes because they shine brightly and, therefore, use up their energy at a rapid rate. After they exhaust their energy supplies, they explode in catastrophes known as supernovas. It is possible that the explosions could leave behind enough material, which could contract gravitationally to the center of the galaxy and could form a black hole. The black hole would then grow by accreting the surrounding matter and eventually could form a bright, powerful accretion disk. In this way, it is possible theoretically for spiral galaxies in the past to have evolved into quasars.

Applications

Quasars are often used in the study of cosmology, which is the study of the structure and evolution of the entire universe. The most fundamental question in cosmology concerns the fate of the universe. It is generally accepted that the universe began with a big bang and is presently expanding. The question that remains is whether the universe will expand forever or whether it will contract eventually upon itself. A universe that will contract eventually is called a "closed" universe, which will end in a "big crunch" as all matter collapses upon itself. A universe that will expand forever is known as an "open" universe. The criterion that determines whether the universe is open or closed is the density of the universe. If there is sufficient density, gravity will slow the rate of expansion and cause eveything to collapse upon itself eventually.

The problem of the fate of the universe reduces to a problem of measuring the mass within the universe. While it may be theoretically possible to count all the galaxies that emit light and measure their masses, a major problem is that not all the mass of the universe is in the form of light-emitting matter. Most is in the form of dark matter. The dark matter cannot be detected by itself but can be detected with the help of the background quasar light.

If light from the distant quasars passes through intervening gas, the gas will absorb light. When gases absorb light, they do not absorb all the colors of light. The absorbed light goes into "exciting" the electrons in the atoms in the gas; the electrons gain energy by moving to outer orbits in their atoms. The orbits of electrons in atoms are discrete, which means that only certain energies are needed to move an electron from one energy level to another. Therefore, only certain wavelengths of light are absorbed. If this light is examined through a prism, it will resemble a rainbow of colors with some dark lines where certain colors are missing. These dark lines are referred to as absorption lines. The observations of many quasars reveal additional absorption lines that are not associated with the quasar itself. Analysis of the absorption lines gives information about the intervening matter, which consists primarily of low-density gas that is arranged in discrete clouds. The absorption lines in the clouds have different Doppler shifts than the quasars, which means that they are not associated with the quasars but, rather, are between the quasars and Earth. By examining the differences between Doppler shifts, it is possible to distinguish one cloud from another and discover the motion of each cloud. The universe appears to be filled with these clouds, which would be difficult to locate without the light of the distant quasars.

Besides the low-mass gas clouds, quasars also can be used to investigate very massive bodies that are between the quasar and Earth through an effect known as a gravitational lens.

According to the theory of general relativity developed by Albert Einstein, light rays will bend as they travel near massive bodies in a manner similar to the bending of light with an optical lens.

The images of many quasars are seen through gravitational lenses and therefore are distorted.

The most common effect of these lenses is to produce a double or multiple image of the quasar.

There are many quasars that appear in pairs that are located very close to one another in the sky.

Close examination reveals that these are, in fact, the same object seen twice. Quasars are the only objects in the universe that are so distant that there is enough matter between the quasars and Earth for there to be significant numbers of gravitational lenses. Therefore, that is the only available information about these very massive objects, since they are generally unseen and do not appear to be emitting any substantial light of their own. Although the exact nature of the objects responsible for the gravitational lenses is unknown, it is possible that they are old galaxies that have used up their energy and are, therefore, dark.

Besides studying the nature of the universe by observing how the quasar light is affected by intervening matter, there is another method that involves observations of the quasars themselves. Theories of cosmology predict that the universe will have a different "curvature" depending on whether it is open or closed. If the universe is open, there will be an excess of distant objects compared to the number if the universe is closed. Since quasars are the most distant objects, they are the objects that are studied. Many studies are concerned with counting the quasars and determining how the count varies with distance. Because distant quasars are difficult to find and because the differences between the expected numbers are slight between open and closed universes, this problem has not yet been solved.

