Quasars


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Quasars

 

or quasi-stellar radio sources, celestial objects that resemble stars in optical appearance and gaseous nebulas in the character of their spectra; quasars also exhibit significant red shifts (exceeding the largest known red shifts of galaxies by up to a factor of 6). The last property determines the important role of quasars in astrophysics and cosmology.

The discovery of quasars was the result of increased precision in the determination of the coordinates of extragalactic sources of radio emission, which has led to a great increase in the number of radio sources identified with celestial objects seen optically. The first identification of a radio source with a starlike object was made in 1960. In 1963, when the American astronomer M. Schmidt identified the spectral lines of these objects, which had shifted owing to the effect of the red shift, they were isolated as a special class of cosmic objects—quasars. Thus, quasars with strong radio emission were discovered first; however, quasars with weak radio emission were also subsequently discovered (about 98.8 percent of all quasars accessible to observation). This numerous variety of quasar are called radio-quiet quasars, quasar-galaxies (“quasags”), interlopers, and sometimes, blue starlike objects. The total number of quasars accessible to observation is about 105; of these about 1,000 have so far been identified with optical objects, but using spectral criteria only approximately 200 have been established as reliably belonging to quasars.

Intense ultraviolet radiation and broad bright lines are revealed in the spectra of quasars. These are characteristic for hot gaseous nebulas (temperatures around 30, 000°C) but are significantly shifted toward the redo end of the spectrum. At red shifts exceeding 1.7, the La 1216 A resonance line of hydrogen even becomes visible on photographs of quasar spectra. From time to time, narrow dark lines, caused by the absorption of light in the intergalactic gas surrounding the quasars, are observed in the spectra of quasars. On photographs, quasars look like stars, and thus their angular diameter is less than 1”. Only the nearest quasars reveal optical features: an elliptical form of starlike image and gas ejections. Quasars are distinguished on photographs from normal stars by their intense ultraviolet radiation, which is characterized by a blue color index, and from white dwarfs, by excessive infrared radiation, even if the quasars do not exhibit radio emission.

Variations in the brightness of many quasars are, evidently, one of the fundamental properties of quasars (the shortest variation has a period τ ≈ 1 hr, the maximum changes in brightness are by a factor of 25). Since the size of an object of variable brightness cannot exceed cτ (c is the velocity of light), the size of quasars cannot be more than 4 × 10” m (less than the diameter of the orbit of Uranus). Only with the motion of the matter with a velocity close to that of light can this size be larger. In contrast to continuous radiation, variations in the intensity of the spectral lines are rare.

As radio sources, quasars are similar to radio galaxies. Quasars are often observed with two extended radio sources, not necessarily identical in intensity, which are located at significant distances in different directions from the optical object. The mechanism of radio emission in these and other quasars is synchrotron radiation. In addition, compact radio sources are observed in quasars, which give rise to variations in radio emission at centimeter wavelengths; these sources are expanding clouds of relativistic particles existing for several years. The mechanism of their radio emission is linked, evidently, with plasma oscillations.

Little has been learned about the nature of quasars. Depending on the interpretation of the nature of the red shift in the spectra of quasars, three hypotheses are under consideration (as of the early 1970’s). The most plausible is the cosmological hypothesis, according to which the large red shifts indicate that quasars are located at enormous distances (up to 10 gigaparsecs) and are taking part in the expansion of the metagalaxy. Determinations of the distances to quasars (according to their red shifts) as well as estimates of their masses and luminosities are based on this assumption. In the cosmological hypothesis, quasars, according to their absolute magnitudes (—27) and masses (about 1038 kg, that is, 108 solar masses), are actually superstars. The physical nature of a quasar in this case is connected with the gravitational collapse of a mass of gas, which is stopped by magnetic turbulence or the rotation of the quasar.

The large output of energy in all forms of electromagnetic radiation, according to this hypothesis, limits the active stage of a quasar to 104 years. In the power of their radio emission (1045 W), quasars are comparable to radio galaxies. It is supposed that quasars are supermassive stars with radii on the order of 1012 m, whose plasma continuously and by means of great explosions ejects streams of particles of different energies. At a radius of about 1016 m, the quasars are surrounded by clouds of ionized gas, which produce the bright lines in the spectra of quasars, and at distances on the order of 1019 m are located clouds of relativistic particles, trapped in weak magnetic fields—the radio-emitting clouds of the quasars.

The nearest quasars are located at more than 200 megapar-secs. Their relative scarcity and the short duration of their existence corroborates the assumption that quasars constitute a stage in the evolution of large cosmic masses, for example, galactic nuclei. Thus, there exists a nonaccidental similarity between quasars and N-galaxies, Seyfert galaxies, and blue compact galaxies in the character of their spectra, variations of brightness, and radio emission. The nearest quasars, whose structures have been observed on photographs, have proved to be N-galaxies, on the basis of which they have been grouped into one class of compact, very bright objects. The nature of the object BL Lacer-tae (and several more) is puzzling. In its brightness variations, radio emission, color index, and optical structure it appears to be a typical quasar; yet at the same time it has no lines in its spectrum.

According to another hypothesis, quasars with velocities close to the velocity of light are flying out as a result of an explosion (which occurred several million years ago in the center of the galaxy) of matter with a mass of about 1040 kg and the subsequent ejection of this matter. According to this hypothesis, the masses of quasars are about 1031 kg (5 solar masses), and the distances to them range from 60 to 600 kiloparsecs. However, the physical processes that would generate the energy necessary for the explosion (1058 J) are not known.

The third hypothesis assumes that quasars are compact gaseous objects with sizes of 1016-1017 m and masses of 1042—1043 kg, in whose spectra the lines have a large red shift of a gravitational nature.

REFERENCE

Burbidge, G., and M. Burbidge. Kvazary. Moscow, 1969. (Translatedfrom English.)

IU. P. PSKOVSKII

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