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astrophysics,

application of the theories and methods of physics to the study of stellar structurestellar structure,
physical properties of a star and the processes taking place within it. Except for that of the sun, astronomers must draw their conclusions regarding stellar structure on the basis of light and other radiation from stars that are light-years away; this light
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, stellar evolutionstellar evolution,
life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable configuration.
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, the origin of the solar systemsolar system,
the sun and the surrounding planets, natural satellites, dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass.
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, and related problems of cosmologycosmology,
area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe. Modern Cosmological Theories
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. The distinction between astrophysics and modern astronomyastronomy,
branch of science that studies the motions and natures of celestial bodies, such as planets, stars, and galaxies; more generally, the study of matter and energy in the universe at large. Ancient Astronomy

Astronomy is the oldest of the physical sciences.
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 is disappearing in scientific usage.

astrophysics

(ass-troh-fiz -iks) The study of the properties, constitution, and evolution of celestial bodies and the intervening regions of space. It is concerned in particular with the production and expenditure of energy in systems such as the stars and galaxies and in the Universe as a whole and with how this affects the evolution of such systems. Astrophysics developed in the 19th century with the application of spectroscopy to the study of light from celestial bodies. It is also now closely related to particle physics, plasma physics, thermodynamics, solid-state physics, and relativity. Cosmology, radio astronomy, X-ray, gamma-ray, infrared, and ultraviolet astronomy are usually considered subsections of astrophysics.

Astrophysics

 

a branch of astronomy that studies the physical phenomena occurring in celestial bodies and their systems and in cosmic space, as well as the chemical processes occurring in them. Astrophysics includes the development of methods for obtaining information on physical phenomena in the universe, the gathering of that information—mainly by means of astronomical observations—and its scientific analysis and theoretical generalization. Theoretical astrophysics, which deals with generalizations and explanations of factual data obtained by observational astrophysics, uses the laws and methods of theoretical physics. The methodology of observational astrophysics is often called practical astrophysics.

In contrast to physics, which is based on experiments where conditions under which phenomena occur can be arbitrarily altered, astrophysics is based primarily on observations during which the investigator has no opportunity to influence the course of the physical process. However, when studying a particular phenomenon, it is usually possible to observe it in many celestial objects under different conditions so that in the final analysis astrophysics turns out to be in a position no less favorable than experimental physics. In many cases the conditions under which matter in celestial bodies and systems exist differ greatly from those attained in modern physics laboratories (extremely high or low densities, high temperatures, and so forth). As a result, astrophysical research frequently leads to the discovery of new physical laws.

Historically, the division of observational astrophysics into independent disciplines was based on two factors: the methods of observation and the objects being observed. Such disciplines as astronomical photometry, astronomical spectroscopy, astrospectrophotometry, astropolarimetry, astrocolorimetry, X ray astronomy, gamma astronomy, and others use different methods of observation. Examples of disciplines classified by the object of investigation are solar physics, physics of the planets, nebular physics, physics of galactic nebulas, and stellar physics.

As the technology of space flight develops, an increasingly greater role in astrophysics is being played by extra-atmospheric astronomy, which is based on observations made by instruments placed on artificial earth satellites and space probes. As astronautics developed, the possibility arose of setting up such instruments on other celestial bodies (first of all on the moon). The growth of experimental astronomy is predicated on this very basis. Radio astronomy (radiolocation of meteors, the moon, and planets closest to the earth) and laser astronomy border on experimental and observational astronomy. Radiolocation and laser astronomy obtain information, used in astrophysics, about celestial bodies by artificially irradiating them with beams of electromagnetic waves.

Astrophysical discoveries that disclose new forms of the existence of matter and new forms of its natural organization are a brilliant confirmation of the fundamental thesis of dialectical materialism about the qualitative inexhaustibility of matter.

