B²FH

B²FH (pronounced "B squared F H") is the initials of Geoffrey Burbidge, Margaret Burbidge, William Fowler and Fred Hoyle, and is used to reference a well-known astrophysics paper titled Synthesis of the Elements in Stars published in 1957 [E. M. Burbidge, G. R. Burbidge, W. A. Fowler, and F. Hoyle. Reviews of Modern Physics, 29 (1957) 547.] ; the article is so famous that it is typically referred to only as B²FH. This paper is credited as the origin of what is now the theory of stellar nucleosynthesis.

Physics in 1957

At the time of the publication of B²FH, George Gamow advocated a theory of the universe where virtually all elements, or atomic nuclei, were synthesized during the big bang. The implications of Gamow's nucleosynthesis theory (not to be confused with present-day nucleosynthesis theory) is that nuclear abundances in the universe are largely static. Hans Bethe and Charles L. Critchfield together derived the pp-chain in 1938 [C. L. Critchfield, H. A. Bethe. Physical Review 54 (1938) 248.] . and Carl von Weizsäcker [C. F. von Weizsäcker. Physik. Zeitschr. 39 (1938) 633.] and Hans Bethe [H. A. Bethe. Physical Review 55 (1939) 436.] independently derived the CNO cycle in 1938 and 1939, respectively, to show that nuclear fusion could account for stellar energy production by converting hydrogen to helium. So, it was known by Gamow and others in 1957 that the abundances of hydrogen and helium were not perfectly static. However, at the time, stellar fusion theories did not show how to create any elements heavier than helium, and so Gamow advocated for a theory where all elements were residual from the big bang, allowing for slight changes in the ratios of hydrogen and helium. B²FH gives a different account for the origin of all the observed heavy elements, suggesting that all atomic nuclei heavier than lithium up through uranium must be synthesized in stars and not during the big bang. Both theories agree that the some light nuclei, between hydrogen and lithium, are not created in stars, and this led to the now-accepted theory of big bang nucleosynthesis.

Physics in the paper

Because B²FH argued that a majority of all elements, including the carbon and oxygen we humans are made of, must come from stars, their ideas are called the theory of stellar nucleosynthesis. An often quoted statement of Carl Sagan, "We are all star dust," is simply his paraphrase of this review paper's thesis. The key difference between this theory of stellar nucleosynthesis and all previous accounts for the origin of the elements is that B²FH predicts chemical evolution of the universe, which is testable by looking at stellar spectral lines. Quantum mechanics explains why different atoms emit light at characteristic wavelengths, and so by studying the light emitted from different stars, one may infer the atmospheric composition of individual stars. However, upon undertaking such a task, observations indicate a strong correlation between a star's heavy element content (metallicity) and its age (red shift).

Big bang nucleosynthesis tells us that the early universe consists of only the light elements, and so one expects the first stars to be composed of hydrogen, helium, and lithium, the three lightest elements. Stellar structure and the H-R diagram indicate that the lifetime of a star depends greatly on its initial mass, so that massive stars are very short-lived, and less massive stars are longer-lived. As a star dies, B²FH argues that it will enrich the interstellar medium in 'heavy elements' (in this case all elements heavier than lithium, the third element), from which new stars are formed. This account is consistent with the observed correlation between stellar metallicity and red shift.

However, the theory of stellar nucleosynthesis advocated by B²FH would not be very convincing if it could not also detail the nuclear physics and astrophysics involved. By carefully scrutinizing the table of nuclides, the authors were able to predict the existence of different stellar environments which could produce the observed isotopic abundances, and the nuclear processes which must occur in these stars. In this paper, among other things, the authors predicted the existence of the p-process, r-process, and s-process to account for many of the elements heavier than iron, and these idea have since come to bear much fruit.

Writing of the paper

Margaret and Geoffrey Burbidge wrote the first draft of the paper, deliberately incorporating extensive observations and experimental data to support the theory. Both Hoyle and Fowler worked extensively on the early draft. Geoffrey Burbidge says that it is a misconception some have had that Fowler was the leader of the group. "There was no leader in the group," he wrote in 2008, "We all made substantial contributions."G. Burbidge, [ "Hoyles Role in B²FH"] , "Science", 319 (2008) 1484]

Recognition

Because this work firmly established the field of nuclear astrophysics, William Fowler was awarded half of the 1983 Nobel Prize in Physics for his contributions; some believe that Fred Hoyle also deserved similar recognition for his scholarship on this topic, and contend that his unorthodox views concerning the big bang played a role in his lack of a Nobel Prize.

Geoffrey Burbidge wrote in 2008, "Hoyle should have been awarded a Nobel Price for this and other work. On the basis of my private correspondence, I believe that a major reason for his exclusion was that W.A. Fowler was believed to be the leader of the group." Burbidge states that this perception is not true and also points to Hoyle's earlier papers from 1946 [F. Hoyle, "Mon. Not. R. Astron. Soc.", 106' (1946) 343] and 1954F. Hyle, "Astophys. J. Suppl", 1 (1954) 121] as indicators of Hoyle's authorship of the theory of stellar nucleosynthesis. Burbidge explains, "Hoyle's work has been undercited in part because it was published in an astrophysical journal, and a new one at that (the very first volume, in fact), whereas "B²FH" was published in a well-established physics journal, "Reviews of Modern Physics". When "B²FH" was first written, preprints were widely distributed to the nuclear physics community. Willy Fowler was very well known as a leader in that community, and the California Institute of Technology already had a news bureau that knew how to spread the word.

In 2007 a conference was held in Pasadena, California to commemorate the 50th anniversary of the publication of this influential paper. [ [http://www.na2007.caltech.edu/ Nuclear Astrophysics 1957-2007: Beyond the first 50 years] , California Institute of Technology, Pasadena, CA, July 23-27, 2007]

References