Pressure of Calcium in the Sun's Atmosphere

Abstract

IN a recent interesting investigation, A. Unsöld has estimated the total number of atoms of various kinds in the solar atmosphere which give rise to Fraunhofer lines of the observed width and depth (Zeits. für Phys., 46, 765; 1928). The theoretical curves he has calculated reproduce with remarkable fidelity the contours of such lines as those of sodium, calcium, strontium, and barium. They give, it is true, too small values for the residual intensities in the centres of the lines, but in the wings, where the theory seems unquestionably trustworthy, they agree exceedingly well with observation. With the aid of the Saha theory of ionisation, Unsöld has deduced the total number of atoms of a given species per square centimetre column above the ‘photosphere.’ The results as regards the relative abundances of atoms of different kinds are in agreement with Miss Payne's estimates of relative abundances in stellar atmospheres, but Unsöld's method leads to an absolute determination. For example, for calcium he finds 2.3 × 10¹⁰ atoms per cm.².