The leading centers of astrophysical research in the USSR are the Crimean Astrophysical Observatory of the Academy of Sciences of the USSR, the Central Pulkovo Astronomical Observatory of the Academy of Sciences of the USSR, the Abastumani Astrophysical Observatory of the Academy of Sciences of the Georgian SSR, and the Biurakan Astrophysical Observatory of the Academy of Sciences of the Armenian SSR. Important work in astrophysics is also carried out at Moscow and Leningrad universities. Astrophysical research is developing in astronomical institutions in Alma-Alta, Dushanbe, Shemakha, and Riga. One of the oldest observatories in our country, in Tartu (presently in Toravere), which has resumed work in the past decades, is primarily occupied with astrophysical research. Astrophysical research is also conducted at the Zimenki and Serpukhov radio astronomical observatories. Among the most prominent foreign research institutions conducting astrophysical work are the Mount Palomar Astronomical Observatory and the Lick Astronomical Observatory in the USA, the San Michel Observatory and the Paris Astrophysical Institute in France, the Ondřejov Astronomical Observatory in Czechoslovakia, the Konkoly Astronomical Observatory in Hungary, the radio astronomical observatories of Cambridge and Jodrell Bank in Great Britain, and Parkes Observatory in Australia.

History. As early as the second century B.C, stars that were visible to the naked eye were divided into six classes (stellar magnitudes) based on their brightness. Actually, this division was later refined and expanded to include fainter stars and nonvisual methods of receiving radiation and became the basis of contemporary astronomical photometry. Even before the invention of the telescope, solar prominences were described in Russian chronicles (12th century). In addition, bright comets as well as novas and supernovas in our galaxy were discovered; in particular, detailed observations of Supernova 1572 in Cassiopeia were made by the Dane Tycho Brahe and the Prague astronomer T. Hagek. The invention of the telescope enabled man to acquire valuable information about the sun, moon, and planets. The discovery of the phases of Venus by Galileo and the atmosphere of Venus by M. V. Lomonosov was of great importance in understanding the nature of planets. The German scientist J. Fraunhofer’s detailed studies of the dark lines in the sun’s spectrum (1814) became the first step in obtaining extensive spectral information on celestial bodies. The value of this information was appreciated after the work of G. Kirchhoff and P. Bunsen (Germany) on spectral analysis (1859–62). Beginning in the early 1890’s most of the world’s largest telescopes were equipped with slit spectrographs for studying the spectra of stars with high dispersion. The photography of the spectra of stars and other celestial bodies became the main part of observation programs with these instruments. Such pioneers of modern astrophysics as the Russian astronomer A. A. Belopol’skii, H. Vogel of Germany, and W. Campbell and E. Pickering of the USA dedicated their efforts to this work. As a result of their research, the radial velocities of many stars were determined, spectrally binary stars were discovered, changes in the radial velocities of cepheids were found, and a basis for the spectral classification of stars was established.

In the first half of the 20th century, the rapid growth of laboratory spectroscopy and the theory of atomic and ionic spectra based on quantum mechanics permitted an interpretation of stellar spectra and the development of stellar physics, particularly that of stellar atmospheres. In the first quarter of the 20th century, the Indian physicist M. Saha formulated the basis of the theory of ionization in stellar atmospheres.

Theoretical astrophysics, founded in the first quarter of the 20th century by the German astronomer K. Schwarz-child and the English astronomer A. Eddington and concentrating on stellar atmospheres and structures, spurred interest in the study of stellar spectra. The process continued until the middle of the century when, along with spectral studies, methods of radio astronomy, extragalactic astronomy, and extra-atmospheric astronomy began to play an important role in astronomical investigations.

Beginning in the second quarter of the 20th century, as a result of the identification of forbidden lines in the spectra of gaseous nebulas and the expansion of studies of interstellar absorption, first investigated by the Russian astronomer V. Ia. Struve in 1847, the physics of interstellar matter began to develop rapidly. Methods of radio astronomy opened up for this area of astrophysics limitless possibilities—for example, observation of radio emissions of neutral hydrogen at a wavelength of 21 cm.

