A quasar is any of a class of rare cosmic objects of high luminosity as well as strong radio emission observed at extremely great distances. The term is also often applied to closely related objects that have the same optical appearance but that are radio quiet--the so-called QSOs, which stands for "quasi-stellar objects."

Quasars were first detected as unresolved sources in surveys conducted during the 1950s by radio astronomers in Cambridge, Eng. Optical photographs subsequently taken of their spectra showed locations for emission lines at wavelengths that were at odds with all celestial sources then familiar to astronomers. The puzzle was solved by the American astronomer Maarten Schmidt, who announced in 1963 that the pattern of emission lines in 3C 273 could be understood as coming from hydrogen atoms that had a redshift of 0.158. In other words, the wavelength of each line was 1.158 times longer than the wavelength measured in the laboratory where the source is at rest with respect to the observer. (The general formula is that, if the factor is 1 + z, astronomers say the astronomical source has a redshift of z. If z turns out to be negative [i.e., if 1 + z is less than 1], the source is said to be "blueshifted.")

Schmidt's discovery raised immediate excitement, since 3C 273 had a redshift whose magnitude had been seen theretofore only among the most distant galaxies. Yet it had a starlike appearance, with an apparent brightness (but not a spectrum) in visible light not very different from that of a galactic star at a distance of a few thousand light-years. If the quasar lay at a distance appropriate to distant galaxies a few times 10 light-years away, then the quasar must be 10¹² times brighter than an ordinary star. Similar conclusions were reached for other examples. Quasars seemed to be intrinsically brighter than even the most luminous galaxies known, yet they presented the pointlike image of a star.

A hint of the actual physical dimensions of quasars came when sizable variations of total light output were seen from some quasars over a year or two. These variations implied that the dimensions of the regions emitting optical light in quasars must not exceed a light-year or two, since coherent fluctuations cannot be established in any physical object in less time than it takes photons, which move at the fastest possible speed, to travel across the object. These conclusions were reinforced by later satellite measurements that showed that many quasars had even more X-ray emission than optical emission, and the total X-ray intensity could vary in a period of hours. In other words, quasars released energy at a rate exceeding 10¹² suns, yet the central machine occupied a region only the size of the solar system.

Understandably, the implications were too fantastic for many people to accept, and a number of alternative interpretations were attempted. An idea common to several of the alternatives involved the proposal that the redshift of quasars arose from a different (i.e., noncosmological) origin than that accepted for galaxies. In that case, the distance to the quasars could be much less than assumed to estimate the energy outputs, and the requirements might be drastically relaxed. None of the alternative proposals, however, withstood close examination.

In any case, there now exists ample evidence for the validity of attributing cosmological distances to quasars. The strongest arguments are the following. When the strong nonstellar light from the central quasar is eliminated by mechanical or electronic means, a fuzzy haze can sometimes be detected still surrounding the quasar. When this light is examined carefully, it turns out to have the colour and spectral characteristics appropriate to a normal giant galaxy. This suggests that the quasar phenomenon is related to nuclear activity in an otherwise normal galaxy. In support of this view is the observation that quasars do not really form a unique class of objects. For example, not only are there elliptical galaxies that have radio-emission characteristics similar to those of quasars, but there are weaker radio sources among spiral galaxies (called Seyferts after their discoverer, the American astronomer Carl K. Seyfert), which have bright nuclei that exhibit qualitatively the same kinds of optical emission lines and nonstellar continuum light seen in quasars. There also are elliptical galaxies, N galaxies, and the so-called BL Lac objects, which have nuclei that are exceptionally bright in optical light. Plausible "unification schemes" have been proposed to explain many of these objects as the same intrinsic structure but viewed at different orientations with respect to relativistically beamed jets or with obscuring dust tori surrounding the nuclear regions or both. Finally, a number of quasars--including the closest example, the famous source 3C 273--have been found to lie among clusters of galaxies. When the redshifts of the cluster galaxies are measured, they have redshifts that bracket the quasar's, suggesting that the quasar is located in a galaxy that is itself a cluster member.

Excerpt from the Encyclopedia Britannica without permission.