[68] Saunders, quoted by Russell and Stewart, Ap. J., 59, 197, 1924. [69] Payne, H. C. 256, 1924. [70] A. Fowler, M. N. R. A. S., 80, 692, 1920. [71] Stewart, Ap. J., in press. [72] Ap. J., 59, 197, 1924. [73] Chapter VI, p. 99. [74] Chapter IX, p. 137. [75] Shapley, H. B. 805, 1924. [76] Chapter XIII, p. p. 188. [77] Chapter V, p. 56. [78] Ap. J., 59, 197, 1924. [79] Eddington, Zeit. f. Phys., 7, 371, 1921. [80] Russell and Stewart (loc. cit.) show that there are about 0.4 grams of matter above the photosphere per square centimeter of surface. [81] Phil. Mag., 26, 9, 1913. [82] Phil. Mag., 37, 456, 1919. [83] Zeit. f. Phys., 1, 1, 1920. [84] Wright, Nature, 109, 810, 1920. [85] Saha, Nature, 114, 155, 1924. [86] Nicholson, M. N. R. A. S., 85, 253, 1925. [87] Unpublished. [88] Ap. J., 59, 197, 1924. [89] M. N. R. A. S., 83, 403, 1923; 84, 499, 1924. [90] H. C. 256, 1924. [91] H. C. 258, 1924. [Pg 46] CHAPTER IV THE SOURCE AND COMPOSITION OF THE STELLAR SPECTRUM THE spectrum of a laboratory source offers a somewhat inadequate comparison with the spectrum of a star. Matter can be studied terrestrially in small quantities only, and when a laboratory source is used in obtaining a spectrum, all the contributing material is collected into a very small region. With the stellar source it is quite otherwise. An enormous mass of matter, spread over a very large region, gives rise to the spectrum, and probably widely different physical conditions prevail at the origin of light of different wave-lengths. The present chapter contains a brief survey of the chief components which go to make up the stellar spectrum. The spectrum of a star nearly always consists of a continuous background, in which the energy distribution corresponds more or less to that of a black body, and of absorption and emission lines and bands. The observed stellar spectrum is the integrated contribution from all parts of the disc, the unlined portion representing radiation that passes undisturbed from the photosphere through the reversing layer, and the light within any individual absorption line coming from the greatest depth in the reversing layer that can be penetrated by light of the corresponding frequency. This depth, which is a function of the monochromatic coefficient of absorption for the wave-length considered, is negligible when compared with the radius of the star. DESCRIPTIVE DEFINITIONS