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  • Writer's pictureDale DeBakcsy

Margaret Burbridge and the Dawn of Nucleosynthesis Theory

If you had asked a random astronomer in the 1930s how all of the elements in the universe were produced, they would have had a ready and instant answer: during the Big Bang event. Hypothesised in 1927 by Georges Lemaitre and heavily reinforced by Edwin Hubble’s 1929 interpretation of stellar red-shifts as evidence of an expanding universe, the Big Bang seemed an ideal explanation of how the universe’s elements arose – all produced at once in one grand moment of matter creation. That explanation, however, did not sit well with certain portions of the astronomical community, some of whom continued to hold to a ‘steady-state’ model of the cosmos, in which the universe was eternal, and the elements were produced in a series of waves rather than all once, and others of whom were simply ill at ease with the variation in element distributions across different stars that was difficult to explain by a single atomic origin event.


In 1946, the astronomer Fred Hoyle first proposed the idea, as a buttress to his steady-state theory of the cosmos, that new elements could be created inside of stars, but that idea did not get much traction in the scientific community until Hoyle teamed up with William Fowler, Margaret Burbridge (1919–2020), and Geoffrey Burbridge to produce an exhaustive 100 page paper of such import and impact that it today is known simply as the B^2FH paper, which definitively laid out the modern theory of nucleosynthesis. The B^2 in that paper is a reference to the two Burbridges whose observations of stellar spectra and evolving theories about neutron capture as a motivator for the creation of new elements lay at the centre of the new theory, and whose path to the upper echelons of the astronomical community had been anything but a sure one.


Margaret was born Margaret Peachey (a family name originating, so Margaret believed, in Huguenot fisher, or ‘pecheur’, ancestors who came to England to escape religious persecution) nine months and one day after the signing of the armistice ending World War I, a subject of much speculation on Margaret’s part as to the circumstances of her conception, which her mother very properly declined to confirm or deny. