Stars and Atoms [Illustration] [Illustration: Fig. 1. THE SUN. Hydrogen photograph] STARS AND ATOMS BY A. S. EDDINGTON M.A., D.Sc., LL.D., F.R.S., Plumian Professor of Astronomy in the University of Cambridge NEW HAVEN: YALE UNIVERSITY PRESS LONDON: HUMPHREY MILFORD, OXFORD UNIVERSITY PRESS 1927 Ich häufe ungeheure Zahlen, Gebürge Millionen auf, Ich setze Zeit auf Zeit und Welt auf Welt zu Hauf. A. VON HALLER. PREFACE ‘STARS and Atoms’ was the title of an Evening Discourse given at the meeting of the British Association in Oxford in August 1926. In adapting it for publication the restrictions of a time limit are removed, and accordingly it appears in this book as three lectures. Earlier in the year I had given a course of three lectures in King’s College, London, on the same topics; these have been combined with the Oxford lecture and are the origin of most of the additions. A full account of the subject, including the mathematical theory, is given in my larger book, _The Internal Constitution of the Stars_ (Camb. Univ. Press, 1926). Here I only aim at exposition of some of the leading ideas and results. The advance in our knowledge of atoms and radiation has led to many interesting developments in astronomy; and reciprocally the study of matter in the extreme conditions prevailing in stars and nebulae has played no mean part in the progress of atomic physics. This is the general theme of the lectures. Selection has been made of the advances and discoveries which admit of comparatively elementary exposition; but it is often necessary to demand from the reader a concentration of thought which, it is hoped, will be repaid by the fascination of the subject. The treatment was meant to be discursive rather than systematic; but habits of mind refuse to be suppressed entirely and a certain amount of system has crept in. In these problems where our thought fluctuates continually from the excessively great to the excessively small, from the star to the atom and back to the star, the story of progress is rich in variety; if it has not lost too much in the telling, it should convey in full measure the delights--and the troubles--of scientific investigation in all its phases. Temperatures are expressed throughout in degrees Centigrade. The English billion, trillion, &c. (10^{12}, 10^{18}, &c.) are used. A. S. E. CONTENTS LECTURE I. THE INTERIOR OF A STAR 9 Temperature in the Interior 11 Ionization of Atoms 17 Radiation Pressure and Mass 24 The Interior of a Star 26 Opacity of Stellar Matter 28