Henry Norris Russell observed that the relationship in the Hertzsprung–Russell diagram suggested that a star's heat came from a hot core rather than from the entire star. Eddington used this to calculate that the temperature of the core would have to be about 40 million K. This became a matter of debate, because the value is much higher than astronomical observations that suggested about one-third to one-half that value. George Gamow introduced the mathematical basis for quantum tunnelling in 1928. In 1929 Atkinson and Houtermans provided the first estimates of the stellar fusion rate. They showed that fusion can occur at lower energies than previously believed, backing Eddington's calculations.