E. Hubble of the USA, as early as the 1920’s, conclusively proved the extragalactic nature of spiral nebulas. These celestial objects—galaxies—which seem to be gigantic agglomerations of stars and interstellar matter, are studied by optical as well as radio astronomical methods. Both methods give equally important and mutually complementary information, although the latter yields less information. Beginning in the 1940’s, large reflectors began to be used to photograph the sky. These reflectors have wide fields of view (Schmidt and Maksutov telescopes) which permit the massive study of galaxies and their clusters. Research conducted at the Mount Palomar Observatory (USA) by W. Baade, F. Zwicky, and A. Sandage; at the Biurakan Astrophysical Observatory of the Academy of Sciences of the Armenian SSR by V. A. Ambartsumian, B. E. Markarian, and others; and at the P. K. Shternberg Astronomical Institute in Moscow by B. A. Vorontsov-Bel’iaminov, as well as observations made at radio astronomical obervatories in Cambridge (Great Britain) and Parkes (Australia), have uncovered great varieties of galactic forms and physical processes occurring within them. In the second half of the 1950’s, following the discovery of immense explosive processes demonstrating the activity of the nuclei of galaxies, theoretical astrophysics was faced with the task of explaining them. Quasi-stellar radio sources (quasars) were discovered during the first half of the 1960’s. The study of quasars and the nuclei of galaxies showed that their basic nature differs from that of the stars, planets, and interstellar dust or gas. Because the new phenomena observed in them are unique, existing physical concepts cannot always be applied to them. As a result of these and other discoveries, astrophysics is actually experiencing a revolution comparable in its importance to the revolution during the times of Copernicus, Galileo, Kepler, and Newton and comparable to the revolution that physics experienced in the first third of the 20th century. The development of extra-atmospheric astronomy has significantly enriched the methods of planetary astronomy. Its first outstanding results include the photography of the far side of the moon (1959, USSR), the first launching of scientific instruments to the moon and reception of pictures of the lunar landscape (1966, USSR), photographs of Mars taken at close range (1965, USA), the penetration by Soviet space probes of the lower layers of the atmosphere of Venus (1967, USSR), and the landing of astronauts on the moon and the beginning of direct studies of the lunar soil (1969, USA).

Studies of bodies of the solar system. The earth, which is the subject of geophysics, is the most fully studied of all the planets. Until the middle of the 20th century, data on the other eight planets remained relatively scarce. However, the research based on observations with space probes will change this situation in the near future. In solving with terrestrial methods different problems in structure and composition of planetary atmospheres, astrophysics frequently uses the same observational and theoretical methods as geophysics, in particular, the methods of investigating the upper layers of the earth’s atmosphere. Of special interest are spectral investigations of planets endowed with an atmosphere. As a result of such investigations, fundamental differences in the composition of planetary atmospheres have been established. In particular, it has been shown that the main component of Jupiter’s atmosphere is ammonia and that of Venus is carbon dioxide, whereas on the earth molecular nitrogen and oxygen predominate. The discovery of large crater-like formations on Mars, with the aid of the space probe Mariner (USA), has necessitated the formulation of a general theory of the origin of the topography on the planets and the moon. Two opposing theories exist regarding the origin of craters on the moon and Mars. One attributes their formation to volcanic processes, the other, to the impacts of gigantic meteors. Since the discovery of new evidence in favor of volcanic activity on the moon, the first theory is gaining more and more supporters. Radar observations supply information on the peculiarities of planet topographies as well as on the laws of their motions (V. A. Kotel’nikov of the USSR and others).

Most planet satellites, like all asteroids, do not have atmospheres, since the gravitational force on their surfaces is insufficient to retain gases. Moreover, the small angular sizes of these bodies prevent the study of their surface details. Therefore, the only information about the physics of these bodies is based upon the measurements of their integral reflectivity in different spectral regions. Measurements of their brightness give us information about their rotation.

