STAR CLUSTERS Thirty-four Globular Cliutterv in ike Southern Milky Way. HARVARD OBSERVATORY MONOGRAPHS No. 2 STAR CLUSTERS BY HARLOW SHAPLEY Published for the Observatory by the McGRAW-HILL BOOK COMPANY, INC. NEW YORK 370 SEVENTH AVENUE LONDON 6 8 BOUVERIE ST, E. C. 4 1930 PREFACE FOR more than five years the writing of this second monograph of the Harvard Observatory series has been in progress. Several factors have contributed to the postponement of its publication, but the chief cause of delay has been the desire to provide a revised system of parallaxes for globular clusters. All considerations of the dimensions, star densities, and lumin osities of clusters, and most of the conclusions concerning the dimensions of the Galaxy and the distance to external systems, depend at the present time on the period-luminosity relation of Cepheid variable stars. In the preparation of a monographic treatment of star clusters much preliminary work on Cepheids has therefore been necessary., At the McCormick and Mount Wilson observatories investigations of proper motions have been undertaken in order to fix the zero point of the period luminosity curve. At Harvard we have determined periods and magnitudes of many variable stars in the Large and Small Clouds of Magellan to provide a firmer photographic connection between period and luminosity. The revision of the parallaxes of globular clusters has also depended largely on the study of individual systems. In my determination in 1917 of the parallaxes of sixty-eight globular clusters, there were only five systems for which the variables had been sufficiently studied to enter directly into the measure ment of cluster distances. We now have nineteenclusters in which variable stars have been adequately studied the periods and median magnitudes of more than five hundred individual variables have been derived. In 1917 the magnitudes of the high luminosity stars had been measured for twenty-eight globular clusters the number is now increased to forty-eight. viii PREFACE Much of the recent investigation of magnitudes in clusters is the work of Miss Helen Sawyer at the Harvard Observatory my earlier photometric studies preliminary to the revision of cluster distances were carried on at Mount Wilson and Harvard with the aid of several assistants. Probably one of the most satisfactory features of the present volume is the bibliography in Appendix C. It is essentially complete for all papers bearing directly on star clusters pub lished during the past fifty or sixty years. Particular attention should be directed to the treatises by ten Bruggencate and Parvulesco, who handle some special subjects in the field of clusters more fully than they are treated here. I am indebted to Dr. Cecilia Payne for extensive assistance in the preparation of manuscript and bibliography and in other details. In the interest of this study of clusters Dr. Walter S. Adams has generously transferred to the Harvard Observatory the photographs I made with the 6o-inch and loo-inch reflectors at Mount Wilson. Miss Jenka Mohr has assisted in the prep aration of the manuscript and in editorial matters. To several members of the Observatory staff I am indebted for incidental or particular assistance with illustrations, com putations, and routine work on manuscript and proof. My greatest debt is to the star clusters themselves, which have provided persistent excitementand inspiration. H. S. CAMBRIDGE, MASS., June, 1930. CONTENTS PAOE PREFACE . .... vu CHAPTER I INTRODUCTORY SURVFY i 1. The Significance of Clusters . . i 2. Historical Notes on Clusters . . . . 3 II. CLASSIFICATION, NUMBER, AND DISTRIBUTION 6 Q A Comparison of Galactic and Globular Clusters .... 6 tAA Classification of Galactic Clusters ... 8 j Classification of Globular Clusters. n o The Number of Clusters . . 14 fy The Apparent Distuibution of Galactic Clusters 1 7 18 NGC 5053 and NGC 2477 18 The Apparent Distribution of Globular Clusters . . 20 10...

The phenomenal growth of modern astronomy, including the invention of the coronagraph and major developments in telescope design and photographic technique, is unparalleled in many centuries. Theories of relativity, the concept and measurement of the expanding universe, the location of sun and planets far from the center of the Milky Way, the exploration of the interiors of stars, the pulsation theory of Cepheid variation, and investigations of interstellar space have profoundly altered the astronomer's approach. These fundamental discoveries are reported in papers by such eminent scientists as Albert Einstein, Sir Arthur S. Eddington, Henry Norris Russell, Sir James Jeans, Meghnad Saha, Otto Struve, Fred L. Whipple, Bernard Lyot, Jan H. Oort, and George Ellery Hale. The Source Book's 69 contributions represent all fields of astronomy. For example, there are reports on the equivalence of mass and energy (E = mc(2)) of the special theory of relativity; building the 200-inch Palomar telescope; the scattering of galaxies suggesting a rapidly expanding universe; stellar evolution; and the Big Bang and Steady State theories of the universe's origin.