The Structure of Narrow Shells in Red Giants

Stars of moderate mass (1–2 $$M_\odot$$ ) which have reached the red giant stage contain a helium core surrounded by a thin hydrogen burning shell, beyond which there is a region in radiative equilibrium. A method is given for computing in some detail the structure of the shell source, largely independently of the rest of the star, and the results of this method are compared with the zero-order approximation (Schwarzschild 1958) in which the shell source is treated as a jump discontinuity in composition and luminosity. A similar method is given for obtaining the energy released by gravitational contraction in the narrow shell of non-degenerate helium immediately below the hydrogen burning shell.