Extragalactic Astronomy


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Extragalactic Astronomy

 

a branch of astronomy studying celestial bodies and systems that lie beyond our stellar system, the Milky Way galaxy. The formation of this branch of astronomy was preceded by a long period for determining what types of celestial bodies make up our stellar system and what types are found outside it. At the end of the first quarter of the 20th century it was conclusively established that our stellar system has finite dimensions and at the same time does not exhaust the entire stellar universe. It was called the Galaxy (the Milky Way galaxy). Also established was the existence of other stellar systems which, because of their closed nature and independent position in space, were called galaxies. The totality of all galaxies, called the metagalaxy, is the most extensive system known to science. The most distant of the brightest galaxies, whose distances it has been possible to establish, are located over 1 billion par-sees away from us. The exact value of this maximum distance cannot be indicated since, first, more and more remote objects become known almost annually and, second, the result of computing distances based on quantities obtained directly from observations depends on the assumed proper-ties of space in the metagalaxy, which have not been sufficiently studied. Nevertheless, it may be asserted that the most distant of the known galaxies are not at the limits of the metagalaxy.

The results of investigations obtained by extragalactic astronomy are the main observational material for cosmology. In studying natural phenomena on a very large scale, extragalactic astronomy encounters new, previously unknown, phenomena and perhaps even new laws of nature. The results of extragalactic astronomy greatly assist the study of our galaxy. This is conditioned by the fact that we observe other galaxies from the outside and as a whole, but we must study our own galaxy from within. This is more difficult for a number of reasons. The solar system is located within the dusty equatorial layer of our galaxy, which severely reduces our zone of visibility, especially in directions close to the plane of the galactic equator. Other galaxies are seen as a whole and from various points of view depending on their random orientation with respect to our line of sight. But be-cause of the great distances to the galaxies, the various-type stars that make up these galaxies can almost never be ob-served separately. However, data on the types of stars and their motions in our galaxy contribute to a better understanding of other stellar systems.

The distribution of galaxies in space is not uniform. Most of them are concentrated in compact or scattered clusters of galaxies containing from dozens to tens of thousands of members. The rates of motion of galaxies in clusters, measured by spectrograms based on the Doppler effect, are random in direction and may be as high as 2,000 km/sec. In some cases these velocities are so great that they may prove sufficient for galaxies to leave a cluster. The question as to what extent the distribution of galactic clusters in the metagalaxy may be considered uniform has not yet been answered. On the one hand, the majority of galaxies are concentrated in randomly scattered clusters and, on the other, no marked asymmetry in the distribution of clusters and no tight crowding are observed. The question of whether the real universe is uniform or nonuniform is important to cosmology.

The metagalactic space between galaxies is not empty. There are many small stellar systems, individual stars, rarefied gas, and cosmic dust in it as well as cosmic rays. Moreover, the intensity of the fields—including the gravitational and magnetic fields—are nonzero. The study of these fields is also part of the task of extragalactic astronomy.

At the turn of the 19th century, the English astronomer W. Herschel was the first to compile extensive catalogs of the bright nebulous spots visible in the sky. Investigations showed that some of them, when viewed through strong tele-scopes, proved to consist of stars. At the same time, how-ever, the existence of nebulas consisting of a continuous dif-fuse medium was recognized. This was finally proved in the second half of the 19th century by spectral analysis. The spectrum of some nebulas proved to consist of bright lines that belonged to rarefied gases. For other nebulas it resembled the spectrum of star clusters—continuous, with absorption lines; such nebulas constituted an overwhelming majority. Later it was learned that a small percentage of nebulas with such a spectrum do not constitute stellar systems but are clouds of cosmic dust shining with the reflected light of bright stars. In the 1920’s, E. Hubble (of the United States) was able to prove that gaseous and dust nebulas are found even among objects comparatively close to us. Somewhat earlier H. Shapley succeeded in determining the distances to globular star clusters, of which the most distant “resolve” into stars only with difficulty, even through the most powerful telescopes.

