Diffusion in the Sun.

The gradients of pressure and temperature in the sun set up an electrostatic field that maintains electrical neutrality to a high degree of approximation. The gradients and the field cause the heavier ions to diffuse downward. The rate of this diffusion is estimated for different depths, on the basis of a model (III) obtained by Weymann. The diffusive drainage of the heavier elements from the convection zone reduces their abundances relative to hydrogen in the sun's atmosphere. It also affects their relative abundances among themselves. The reduction is naturally greater, the shallower the convection region. The low abundance of lithium in the sun's atmosphere is taken to indicate that the convective region extends to a depth at which its destruction by nuclear reactions can occur-namely, down to three-tenths of the solar radius. For this depth the reduction of the abundance of the heavier elements brought about by diffusion during 4.5 X 10 years is about 10 per cent for silicon, 12 per cent for iron, and about 25 per cent for lead. The diffusion will modify the concentration of these elements in the region below, but only by a small percentage.