Context

Like many discoveries in astronomy, the discovery of the first quasi-stellar object was a surprise. In 1963, Maarten Schmidt observed an object that appeared as a bright point, which he thought was a star. The star, however, had properties that were different from those of any previous type of star that he had observed. Schmidt examined the spectrum of the star and tried to identify the spectral lines as lines seen from elements studied in a laboratory. He then discovered that the lines were, in fact, spectral lines that appear from hydrogen atoms; however, these lines had been redshifted to such a degree that he did not recognize them at first. Since the object was much more distant than a star, it had to be much brighter--it had to be brighter than any other known object.

At first, the discovery of quasars was considered quite controversial, because many astronomers did not believe that objects could be so powerful and so distant. As a result, some astronomers developed an alternative hypothesis that the objects were much closer and that the redshift had to be caused by something other than the Doppler effect. Over the years, however, there were no physical mechanisms that could explain the redshifts, other than the quasars being at huge distances. Quasars are readily accepted as very distant objects, and astronomers continue to observe quasars to study the mechanisms that could fuel the most powerful objects in the universe and to probe the nature of the entire universe.

There are thousands of known quasars. Many more quasars are likely to be discovered through present and future surveys from Earth, as well as from space. In the future, most new data will probably come from space through surveys at a wide range of wavelengths. There are many present and planned missions of the National Aeronautics and Space Administration (NASA). These include the High Energy Astrophysical Observatory (HEAO) and the Advanced X-ray Astrophysics Facility (AXAF) for detection of X rays; the International Ultraviolet Explorer (IUE), the Infrared Astronomical Satellite (IRAS), and the planned Space Infrared Telescope Facility (SIRTF). With all these future missions, it is likely that there will be more information shed on the nature of the quasars, as well as the nature of the universe through the observation of quasars.

Principal terms

ABSORPTION LINES: dark spectral lines that occur when light of a particular wavelength is absorbed by an atom

ACCRETION DISK: a host disk of material that spirals in toward a black hole or other compact object

BLACK HOLE: a massive point source that is so dense that nothing can escape from it, including light

COSMOLOGY: the study of the universe as a whole, including its origin and evolution

DOPPLER SHIFT: the change in wavelength of light caused by motion toward or away from an observer

GRAVITATIONAL LENS: a large, invisible mass that bends the light from distant objects

SEYFERT GALAXY: a spiral galaxy with an active core that is similar to a quasi-stellar object

SUPERNOVA: a catastrophic explosion that occurs at the end of the life cycle of a massive star

Bibliography

Kaufmann, William J., III. DISCOVERING THE UNIVERSE. New York: W. H. Freeman, 1990. Takes a textbook approach to astronomy beginning with the solar system and moving outward. Chapter 18 focuses exclusively on quasars, and chapter 15 concentrates on black holes.

Rees, Martin J. "Black Holes in Galactic Centers." SCIENTIFIC AMERICAN 263 (November, 1990): 56. Describes how black holes form in active galactic nuclei in general. Gives substantial discussion to the black holes in the cores of quasars and explains how quasars are powered.

Schwarzschild, Bertram. "Probing the Early Universe with Quasar Light." PHYSICS TODAY 40 (November, 1987): 17. Describes the important applications for studying quasars such as probing the nature of the universe. Concentrates on how the light from background quasars is affected by intervening dust clouds, and discusses absorption lines not associated with the quasars themselves.

Seeds, Michael A. FOUNDATIONS OF ASTRONOMY. Belmont, Calif.: Wadsworth, 1990. A textbook approach to astronomy, with thorough development of the physical principles and observations involved. Chapter 17, "Peculiar Galaxies," describes active galactic nuclei and quasars, including their historical development and physical properties. Chapter 18, "Cosmology," describes how quasars can be used to probe the nature of the universe.

Wright, Alan, and Hilary Wright. AT THE EDGE OF THE UNIVERSE. New York: Halsted, 1989. Focuses on quasars from the observational point of view. Relates the observations directly to the physical properties of quasars.

The Evolution of the Universe

The Expansion of the Universe