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 Sun’s 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 99 Evolution of the Stars 106 Radiation of Mass 113 APPENDIX Further Remarks on the Companion of Sirius 122 LIST OF ILLUSTRATIONS FIG. 1. The Sun. Hydrogen Spectroheliogram. (J. Evershed) 2. Solar Vortices. Hydrogen Spectroheliogram. (Mount Wilson Observatory) 3. Tracks of Alpha Particles (helium atoms). (C. T. R. Wilson) 4. Tracks of Beta Particles (electrons). (C. T. R. Wilson) 5. Ionization by X-rays. (C. T. R. Wilson) 6. Ions produced by Collision of a Beta particle. (C. T. R. Wilson) 7. The Mass-luminosity Curve. 8. The Ring Nebula in Lyra. Slitless Spectrogram. (W. H. Wright) 9. Flash Spectrum of Chromosphere showing Head of the Balmer Series. (British Eclipse Expedition, 14 Jan. 1926) 10. Solar Prominence. (British Eclipse Expedition, 29 May 1919) 11. Star Cluster ω Centauri. (Cape Observatory) LECTURE I THE INTERIOR OF A STAR THE sun belongs to a system containing some 3,000 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. Imagine thirty cricket balls roaming the whole interior of the earth; the stars roaming the heavens are just as little crowded and run as little risk of collision as the cricket balls. We marvel at the grandeur of the stellar system. But this probably is not the limit. Evidence is growing that the spiral nebulae are ‘island universes’ outside our own stellar system. It may well be that our survey covers only one unit of a vaster organization. A drop of water contains several thousand million million million atoms. Each atom is about one hundred-millionth of an inch in diameter. Here we marvel at the minute delicacy of the workmanship. But this is not the limit. Within the atom are the much smaller electrons pursuing orbits, like planets round the sun, in a space which relatively to their size is no less roomy than the solar system.