Of great interest are phenomena occurring when comets near the sun. Because of sublimation, which occurs under the influence of solar radiation, the kernel of the comet emits gases forming the broad comet head. Solar radiation and, probably, solar wind foster the formation of the comet’s tail, which can sometimes reach a length of millions of kilometers. The emitted gases escape into interplanetary space. As a result, every time the comet nears the sun, it loses a significant part of its mass. In connection with this, comets, especially short-period ones, are considered as objects having small life spans measured in millennia or even centuries (S. K. Vsekhsviatskii and others). The study of the origin and development of the system of comets has allowed us to make conclusions regarding the evolution of the entire solar system.

Solar physics. Physical processes occurring within the sun are practically independent of the influence of the surrounding medium. The sun’s development, at least in the current epoch, is governed by its own internal laws. It has been shown that the sun’s interior, like the interior of all stars, has sources of thermal energy (nuclear in nature), because of which the mass of the sun (stars) heats up to high temperatures. This results in the emission of radiant energy from the surface. A balance is established between the intensity of the solar (stellar) radiation and the total intensity of the sources of thermal energy within the sun. At the same time, the manifestation of solar activity—the sun’s radiation, the sun’s emission of fluxes of particles having “frozen in” magnetic poles—significantly affects the development of all bodies in the solar system. Objects of detailed study are such varied formations in the solar atmosphere as sunspots, faculae, and prominences. Of special interest are the short chromo-spheric flares, usually lasting only dozens of minutes and accompanied by the emission of a significant quantity of energy. Corpuscular streams, which are connected with active solar regions, were studied at the Crimean Astrophysical Observatory of the Academy of Sciences of the USSR by E. R. Mustel’. Constant changes of magnetic fields are occurring in the sun’s outer layers. Investigations conducted at the same observatory by A. B. Severnyi made it possible to establish the connection between the solar flares and rapid changes in the magnetic field’s structure in a given portion of the solar surface. Theoretical research has shown that energy is transferred in the sun, as in the stars, primarily by the emission and absorption of radiation. The theory of solar radiation balance is based on this conclusion. This theory is valid for both the outer and inner layers of the sun.

The most important question of solar and stellar physics is the nature of energy sources. The energy due to gravitational contraction is inadequate. The hypothesis that the source of solar energy is a thermonuclear reaction can provide a satisfactory quantitative explanation of radiation over billions of years. Nevertheless, this theory needs conclusive proof. The complete explanation of the nature of solar and stellar energy sources will have great importance in answering questions related to the evolution of the sun and stars.

Because of the scientific significance of the study of physical processes occurring on the surface layers of the sun and their effects on the upper layers of the earth’s atmosphere, the observatories of many countries have united to observe these processes systematically, using all possible methods, by organizing the around-the-clock Solar Activity Service.

Stellar physics. Theories of the sun’s structure play an important role in the study of the stars. These theories are modified in such a way as to be consistent with photometric and particularly spectral data on the stars. The varied character of spectral information permits, in the final analysis, a singular solution of this problem. Today, more than a million spectra of stars have been classified. Spectral classification of stars was first developed at the beginning of the 20th century at the observatory at Harvard University (USA) and was later perfected and refined. The chief characteristic of this classification is the presence of particular spectral lines and their relative intensities.

Of great interest are the so-called white dwarfs, which have relatively high surface temperatures (7000°-30,000° C) and low luminosities, many times less than the luminosity of the sun. The average density of some white dwarfs exceeds that of water by more than a million times. Later, the possibility of the configurations of stellar masses consisting of the degenerated gases of neutrons and even hyperons was theoretically established. The density of such configurations must reach 1014-1015 times that of water. However, over the years, these configurations could not be detected. Only in 1967, pulsars—objects emitting periods of variability ranging from seconds to fractions of a second—were discovered. There are significant reasons to believe that these are the superdense configurations.

