Prior to the publication of the B FH paper, George Gamow advocated a theory of the Universe in which almost all chemical elements, or equivalently atomic nuclei, were synthesized during the Big Bang. Gamow's theory (which differs from present-day Big Bang nucleosynthesis theory) would imply that the abundance of the chemical elements would remain mostly static over time. Hans Bethe and Charles L. Critchfield had shown that the conversion of hydrogen into helium by nuclear fusion could provide the energy required to power stars, by deriving the proton-proton chain (pp-chain) in 1938. Carl von Weizsäcker and Hans Bethe had independently derived the CNO cycle in 1938 and 1939, respectively. Thus, it was known by Gamow and others that the abundances of hydrogen and helium were not perfectly static. According to their view, fusion in stars would produce small amounts of helium, adding only slightly to its abundance from the Big Bang. This stellar nuclear power did not require substantial stellar nucleosynthesis. The elements from carbon upward remained a mystery.