The red giant problem

While the understanding of main-sequence stars proceeded apace through the 1920s and 1930s, there remained the problem of accounting for the red giant stars, which are very much more luminous than main-sequence stars at the same effective temperatures. Russell adopted the position that matter existed in different states in the dwarf and giant stars, what he termed ‘giant stuff’ and ‘dwarf stuff’. Atkinson assumed that different nuclear processes were responsible for the luminosities of the giant stars.

The stellar models of Eddington are homogeneous, and it was assumed that homogeneity was maintained, probably by large-scale meridional circulation driven by the internal rotation of the star. It was only in the early 1950s, that a number of astrophysicists, Peter Sweet (1921–2005), Martin Schwarzschild, Ernst Öpik and Leon Mestel, showed that the mixing assumption was highly implausible.

The solution to the red giant problem was discovered in 1938 by the Estonian astrophysicist Ernst Öpik (1893–1985), then working at the University of Tartu (Öpik, 1938). Öpik realised that if the stars are not well mixed, it is inevitable that they become inhomogeneous. Within the central core of the star, nuclear burning of hydrogen into helium leads to the depletion of the nuclear fuel in the core. In Öpik's model it was assumed that the central core of the star was maintained in convective equilibrium, resulting in a uniform depletion of hydrogen in this region.