Robert d'Escourt Atkinson and Fritz Houtermans, at Göttingen, apply the barrier-penetration formula (quantum tunneling) Gamow had derived a year earlier for alpha decay to the inverse problem: the fusion of light nuclei inside stars. They calculate the probability that a proton, at stellar temperatures of about 40 million Kelvin, penetrates the Coulomb barrier of a light nucleus despite lacking sufficient classical kinetic energy. They conclude that these penetration probabilities are high enough that sequential fusion of protons into light nuclei could be the energy source of stars, offering the first quantitative estimate of stellar thermonuclear fusion rates — a decade before Hans Bethe formalized the CNO cycle.