Stars and Atoms Stars and Atoms A. S. EDDINGTON M. A., D. Sc., LL. D., F. R S., Flumian Professor of Astronomy in the University of Cambridge OXFORD AT THE CLARENDON PRESS 1927 PREFACE STARS and Atoms was the title of an Evening Dis course 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 accord ingly it appears in this book as three lectures. Earlier in the year I had given a course of three lectures in Kings 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 mathe matical 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 con ditions 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 detnand 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 6 Preface 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. io l8 , c. are used. A. S. E. CONTENTS LECTURE I. THE INTERIOR OF A STAR . 9 Temperature in the Interior . .11 lonization of Atoms , 17 Radiation Pressure and Mass . . 24 The Interior of a Star . . 26 Opacity of Stellar Matter . . 28 The Relation of Brightness to Mass . 31 Dense Stars . . 36 LECTURE II. SOME RECENT INVESTIGATIONS 42 The Story of Algol . , 42 The Story of the Companion of Sirius . . 48 Unknown Atoms and Interpretation of Spectra . 53 Spectral Series . . 59 The Cloud in Space . . 63 The Suns Chromosphere . . 70 The Story of Betelgeuse . .76 LECTURE III. THE AGE OF THE STARS . 85 Pulsating Stars , . 85 The Cepheid as a Standard Candle .90 The Contraction Hypothesis . . 94. Subatomic Energy . t oo Evolution of the Stars . .106 Radiation of Mass . - in APPENDIX Further Remarks on the Companion of Sirius . 122 LIST OF ILLUSTRATIONS FIG. 1. The Sun. Hydrogen Spectroheliogram. J. Evershed . . Frontispiece 2. Solar Vortices. Hydrogen Spectroheliogram. Mount Wilson Observatory . To face page 10 3. Tracks of Alpha Particles helium atoms. C. T. R. Wilson . . 18 4. Tracks of Beta Particles electrons. C. T. R. Wilson . . 18 5. lonization by X-rays. C. T. R. Wilson , 24 6. Ions produced by Collision of a Beta particle C. T. R Wilson . . 24 7. The Mass-luminosity Curve . . page 33 8. The Ring Nebula in Lyra. Slitless Spectro gram. W. H. Wright . . To face page 4 9. Flash Spectrum of Chromosphere showing Head of the Balmer Series British Eclipse Expedition, 14 Jan. 1926 . 54 10. Solar Prominence. British Eclipse Expedi tion, 29 May 1919 . . 70 11. Star Cluster a Centauri. Cape Observatory 70 LECTURE I THE INTERIOR OF A STAR sun belongs to a system containing some 3,000 JL million stars. The stars are globes comparable in size with the sun, that is to say, of the order of a million miles in diameter. The space for their accommodation is on the most lavish scale...