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.

PREFACE TO THE SECOND IMPRESSION

Since the first edition was published the principal event in astrophysics has been the identification of Nebulium (see page 55) by Dr. I. S. Bowen in the autumn of 1927. Nebulium turns out to be an oxygen atom with two electrons missing. Singly ionized a toms of oxygen and nitrogen are responsible for the other prominent lines in the nebular spectrum not previously identified. These lines have not been reproduced terrestrially; but it has been calculated that their wave-lengths correspond exactly to transitions between certain states of the oxygen and nitrogen atoms, the states (but not these direct transitions) being familiar experimentally. Confidence that the mysterious substance producing the light of the nebulae would prove to be quite familiar has been justified; it is air. A.S.E.

PREFACE TO THE THIRD IMPRESSION

An Appendix on 'The Identification of Nebulium' has been added. This deals with recent developments in a subject touched on in the lectures. Otherwise no change has been made as there appears to be nothing important to add or to modify. A.S.E.