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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.
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