The study of quasars goes back quite a ways, and it’s not until recently observations with more sensitive and advanced equipment made it easier to ensure the accuracy of measurements.
Quasars have been detected in essentially every wavelength band used by astronomers from radio to [gamma]-rays. Extensive radio, infrared, optical, and x-ray surveys have been carried out. Thus the purely empirical properties of quasars are fairly well known.
Quasars are essentially defined by their optical properties to be unresolved or nearly unresolved (that is, having nearly all of their flux in a component unresolved at the [is approx.] 1″ limit imposed by atmospheric blurring) objects with spectra containing highly red-shifted emission lines (7). These red shifts (= [delta][lambda]/[lambda]) generally lie in the range from 0.1 to nearly 3.8 with many values near 2.0. If not completely stellar (unresolved) in appearance, quasars generally show faint and frequently asymmetric halos with sizes of the order of a few arc-seconds. In addition to these defining optical features, many quasars share several other optical characteristics. The observed emission lines may generally be identified with resonance lines of hydrogen or of ions of heavy elements present in high cosmic abundance. The two most common strong quasar emission lines are Lyman-[alpha] (hydrogen) and C IV (triply ionized carbon) with rest wavelengths in the vacuum ultraviolet. In addition to exhibiting large red shifts, quasar emission lines are generally very broad with widths up to several percent of their observed wavelengths. The continuum emission from quasars is usually a nonthermal spectrum with strong ultraviolet and infrared excesses relative to a thermal Planck spectrum; in many cases the flux per unit frequency may be approximated by a v.sup.-1 power law, although there are wide variations about this mean slope. Almost all quasars show slow (over years) continuum flux variations of a few tens of percent, and a few vary rapidly (in as little as a few days) by factors up to 100 or more. Large and variable polarization has also been seen in some of the optically active quasars. Finally, most quasars of high red shift show a multitude of narrow and weak absorption lines when observed at high spectral resolution with good signal-to-noise ratio. It is only possible to identify these lines by associating them with a number of discrete objects each having a different red shift (in almost all cases smaller than the emission-line red shift) through which the quasar continuum source is being seen.
Only a small fraction ([is approx.] 3 percent) of quasars are radio sources; however, radio quasars are strongly overrepresented among known (catalogued) and well-studied quasars because, until fairly recently, the optical identification of radio sources was by far the easiest way to locate quasars seen in projection among the ordinary stars in our Galaxy. Those quasars that are radio sources are frequently very powerful and, in fact, number among the brightest radio objects in the sky. Radio quasars generally contain a compact central source with structure frequently extending down to scales of milliarc-seconds, emitting a flat (spectral index [is approx.] -0.25) power-law spectrum over a wide range of frequencies. Strong variability both in flux and in milliarc-second image structure on a time scale of months to years is common. In many cases the central source will be flanked by much larger nonvariable double radio lobes with a steep spectrum, reminiscent of typical radio galaxy structure.
X-ray surveys of quasars made with the Einstein satellite (3) have established that many quasars are strong x-ray sources and that their combined emission must account for at least a modest fraction of the cosmic x-ray background. There are indications in the data that quasar x-ray emission may be well correlated with optical emission, although selection effects and other difficulties with the x-ray sample cast some doubt on this and other statistical inferences about x-ray emission (8).
Turner, Edwin L. “Quasars and gravitational lenses.” Science 223 (1984): 1255+.