The hydrogen abundance in stars : a first major step for quantitative astrophysics

Historiography has recognized that Saha’s work in the early 1920s was the beginning of a quantitative era in astrophysics, and the deduction of the large hydrogen abundance in stars around 1930 was a major outcome of Saha’s theory. In this paper, the development of stellar physics in these years is analysed, and the recognition of the hydrogen abundance is pointed out as the first major achievement of the quantitative era. This idea is sustained from two different points of view. First, there exists a tight scientific continuity from Saha’s investigative papers up to Russell’s 1929 paper where the hydrogen abundance was clearly worked out: the whole of the 1920s should therefore be considered as a scientific discontinuity that paved the way for modern stellar spectroscopy. Second, in 1932 the same conclusion was reached by Strömgren and Eddington, who were working on the problem of internal stellar structure. Thus, the hydrogen abundance can be viewed as the first major step of the quantitative era, as it led to the first sound theory of stellar structure, both for the inner and the surface regions of stars.