STAR CLUSTER. In surveying the sky we find regions here and there where the stars are much more richly strewn than usual. Some of these are clusters, where hundreds or thou sands of stars are seen concentrated together in a small field of view, providing show-objects—beauty spots of the celestial landscape. Not only are they impressive to observe, but they have led to remarkable advances in our knowledge. The meas urement of the extent of the visible universe and an indication of its organization, together with intimations of its mass, popula tion, and past duration, are among the contributions to astron omy and cosmogony that have been made chiefly through the study of star clusters. The researches have been descriptive, astrometric, photometric, spectroscopic, statistical, mathematical, and speculative—often complicated, and more often very tedi ous. In a three-page account of these ancient and instructive cluster systems, the reader can be and should be spared the details of method and the evidence of observatory labours. Per haps the most important item will be the short bibliography, which will guide the reader who desires more information. The aim of the following treatment is a general survey of the subject and a statement of current results ; we take up, successively, (I) introductory remarks on the frequency and variety of clusters.
(2) the naked-eye systems and their measurable motions, (3) classification and distribution, (4) the forms of globular clusters, (5) variable stars in clusters, (6) distances and dimensions, and (7) the role of clusters in the evolution of stars and galaxies.
I. Introduction.—The grouping and clustering of astronom ical bodies are so common that we may infer from available observation that most of them are in close gravitational associa tion. The earth-moon system, the sun and the planets, and the close double stars are examples of physically associated groups, probably all resulting from subdivision. Wide binary stars, typical star clusters, and star clouds are further examples of gravitational organization, but possibly they represent the accumulation of once isolated bodies and not the fission of originally larger masses. Organized systems smaller than any of these are the showers of meteors; and larger groups are the galaxies, and the clusters of "island universes." It is not easy to draw a dividing line between multiple stars and star clusters. The brightest star, Sirius, is double; the near
est star, a Centauri, is triple; possibly Polaris is quadruple, and Castor is sextuple. The trapezium of the Orion Nebula, 0 Orionis, is actually the many-starred nucleus of a widely spread system of stars. For the convenience of the present discussion we shall call the Ursa Major group of bright stars, the Great Bear, a cluster. All groups of equal or greater population will fall within the cluster category, up to the richest of globular clusters, where we shall limit the family, without including the spheroidal extra-galactic nebulae (which may be merely globular clusters of superior size), or the star clouds and the spiral nebulae.
It will be convenient to divide the star clusters into a globular class and a galactic class ; the one is named from its form, the other from its apparent and real association with the Milky Way. The former can be limited fairly well to one hundred objects, only a dozen of which approach in structure, and in paucity of stars, the more open clusters of the galactic class ; but the galactic clusters themselves are so varied in form, population, and angular and linear dimensions, that no distinct limits can be set, at least between them and the chance aggregations of stars in the Milky Way. A current compilation of well-defined galactic clusters, made at the Harvard Observatory, contains about 25o entries.
2. Conspicuous Star Clusters.—The stellar groupings that have led in ancient and modern times to the establishment of constellations frequently bring together stars that are not actu ally associated in space. A few, however, involve bright stars that are physically allied—notably Orion, Ursa Major, Scorpio, and Perseus. Thus, five of the seven conspicuous stars of Ursa Major are now known to be closely alike in distance, in amount of motion, and in spectral class; and though they are widely separated in the sky, Sirius and other bright stars are also members of the same Ursa Major family. These stars move in parallel paths through space, and probably they have consti tuted a cluster throughout their whole past history as stars, and will continue to be associated in the future for a period indefi nitely long. Ultimately, through encounters with other stars, the members of such groups may be diverted from essentially parallel motion, for they move through the highly populated regions of the Milky Way.