Of special interest are variable stars whose brightness and spectra change. In those cases when such changes have a periodic or almost periodic nature, they are explained by pulsations—that is, successive expansions and contractions of stars. Greater changes occur in nonstationary stars, many of which are young stars in the process of formation. Stars of the type RW in Auriga are of great importance. They display completely irregular changes in brightness and belong to the T associations, which are not more than 10 million years old. At a later stage of development, many of these stars have a normally constant brightness and experience flashes from time to time lasting only several minutes, during which their luminosity increases several times and sometimes (in the short-wave region of the spectrum) a hundredfold. The variable star UV in Cetus is an example of a star at this stage. While the normal radiation of stars has a purely thermal nature, the energy emitted during flashes has an obviously nonthermal nature. Even more massive processes of energy emission occur during the explosions of novas and supernovas. During the explosions of supernovas, energy on the order of 1042 joules (1049 ergs) a month is released. During explosions of novas and supernovas, the expanding gaseous shells are expelled. The outburst of so-called nova-like variable stars, in part like the stars of the type SS in Cygnus, occupy, according to their size, an intermediate place between explosions of novas and stars of the type UV in Cetus.

Nebular physics. The physical processes occurring in gaseous nebula irradiated by hot stars have been studied in some detail. These processes are, in essence, fluoresence under the influence of ultraviolet radiation of the hot stars. Gaseous nebulas not irradiated by hot stars can be studied because they emit a 21-cm wavelength radiation due to hydrogen atoms. In most gaseous nebulas there also exist dust substances composed of hard particles. If a gas-dust nebula is irradiated by a relatively low-temperature star, whose radiation does not cause fluorescence of the gas, the light of the illuminating star is reflected by the nebula’s dust components and is observed. In these cases, the nebula’s spectrum is a reproduction of the star’s spectrum. In our galaxy, radio nebulas emitting continuous spectra in the radio frequency range are also observed. Such radiation is connected with the deceleration of relativistic electrons in the magnetic fields—the so-called synchroton radiation investigated by the Soviet astronomer I. S. Shklovskii and others. These nebulas appeared as a result of the expolsions of supernovas; such is the nature of the Crab nebula and the radio source Cassiopeia A. Their life-span is measured in thousands and sometimes only hundreds of years.

Physics of extragalactic objects. During initial studies, galaxies were considered mechanical agglomerations of stars and nebulas. Therefore, only their internal kinematics and dynamics were discussed. Soon, however, it became clear that a definite connection existed between the form of galaxies (elliptical, spiral, or irregular) and the classes of stars within them (stellar population), in particular, the presence of young stars—the blue giants. In the arms of spiral galaxies, great nonuniformities are observed—O associations—which represent systems consisting of young stars and nebulas. Their appearance is evidently related to profound physical processes during which great masses of prestellar matter are transformed into common stars. The study of these processes is one of the most difficult unsolved problems in astrophysics.

Beginning in the middle of the 20th century, the nuclei of galaxies began to assume a great role in explaining their evolution. Various kinds of activities of the nuclei were established—in particular, gigantic explosions during which huge clouds of relativistic electrons were emitted. As a result of such explosions, ordinary galaxies became radio galaxies. Surges of clouds and streams of ordinary gas also occur. All these phenomena testify to the fact that very profound processes of converting matter and energy occur in nuclei of galaxies.

The discovery of quasi-stellar sources of radio emission (quasars), as well as the discovery of quasi-stellar sources of purely optical objects, led to the discovery of even more profound processes. First, it became evident that among quasars there were objects that emitted radiation 1013 times more powerful than the sun’s and were hundreds of times brighter than supergigantic galaxies.

