The Galaxy

(modern Greek galaktikós —milky, from Greek gala —milk), a vast star system containing the sun and, consequently, our entire planetary system together with the earth. The Galaxy consists of a multitude of various-type stars as well as of star clusters and associations, gaseous and dust nebulas, and individual atoms and particles dispersed in interstellar space. A large portion of all of these occupies a lens-shaped volume with a diameter of about 30 kiloparsecs and a thickness of about 4 kiloparsecs (about 100,000 and 12,000 light-years, respectively). A smaller portion fills an almost spherical volume with a radius of about 15 kiloparsecs (about 50,000 light-years). All the components of the Galaxy are bound up into a single dynamic system revolving around a minor axis of symmetry. To an observer on earth, who is inside the Galaxy, it appears as the Milky Way (hence its name, Galaxy) and the whole totality of individual stars visible in the sky. In this connection the Galaxy is also called the Milky Way System. In contrast to all other galaxies, the one to which the sun belongs is sometimes referred to as “our Galaxy” (the term is always written with a capital G).

Stars and interstellar gaseous and dust matter fill the volume of the Galaxy unevenly: they are concentrated around a plane, which is perpendicular to the Galaxy’s axis of rotation and is the plane of its symmetry (the so-called galactic plane). It is near the line of this plane’s intersection with the celestial sphere (the galactic equator) that the Milky Way is visible; its middle line is almost a major circle, since the solar system is not far from this plane. The Milky Way is a cluster of an enormous number of stars blending into a broad, whitish band. However, the stars that are projected on the sky next to each other are actually enormous distances away from each other in space. This precludes their collisions despite the fact that they are moving at high velocities (tens and hundreds of km/sec) in different directions. The lowest density of distribution of stars in space (spatial density) is observed in the direction of the poles of the Galaxy (its north pole is in the constellation Coma Berenices). The total number of stars in the Galaxy is estimated at 100 billion.

Interstellar matter is also diffused in space unevenly, concentrated primarily near the galactic plane in the form of globules, individual clouds, and nebulas (from 5 to 20-30 parsecs in diameter), and their complexes or amorphous diffuse formations. Particularly large dark nebulas that are relatively close to us appear to the unaided eye as irregularly shaped dark areas against the backdrop of the Milky Way band; the shortage of stars in them is a result of the absorption of light by these nonluminous dust clouds. Many interstellar clouds are illuminated by nearby high-luminosity stars and look like bright nebulas, since they shine either by means of reflected light (if they consist of dust particles) or as a result of the excitation of atoms and their subsequent emission of energy (if the nebulas are gaseous).

The total mass of the Galaxy, including all the stars and interstellar matter, is estimated at 10¹¹ times the sun’s mass, that is, about 10⁴⁴ g. The results of detailed research show that the structure of the Galaxy resembles the structure of a large galaxy in the constellation Andromeda, a galaxy in the constellation Coma Berenices, and others. However, being inside the Galaxy we cannot see its entire structure, and this hampers its study.

The stellar nature of the Milky Way was discovered by Galileo in 1610, but a systematic study of the Galaxy’s structure began only at the end of the 18th century, when W. Herschel, using his “scoop method,” counted the number of stars that were visible in his telescope in various directions. On the basis of the results of these observations, he suggested that the observable stars form a giant system of a flattened shape. In 1847, V. Ia. Struve discovered that the number of stars in a unit of volume increased with proximity to the galactic plane, that interstellar space was not ideally transparent, and that the sun was not in the center of the Galaxy. In 1859, M. A. Koval’skii pointed out the probable axial rotation of the whole system of the Galaxy. The first more or less well-founded estimates of the dimensions of the Galaxy were made by the German astronomer H. von Seeliger and the Dutch astronomer J. Kapteyn in the first quarter of the 20th century. Seeliger, assuming the uneven distribution of stars in space and their different luminosities, concluded that surfaces with the same stellar density were ellipsoids of revolution with an axial ratio of 1:5. However, because of a failure to take into account the distorting influence of the interstellar absorption of the light of stars, many of the first conclusions were erroneous. The dimensions of the Galaxy, in particular, proved to have been exaggerated. In determining the position of the earth’s sun in the Galaxy, most researchers located it in the center of the Galaxy, the main reason for which was also the failure to take into account the influence of light absorption. This view was supported as well by the vitality of the geocentric and anthropocentric conceptions of the world. In the 1920’s the American astronomer H. Shapley proved once and for all the noncentral position of the sun in the Galaxy, in the process determining the direction to the center of the Galaxy (in the constellation Sagittarius).

