Paul W. Hodge large spiral system consisting of several billion stars, one of which is the Sun. It takes its name from the Milky Way, the irregular luminous band of stars and gas clouds that stretches across the sky. Although the Earth lies well within the Galaxy, astronomers do not have as clear an understanding of its nature as they do of some external star systems. A thick layer of interstellar dust obscures much of the Galaxy from scrutiny by optical telescopes, and astronomers can determine its large-scale structure only with the aid of radio and infrared telescopes, which can detect the forms of radiation that penetrate the obscuring matter. large disk-shaped system of stars and interstellar matter of which the Sun is a component. It includes the multitude of stars whose light produces the Milky Way, the highly irregular luminous band that encircles the sky. This band of starlight lies roughly in the plane of the galactic disk. The Milky Way system is one of billions of galaxies that make up the universe. It contains several billion stars and large amounts of interstellar gas and dust. The system is a typical example of a class of galaxies known as barred spirals. It has a nucleus surrounded by a large central bulge with spiral arms coiled around it. These curving arms, which resemble those of a huge pinwheel, are embedded in the galactic disk, which constitutes the main part of the system and measures roughly 70,000 light-years in diameter. The galactic nucleus is obscured by interstellar dust particles, which absorb both the visible and the ultraviolet light radiated by its components. Investigators, however, have been able to record and study emissions from the region at radio, infrared, X-ray, and gamma-ray wavelengths. The strong emissions of infrared radiation and X rays in particular seem to indicate the presence of rapidly moving clouds of ionized gas. These gas clouds are thought to be circling a supermassive object, in all likelihood a black hole with a mass approximately 4,000,000 times that of the Sun. Investigators have determined that the central bulge contains mostly Population II objects (i.e., old stars and star clusters) such as RR Lyrae variable stars and globular clusters. The components of the spiral arms are quite different. The arms are occupied by objects belonging to the extreme Population I variety (i.e., very young, bright stars and open clusters). In addition, this region of the Galaxy contains the highest concentrations of interstellar gas and dust particles and therefore constitutes the most favourable site for new star formation. The Sun is located near the inner edge of one of these armsthe Orion armabout two-thirds of the way from the centre of the Galaxy. The galactic nucleus lies in the direction of the constellation Sagittarius at a distance of about 27,000 light-years from the Sun. Above and below the galactic disk is a spherical region (referred to as simply the spherical component) that is occupied by globular clusters and other very old Population II objectse.g., dwarf stars deficient in the heavy elements. Exterior to the whole visible portion of the Galaxy is the giant massive halo. Its constituents, shape, and extent are still not known. The entire Milky Way system rotates around the galactic centre, but the various constituent objects do not rotate at the same velocity. Stars distant from the centre travel at lower speeds than do those closer to it. The Sun, which is located relatively far from the nucleus, moves at an estimated speed of about 225 km per second (140 miles per second) in a nearly circular orbit. Because of its relatively low velocity, the Sun's period of revolution about the galactic centre is approximately 200,000,000 years. Additional reading Bart J. Bok and Priscilla F. Bok, The Milky Way, 5th ed. (1981), contains a comprehensive and up-to-date account, without mathematics, of modern galactic research. Dimitri Mihalas and James Binney, Galactic Astronomy: Structure and Kinematics, 2nd ed. (1981), is a thorough, mathematically replete textbook on the Milky Way Galaxy and other galaxies, with emphasis on structure and dynamics. Two collections are Hugo van Woerden, Ronald J. Allen, and W. Butler Burton (eds.), The Milky Way Galaxy (1985), symposium articles written at a technical level; and Adriaan Blaauw and Maarten Schmidt (eds.), Galactic Structure (1965), a group of fundamental articles, most of which are still useful sources of basic data, methods, and approaches. Paul W. Hodge Major components Stars and stellar populations The concept of different populations of stars has undergone considerable change over the last several decades. Before the 1940s astronomers were aware of differences among stars and had largely accounted for most of them in terms of different masses, luminosities, and orbital characteristics around the Galaxy. Understanding of evolutionary differences, however, had not yet been achieved, and differences in the chemical abundances in the stars were known but their significance was not comprehended. At this juncture chemical differences seemed exceptional and erratic and remained uncorrelated with other stellar properties. There was still no systematic division of stars even into different kinematic families in spite of the advances in theoretical work on the dynamics of the Galaxy. Principal population types In 1944 Baade announced the successful resolution into stars of the centre of the Andromeda Galaxy, M31, and its two elliptical companions, M32 and NGC 205. He found that the central parts of Andromeda and the accompanying galaxies were resolved at very much fainter magnitudes than were the outer spiral arm areas of M31. Furthermore, by using plates of different spectral sensitivity and coloured filters, he discovered that the two ellipticals and the centre of the spiral had red giants as their brightest stars rather than blue main-sequence stars, as in the case of the spiral arms. This