TheB2FH paper[1] was a landmarkscientific paper on the origin of the chemical elements. The paper's title isSynthesis of the Elements in Stars, but it became known as B2FH from the initials of its authors:Margaret Burbidge,Geoffrey Burbidge,William A. Fowler, andFred Hoyle. It was written from 1955 to 1956 at theUniversity of Cambridge andCaltech and published inReviews of Modern Physics in 1957.
The B2FH paper reviewedstellar nucleosynthesis theory and supported it with astronomical and laboratory data. It identified nucleosynthesis processes that are responsible for producing the elements heavier than iron and explained theirrelative abundances. The paper became highly influential in bothastronomy andnuclear physics.
Prior to the publication of the B2FH paper,George Gamow advocated a theory of the Universe in which almost allchemical elements, or equivalentlyatomic nuclei, were synthesized during theBig Bang. Gamow's theory (which differs from present-dayBig Bang nucleosynthesis theory) would imply that theabundance of the chemical elements would remain mostly static over time.Arthur Eddington had speculated that the conversion ofhydrogen intohelium bynuclear fusion could provide the energy required to powerstars in 1920.[2][3]Hans Bethe andCharles L. Critchfield had shown the mechanism for stellar fusion of helium by deriving theproton–proton chain (pp-chain) in 1938.[4]Carl von Weizsäcker[5] and Hans Bethe[6] had independently derived theCNO 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 substantialstellar nucleosynthesis. The elements from carbon upward remained a mystery.
Fred Hoyle offered a hypothesis for the origin of heavy elements. Beginning with a paper in 1946, and expanded upon in 1954,[7] Hoyle proposed that all atomic nuclei heavier thanlithium were synthesized instars. Both theories agreed that some light nuclei (hydrogen, helium and a small amount of lithium) were not produced in stars, which became the now-accepted theory ofBig Bang nucleosynthesis of H, He and Li.
The B2FH paper was ostensibly areview article summarising recent advances in the theory ofstellar nucleosynthesis.[8] However, it went beyond simply reviewing Hoyle's work, by incorporating observational measurements of elemental abundances published by the Burbidges, and Fowler's laboratory experiments 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 byastronomical spectroscopy. Each element has a characteristic set ofspectral lines, sostellar spectroscopy can be used to infer the atmospheric composition of individual stars. Observations indicate a strongnegative correlation between a star's initial heavy element content (known as themetallicity) and its age. More recently formed stars tend to have higher metallicity.
The early Universe consisted of only the light elements formed duringBig Bang nucleosynthesis.Stellar structure and theHertzsprung–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 theinterstellar 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 thenuclear physics andastrophysics involved in how stars produce these heavy elements. By scrutinizing thetable of nuclides, the authors identified different stellar environments that could produce the observedisotopic abundance patterns and the nuclear processes that must be responsible for them. The authors invoke nuclear physics processes, now known as thep-process,r-process, ands-process, to account for the elements heavier thaniron. 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.[7]
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 thecarbon-burning process, theoxygen-burning process and thesilicon-burning process, each of which contribute to the elements from magnesium to nickel. Hoyle had already suggested thatsupernova nucleosynthesis could be responsible for these in his 1954 paper.[7]Donald D. Clayton has attributed the lower number ofcitations 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 generally; to Hoyle's having described its key equation only in words[9] rather than writing it prominently in his paper; and to Hoyle's incomplete review of the B2FH draft.[10]
The Caltech nuclear physicistWilliam Alfred Fowler used hissabbatical leave to visit Hoyle in Cambridge from 1954 to 1955. The pair invitedMargaret Burbidge andGeoffrey Burbidge to join them in Cambridge, as the couple had recently published extensive work on stellar abundances that would be required to test Hoyle's hypothesis. The quartet collaborated on several projects whilst in Cambridge; Fowler and Hoyle began work on a review that would become B2FH. Fowler returned to Caltech with the work far from complete, and encouraged the Burbidges to join him in California. Both of the Burbidges had temporary positions created for them in 1956 at Caltech by Fowler for this purpose.[citation needed] The first complete draft was completed by the Burbidges in 1956 at Caltech, after adding extensive astronomical observations and experimental data to support the theory. Margaret Burbidge, the paper'sfirst author, completed much of the work whilst pregnant.[11] The final paper is 104 pages long, with 34plots, 4photographic plates, and 22tables; despite this length, it does not have anabstract.[1]
Some have presumed that Fowler was the leader of the group because the writing and submission for publication were done at Caltech in 1956, but Geoffrey Burbidge has stated that this is a misconception. Fowler, though an accomplished nuclear physicist, was still learning Hoyle's theory in 1955 and later stated that Hoyle was the intellectual leader.[12] The Burbidges also learnt Hoyle's theory during 1954–55 in Cambridge. "There was no leader in the group," G. Burbidge wrote in 2008, "we all made substantial contributions".[13]
B2FH drew scientific attention to the field ofnuclear astrophysics. By reviewing the theory of stellar nucleosynthesis and supporting it with observational evidence, B2FH firmly established the theory among astronomers.
Fowler was awarded half of the 1983Nobel Prize in Physics, possibly for his contribution to B2FH. The Nobel committee stated: "Together with a number of co-workers, [Fowler] developed, during the 1950s, a complete theory of the formation of the chemical elements in the universe."[14] Fowler's contributions to B2FH included the nuclear physics of thes-process and ther-process.
Some have argued thatFred Hoyle deserved similar recognition for theoretical work on the topic, and contend that his unorthodox views concerning theBig Bang stopped him being awarded a share of the Nobel Prize.Geoffrey Burbidge, for example, argued in 2008 that "Hoyle should have been awarded a Nobel Prize for this and other work". He also speculated that the reason why Hoyle ended up empty-handed was that "Fowler was believed to be the leader of the group."[13] Burbidge insisted that this perception was false and pointed to Hoyle's earlier foundational papers from 1946[15] and 1954.[7]
Fowler, in his own Nobel lecture, wrote about Hoyle: "Fred Hoyle was the second great influence in my life. The grand concept of nucleosynthesis in stars was first definitely established by Hoyle in 1946."[16]
Hoyle's biographer Mitton has speculated that Hoyle was left out by the Nobel committee because he had earlier spoken out against the injustice of the Nobel committee overlookingJocelyn Bell Burnell for the 1974 prize.[17]
In 2007 a conference was held at Caltech inPasadena, California to commemorate the 50th anniversary of the publication of B2FH,[18] where Geoffrey Burbidge presented remarks on the writing of B2FH.