In the mid 1920’s, G. Strömberg (US), while studying the laws governing the sun’s motion with respect to various groups of stars, discovered the so-called asymmetry of stellar motions, which provided factual material for substantiating many inferences about the complexities of the Galaxy’s structure. The Swedish astronomer B. Lindblad (1920’s), while studying the dynamics and structure of the Galaxy by analyzing the velocities of stars, discerned the complexity of the Galaxy’s structure and the fundamental difference in the spatial velocities of stars populating various parts of the Galaxy, although all of the stars are bound up into a single system that is symmetrical with respect to the galactic plane. In 1927 the Dutch astronomer J. Oort, on the basis of a statistical study of the radial velocities and proper motions of stars, proved the existence of the Galaxy’s rotation around its own minor axis. At the same time it turned out that the inner portion of the Galaxy that is closer to the center rotates more rapidly than the outer portion. At a distance measured from the sun to the center of the Galaxy (10 kiloparsecs), this velocity is about 250 km/sec; the period of a complete rotation is about 180 million years.

The proof of the interstellar absorption of the light of stars (1930, the Soviet astronomer B. A. Vorontsov-Vel’iaminov and the American astronomer R. Trumpler) and its quantitative evaluation and contribution made it possible to determine more accurately the distances to individual galactic objects and the dimensions of the Galaxy and laid the groundwork for ascertaining details of its structure. Numerous investigations of the spatial distribution of various types of stars (the Soviet astronomer P. P. Parenago), the proper motions of stars (early works by S. K. Kostinskii at the Pulkovo Observatory, the American astronomer W. van den Bos), the sun’s motions in space, and the motions of star streams (the Soviet astronomer V. G. Fesenkov, the Dutch astronomer A. Blaauw) and a study of the galactic gravitational field made it possible to discover, on the one hand, many general laws and, on the other, the great diversity in the kinematic, physical, and structural features of the individual components of the Galaxy.

In the 1930’s and subsequently Soviet astronomical observatories achieved considerable success in research on the Galaxy. Important results were obtained in the field of the dynamics of star systems, in observations and the compilation of numerous catalogs of parameters of stars and other galactic objects, in the development of new views, on the nature of the interstellar medium, in the elaboration of new theories and methods making it possible to quantitatively evaluate the parameters characterizing absorption in galactic space, and in ascertaining the connections between stars and interstellar matter. Photometry and the spectral classification of tens of thousands of stars were carried out in selected regions of the Milky Way under a plan by G. A. Shain (USSR) and under a composite plan by P. P. Parenago. The discovery of stellar associations was of huge importance for understanding the developmental processes of the Galaxy. A large role in the study of the Galaxy was played by the successes of Soviet science in the study of variable stars. A comparison of their physical and morphological characteristics with age and spatial parameters made it possible to solve a number of problems regarding the structure and nature of the Galaxy. Research by Soviet and American astronomers made obvious the Galaxy’s complex structure. It turned out that different parts of the Galaxy correspond to different, very specific, elements of their makeup. In 1948, Soviet researchers, as a result of observations in infrared rays, for the first time obtained an image of the nucleus of the Galaxy. Observations in the 1950’s showed that our Galaxy had spiral arms. The study of the Galaxy and its structure and development is the subject matter primarily of three branches of astronomy: stellar astronomy, astrometry, and astrophysics. All these branches played a large role in making our conceptions of the Galaxy more accurate and detailed. Of great importance for investigating the Galaxy was the development of radio astronomy, which obtained much new information about the Galaxy. Radio astronomical observations made it possible to discover a large number of emission sources in the radio-frequency region in the interstellar space of the Galaxy as well as masses of neutral hydrogen and to study their motions and ascertain the general features of the Galaxy’s internal structure.

By the beginning of the 1970’s, as a result of research done in the USSR and abroad, the following conception of the Galaxy had formed. The degree of the overall flattening of the Galaxy—that is, the ratio of the Galaxy’s thickness to its equatorial diameter—is about 1:10, although the Galaxy does not have sharply defined boundaries. The thickness of the layer that lies along the plane of the galactic equator and inside which are most of the stars and interstellar matter is 400 to 500 parsecs. The spatial density of the stars in it is such that one star corresponds to a volume equal to a cube with a side of 2 parsecs. The density is somewhat less in the neighborhood of the sun. It increases substantially with proximity to the Galaxy’s center, which, when observed from the earth, is visible in the constellation Sagittarius. Consequently, the distribution of stars is characterized by a concentration both toward the plane of the Galaxy and toward its center. The total mass of interstellar gas in the Galaxy is about 0.05 of the mass of all the stars, and its average density near the equatorial plane does not exceed 10^-25 or 10^-24g/cm³. Interstellar dust, which consists of tiny solid particles whose radii are on the order of 10^-4-10^-5 cm, is about 100 times smaller than the mass of the gas. While it does not influence the dynamics of the Galaxy because of its tiny mass, the dust nevertheless noticeably influences the visible structure of the Galaxy by diffusing the light of stars passing through its medium. The nucleus of the Galaxy, immersed in relatively dense masses of interstellar matter, is not particularly accessible to optical observations, but radio astronomical observations indicate that the nucleus is active and that there are large masses of matter and sources of energy in it.