finding led Baade to suggest that these galaxies, and also the Milky Way Galaxy, are made of two populations of stars that are distinct in their physical properties as well as their locations. He applied the term Population I to the stars that constitute the spiral arms of Andromeda and to most of the stars that are visible in the Milky Way system in the neighbourhood of the Sun. He found that these Population I objects were limited to the flat disk of the spirals and suggested that they were absent from the centres of such galaxies and from the ellipticals entirely. Baade designated as Population II the bright red giant stars that he discovered in the ellipticals and in the nucleus of Andromeda. Other objects that seemed to contain the brightest stars of this class were the globular clusters of the Galaxy. Baade further suggested that the high-velocity stars near the Sun were Population II objects that happened to be passing through the disk. As a result of Baade's pioneering work on other galaxies in the Local Group (the cluster of star systems to which the Milky Way Galaxy belongs), astronomers immediately applied the notion of two stellar populations to the Galaxy. It is possible to segregate various components of the Galaxy into the two population types by applying both the idea of kinematics of different populations suggested by their position in the Andromeda system and the dynamical theories that relate galactic orbital properties with z distances (the distances above the plane of the Galaxy) for different stars. For many of these objects, the kinematic data on velocities are the prime source of population classification. The Population I component of the Galaxy, highly limited to the flat plane of the system, contains such objects as open star clusters, O and B stars, Cepheid variables, emission nebulas, and neutral hydrogen. Its Population II component, spread over a more nearly spherical volume of space, includes globular clusters, RR Lyrae variables, high-velocity stars, and certain other rarer objects. As time progressed, it was possible for astronomers to subdivide the different populations in the Galaxy further. The Table summarizes the properties and membership of the five subdivisions that were accepted at the time of the Vatican Conference on stellar populations in 1957. These subdivisions ranged from the nearly spherical halo Population II system to the very thin extreme Population I system, and each of the subgroups was found to contain (though not exclusively) characteristic types of stars. It was even possible to divide some of the variable-star types into subgroups according to their population subtype. The RR Lyrae variables of type ab, for example, could be separated into different groups by their spectral classifications and their mean periods. Those with mean periods longer than 0.4 days were classified as halo Population II, while those with periods less than 0.4 days were placed in the disk population. Similarly, long-period variables were divided into different subgroups, such that those with periods of less than 250 days and of relatively early spectral type (earlier than M 5e) were considered intermediate Population II, whereas the longer period variables fell into the older Population I category. An understanding of the physical differences in the stellar populations became increasingly clearer during the 1950s with improved calculations of stellar evolution. Evolving-star models showed that giants and supergiants are evolved objects recently derived from the main sequence after the exhaustion of hydrogen in the stellar core. As this became better understood, it was found that the luminosity of such giants was not only a function of the masses of the initial main-sequence stars from which they evolved but was also dependent on the chemical composition of the stellar atmosphere. Therefore, not only was the existence of giants in the different stellar populations understood but differences among the giants with relation to the main sequence of star groups came to be understood in terms of the chemistry of the stars. At the same time, progress was made in determining the abundances of stars of the different population types by means of high-dispersion spectra obtained with large reflecting telescopes having a coud focus arrangement. A curve of growth analysis demonstrated beyond a doubt that the two population types exhibited very different chemistries. In 1959 H. Lawrence Helfer, George Wallerstein, and Jesse L. Greenstein of the United States showed that the giant stars in globular clusters have chemical abundances quite different from those of Population I stars such as typified by the Sun. Population II stars have considerably lower abundances of the heavy elementsby amounts ranging from a factor of five or 10 up to a factor of several hundred. The total abundance of heavy elements, Z, for typical Population I stars is 0.04 (given in terms of the mass percent for all elements with atomic weights heavier than helium, a common practice in calculating stellar models). The values of Z for halo population globular clusters, on the other hand, were typically as small as 0.003. A further difference between the two populations became clear as the study of stellar evolution advanced. It was found that Population II was exclusively made up of stars that are very old. Estimates of the age of Population II stars have varied over the years, depending on the degree of sophistication of the calculated models and the manner in which observations for globular clusters are fitted to these models. They have ranged from 109 years up to 2 1010 years. Recent comparisons of these data suggest that the halo globular clusters have ages of approximately 1.6 1010 years. The work of Sandage and his collaborators proved without a doubt that the range in age for globular clusters was relatively small and that the detailed characteristics of the giant branches of their colour-magnitude