Quasars experience relatively quick changes of brightness, which accounts for their small diameters; continuous spectra radiate from a volume with a diameter not exceeding 0.2 parsec. In many respects, quasars resemble the more active nuclei of galaxies except that the phenomena in them are on a larger scale. Quasar masses are unknown. However, considering them as extremely large isolated nuclei, it can be assumed that they have 1011 times the mass of the sun or more.

Theoretical astrophysics. The goal of theoretical astrophysics is to explain the phenomena studied in astrophysics using the general laws of physics. Theoretical astrophysics uses the methods already developed by theoretical physics as well as special methods developed for the study of phenomena in celestial bodies and related to the special properties of these bodies. Since all information about astrophysical processes is based on the recording of the radiation that reaches us, the first objective of theoretical astrophysics is the direct interpretation of results of observations and the compilation of an external picture, during their inception of the processes that are taking place—for example, observations of the brightness and spectra of novas were successfully interpreted from the hypothesis that the stars throw off their shells into surrounding space. However, the ultimate goal of theoretical astrophysics is to clarify the mechanism and determine the cause of the phenomenon (in the cited example—the reason for the explosion which leads to the expulsion of the shell). In most cases, the basic distinguishing feature of processes studied in theoretical astrophysics is the vital role of the interaction between matter and radiation. Therefore, theoretical astrophysics, in addition to solving concrete problems, also develops general methods for investigating this interaction. While theoretical physics is concerned with the elementary processes of this type, astrophysics studies the results of repeated and complex interactions in large systems. Thus, the theory of radiation transfer in a material environment, which is used in other branches of physics, reached great sophistication specifically in astrophysics. The successful development of the theory of the transfer of radiation in spectral lines in the works of Soviet astronomers V. V. Sobolev and others made it possible to establish exact laws governing the appearance of lines of absorption and emission lines in stellar atmospheres. Thus, a quantitative interpretation of stellar spectra became possible. General methods of calculating the equilibrium of stellar masses have also been developed. Much work on the equilibrium configurations of gaseous stars has been done by M. Schwarzschild of the USA and A. G. Masevich of the USSR. The theory of degenerate configurations in which the electron gases are considered degenerate was developed in the second quarter of the 20th century by E. Milne of Great Britain and S. Chandrasekhar of India. In the case of superdense configurations in which barion gas is already degenerate, calculations must be made based on the general theory of relativity. These questions as well as theoretical studies related to the process of expansion of the universe as a whole consititute a new branch of theoretical astrophysics called relativistic astrophysics.

The results of astrophysical research are published primarily in the transactions of observatories as well as in special journals. The latter include Astronomicheskii zhurnal (Moscow, since 1924), Astrofizika (Yerevan, since 1965), Astrophysical Journal (Chicago, since 1895), Monthly Notices of the Royal Astronomical Society (London, since 1827), Annales d’astrophysique (Paris, 1938–68), and Zeitschrift fur Astrophysik (Berlin, 1930–44).

REFERENCES

Kurs astrofiziki i zvezdnoi astronomii, vols. 1–3. Moscow-Leningrad, 1951–64.
Sobolev, V. V. Kurs teoreticheskoi astrofiziki. Moscow, 1967.
Ambartsumian, V. A. Problemy evoliutsii Vselennoi. Yerevan, 1968.
Razvitie astronomii v SSSR. Moscow, 1967.
Struve, O. V., and V. Zebergs. Astronomiia 20 v. Moscow, 1967. (Translated from English.)
Zel’dovich, la. B., and I. D. Novikov. Reliativistskaia astrofizika. Moscow, 1968.

V. A. AMBARTSUMIAN

astrophysics

[‚as·trō′fiz·iks]
(astronomy)
A branch of astronomy that treats of the physical properties of celestial bodies, such as luminosity, size, mass, density, temperature, and chemical composition, and with their origin and evaluation.

astrophysics

the branch of physics concerned with the physical and chemical properties, origin, and evolution of the celestial bodies
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