The Galaxy has a pronounced subsystemic structure. Three subsystems are distinguished: the flat, intermediate, and spherical. The flat subsystem is characterized by the presence of young hot stars, variable stars of the long-period cepheid type, stellar associations, diffuse star clusters, and gaseous and dust matter. All of these are concentrated at the galactic plane in the form of an equatorial disk (with a thickness 1/20 of the Galaxy’s diameter). The average age of the disk’s stellar population is about 3 billion years. Concentrated more weakly toward the plane of the Galaxy are yellow and red dwarfs and giant stars, which occupy a volume in the shape of an extremely flattened ellipsoid. All of the subdwarfs, yellow and red giants, variable stars of the short-period cepheid type, and globular star clusters form the spherical component (sometimes called the halo), filling the spherical volume (with an average diameter exceeding 30,000 parsecs, that is, 100,000 light-years) and accompanied by a sharp drop in density upon moving from the central regions toward the edge. The age of the spherical component is more than 5 billion years. The objects of the various components also differ from one another in velocities of motion and chemical composition. The stars of the flat component have higher velocities with respect to the center of the Galaxy and are richer in metals. This indicates that the various types of stars belonging to different subsystems formed under different initial conditions and in different regions of the space occupied by galactic matter. The entire galactic system is immersed in a vast mass of gas that is sometimes called the galactic corona. Extending along the galactic plane from the central region of the Galaxy are the spiral arms, which, bending around the nucleus and branching out, gradually widen and lose brightness. In its spiral structure, which has proved to be a highly characteristic property of galaxies at a certain stage of their evolution, the Galaxy resembles a multitude of other star systems of the same type having the same stellar composition. Gravitational forces and magnetohydrodynamic phenomena evidently play a role in the development of the spiral structure, which at the same time is also influenced by the features of the Galaxy’s rotation. Star formation is taking place along the spiral arms, and they are populated by the youngest galactic objects.

Questions of the evolution of the Galaxy as a whole or of its individual constituent elements are of great significance to the view of the world. For a long time the view prevailed that all the stars and various objects in the Galaxy formed simultaneously. This view was bound up with the recognition of the theory that all the galaxies originated at once in one point of the universe and they subsequently “receded” outward in different directions. However, detailed research, based on numerous observations, led to the conclusion (Soviet astronomer V. A. Ambartsumian) that the process of star formation is continuing even in our epoch.

The problem of the origin and development of stars in the Galaxy is a fundamental one. There exist two principal but opposing points of view on the formation of stars. According to the first, stars form from gaseous matter that is dispersed in substantial amounts in the Galaxy and is observed by optical and radio astronomical methods. Wherever the mass and density of the gaseous matter reach a sufficiently high level, it contracts and condenses under the effect of its own gravitation, forming a cold sphere. In the process of further contraction, the temperature inside it, however, increases to several million degrees; this is enough for the onset of thermonuclear reactions, which together with emission processes are what cause the further evolution of this sphere, that is, of the star. According to the second point of view, stars form from some superdense matter. This kind of superdense matter has not yet been detected and its properties are unknown, but the fact that in the observable universe processes of the flow of masses from stars and the fission and decomposition of systems are observed in many cases and processes of the formation of stars from interstellar matter are not speaks in favor of the second point of view.

It is believed that the Galaxy as a whole developed in the process of condensation of a primary gaseous cloud that was rich in hydrogen; the stars that formed as a result are observed in our epoch as stars of the spherical component, which are poor in metals and the oldest. The primary gaseous cloud, while continuing to contract under the effect of gravitational forces, was enriched by metals through the ejection of matter from the depths of previously formed stars, in which intranuclear reactions had already been proceeding and hydrogen was being transformed into heavier elements for many hundreds of millions of years. Therefore, the later “generation” of stars that formed the disk of the Galaxy proved to be richer in metals. This conception explains the observed distribution of the velocities of stars and the separation of the latter into subsystems. Yet quite a few contradictions remain in the preceding picture. The idea being developed by a number of Soviet astronomers of the role in the evolution of galaxies played by explosive repulsive forces lying in the depths of galaxies may shed new light on the problem of the development of the Galaxy.