diagrams were correlated with age and small differences in chemical abundances. On the other hand, stars of Population I were found to have a wide range of ages. Stellar associations and galactic clusters with bright blue main-sequence stars have ages of a few million years (stars are still in the process of forming in some of them) to 1010 years or more. Studies of the stars nearest the Sun indicate a mixture of ages with a considerable number of stars of great ageon the order of 109 years. Careful searches, however, have shown that there are no stars in the solar neighbourhood and no galactic clusters whatsoever that are older than or even quite as old as the globular clusters. This is an indication that globulars, and thus Population II objects, formed first in the Galaxy and that Population I stars have been forming since. In short, as the understanding of stellar populations grew, the division into Population I and Population II became understood in terms of three parameters: age, chemical composition, and kinematics. A fourth parameter, spatial distribution, appeared to be clearly another manifestation of kinematics. The correlations between these three parameters were not perfect but seemed to be reasonably good for the Galaxy, even though it was not yet known whether these correlations were applicable to other galaxies. The Table illustrates the close correlations, as formulated in the early 1960s, for the stars in the Galaxy and shows that there are many different combinations of these three parameters that seem to be excluded in nature. The many different physical manifestations of these parameters were gradually building up. Methods of determining the abundance of metals in objects by means other than laborious high-dispersion coud spectroscopy became possible. For example, it was found that stars having a low abundance of heavy elements exhibited an easily measurable ultraviolet excess. This is demonstrated when the three colours U, B, and V of the Yerkes system are plotted in a three-colour diagram where the Population II stars all lie distinctly to the left of the normal star, three-colour relationship. The star-formation history of the Milky Way Galaxy. Astronomers devised a graphic way to explain the evolution of the stellar population in the Milky Way Galaxy using a three-dimensional plot in which the age, the abundance of heavy elements, and the rate of star formation are all taken into account. Figure 1 is an example of such a three-dimensional plot. The volume shown in the figure indicates that the rate of star formation about the time the Galaxy originated was somewhat greater than at present but that it has not yet reached zero. As stars formed, the heavy elements were produced in the hot centres of the stars and in supernovas; thus the volume moves forward in the box until the present is reached, and the majority of stars that are now forming have heavy elements in approximately the same amount as the Sun. At any time, t, there is a spread in the abundances of the stars formed, depending on the history of the interstellar material in the region. Complications in scientific understanding of stellar populations first became serious when detailed colour-magnitude diagrams were obtained for star clusters in the Magellanic Clouds during the late 1950s. Arp's work on the clusters of the Small Magellanic Cloud showed that the correlations between the properties of populations found in the Galaxy apparently broke down when other galaxies were examined carefully. Arp suggested that the star clusters of young age that he had observed in the SMC might be examples of young Population II starsi.e., young stars having a low abundance of heavy elements. No such stars were known in the Galaxy. Similarly, Arp found anomalous colour-magnitude diagrams for globular clusters in the SMC and proposed that perhaps this also was the result of abundance differences between the SMC and the Galaxy. At first it appeared that these conclusions were based on detailed comparisons with evolutionary models; however, because of the lack of such models at the time of Arp's observations, it seemed clear that the young star clusters of the SMC were anomalous in many details and that these peculiarities could not easily be accounted for other than by differences in chemical composition. In succeeding years, and as more star clusters in the Magellanic Clouds were measured, investigators were able to make detailed comparisons with models and to conclude that the chemical differences between the Galaxy and the Clouds must be rather small. Many of the star clusters have colour-magnitude diagrams that nearly conform to models calculated on the basis of solar-type abundances. It also is true, however, that many clusters, including those measured with high-detection-efficiency equipment such as the charge-coupled device (CCD), show real differences from colour-magnitude diagrams of galactic clusters, and these differences are still not completely understood. The Andromeda Galaxy has many globular clusters that can be observed with large instruments, and these also show a wider variety of properties than expected on the basis of the local sample. Surveys of the spectra and colours of the Andromeda globulars have demonstrated that there is a considerable spread in heavy-element abundance for these systems and that the close correlation between position and abundance found in the Galaxy fails to materialize in the case of Andromeda. Consequently, the segregation into stellar populations that works so well for the Galaxy is not necessarily a universal system. Moreover, it is possible that most of the correlations are connected specifically to the detailed history and evolution of the Galaxy rather than to fundamental properties that stars in general would be expected to possess.
MILKY WAY GALAXY
Meaning of MILKY WAY GALAXY in English
Britannica English vocabulary. Английский словарь Британика. 2012