The nature of the remaining nebulous spots (and there is a tremendous number of them; the catalogs contain about 30,000 objects up to the 15th visual stellar magnitude) was clarified by the middle of the 1920’s. As early as the middle of the 19th century the English scientist W. Parsons (Earl of Rosse) observed a spiral structure in the largest of them, but the diversity and fineness of the structure of nebulas were brought to light only after the introduction into astronomical practice of photography and telescopes of increased power. The Swedish astronomer K. Lundmark, observing in spiral nebulas the scarcely noticeable nova outbursts, which actually were of tremendous luminosity, concluded that spiral nebulas lie beyond our galaxy. Subsequently it was learned that stars whose explosions were observed in galaxies most often were not new stars but supernovas, hundreds of times brighter, as a result of which the estimates of distances to spiral nebulas made by Lundmark had to be increased. Not a single supernova has been observed in our galaxy since the invention of the telescope. Therefore the study of these interesting celestial bodies rests mainly on the results of extragalactic astronomy.

Later, Hubble determined more precisely the distances and dimensions of the spiral galaxies M31 (the Great Nebula in Andromeda), M33 (in the Triangulum constellation), and NGC 6822 (in Sagittarius). He proved the great similarity of these stellar systems to our galaxy by establishing that they all contain stars of identical types, identical star clusters, diffuse gaseous nebulas, and novas. These discoveries, like many that followed in extragalactic astronomy, were accomplished with the aid of the largest telescopes in the world, mounted in the United States.

In 1924-25 variable stars, including the cepheids, whose luminosity is known to be connected with the period of variation of their brightness, were detected in photographs of nearby spiral galaxies. Thus, by determining the luminosity on the basis of the observed variation in the brightness and by comparing it with the visual stellar magnitude of these celestial bodies, it is possible to estimate distances to the cepheids and hence to the galaxies containing them. (The dimensions of galaxies are small in comparison with the distances to them.) The cepheid method of determining distances to remote stellar systems is most accurate but is applicable only to the closest ones. For more remote systems, including the most distant systems observed at present, the method of determining distances to galaxies from the line shift in the spectrum of the galaxies, the so-called red shift, is best. In 1924, K. Lundmark and K. Wiertz (of Germany) discovered that the greater the distance to a galaxy the more strongly its spectrum is displaced toward the red end. Later, the magnitude of the red shift caused by movement away from us (the Doppler effect) was determined more precisely. In determining distances by this method it is assumed that for each million parsecs of distance the red shift increases by approximately 100 km/sec (Hubble’s law). This systematic shift due to the expansion of the metagalaxy has superposed on it the shift of spectral lines (toward the red or blue end) due to the individual velocities of the galaxies, which do not usually exceed 1,000 km/sec. Because of this, the method of determining distances by the red shift of spectral lines is unreliable when applied to nearby galaxies.

The tasks of extragalactic astronomy are to study photo-graphically the shapes and types of galaxies, to classify galaxies (the foundation for which was laid by Hubble), to measure the stellar magnitudes and colors of galaxies on the whole and of individual sections, and to investigate the principles governing the structure and composition of galactic clusters. The number and distribution of various objects of various luminosities are studied in the closest galaxies. By means of spectral analysis the rates of motion and the laws governing the rotation of galaxies are studied. This provides material for determining their masses. The chemical composition of the stars making up the galaxies is also studied and compared. Electron image intensifies, which reduce the expo-sure time and make it possible to photograph very faint objects, are used in the photography of galaxies.

New possibilities were offered to extragalactic astronomy by the methods of radioastronomy. With their aid, fundamentally new objects and phenomena in the metagalaxy have been discovered. Among such objects are the so-called radio galaxies, characteristic of which is extraordinarily powerful radiation in the radio band apparently originating from elementary particles of tremendous energy moving in the magnetic fields of some galaxies, and quasars, whose nature is still insufficiently studied. On the basis of the very large red shifts in the spectra of most observed quasars, the conclusion is already being drawn, however, that many of them are at distances of several billion parsecs. So-called quasistellar galaxies, which are starlike objects that have no strong and perhaps not even moderate radio emissions, are similar to quasars in luminosity and spectrum. They are dozens of times more numerous than quasars. At the same time there is much in common between the turbulent processes in quasars and the nuclei of some galaxies.

In the USSR the most extensive theoretical and observational investigations in extragalactic astronomy are being conducted at the Biurakan Astrophysical Observatory of the Academy of Sciences of the Armenian SSR and at the P. K. Shternberg State Astronomical Institute of Moscow University.

B. A. VORONTSOV-VEL’IAMINOV

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