on nuclear reactions. The result was a synthesis of theory and observation, which provided convincing evidence for Hoyle's hypothesis. The theory predicted that the abundances of the elements would evolve over cosmological time, an idea which is testable by astronomical spectroscopy. Each element has a characteristic set of spectral lines, so stellar spectroscopy can be used to infer the atmospheric composition of individual stars. Observations indicate a strong negative correlation between a star's initial heavy element content (known as the metallicity) and its age. More recently formed stars tend to have higher metallicity. The early Universe consisted of only the light elements formed during Big Bang nucleosynthesis. Stellar structure and the Hertzsprung–Russell diagram indicate that the length of the lifetime of a star depends greatly on its initial mass, with the most massive stars being very short-lived, and less massive stars are longer-lived. The B2FH paper argued that when a star dies, it will enrich the interstellar medium with 'heavy elements' (in this case all elements heavier than lithium), from which newer stars are formed. The B2FH paper described key aspects of the nuclear physics and astrophysics involved in how stars produce these heavy elements. By scrutinizing the table of nuclides, the authors identified different stellar environments that could produce the observed isotopic abundance patterns and the nuclear processes that must be responsible for them. The authors invoke nuclear physics processes, now known as the p-process, r-process, and s-process, to account for the elements heavier than iron. The abundances of these heavy elements and their isotopes are roughly 100,000 times less than those of the major elements, which supported Hoyle's 1954 hypothesis of nuclear fusion within the burning shells of massive stars. B2FH comprehensively outlined and analyzed the nucleosynthesis of the elements heavier than iron by the capture within stars of free neutrons. It advanced much less the understanding of the synthesis of the very abundant elements from silicon to nickel. The paper did not include the carbon-burning process, the oxygen-burning process and the silicon-burning process, each of which contribute to the elements from magnesium to nickel. Hoyle had already suggested that supernova nucleosynthesis could be responsible for these in his 1954 paper. Donald D. Clayton has attributed the lower number of citations to Hoyle's 1954 paper compared to B2FH as a combination of factors: the difficulty of digesting Hoyle's 1954 paper even for his B2FH coauthors, and among astronomers