AbstractA procedure for inverting helioseismic data to determine the hydrogen abundance in the radiative interior of the sun is briefly described. Using Backus-Gilbert optimal averaging, the variation of sound speed, density and hydrogen abundance in the energy-generating core is estimated from low-degree p-mode frequencies. The result provides some evidence for there having been some redistribution of material during the sun's main-sequence evolution. The inversion also suggests that the

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... ionary age of the sun is perhaps some 10 per cent greater than the generally accepted value, and that the solar neutrino flux, based on standard nuclear and particle physics, is about 75 per cent of the standard-model value.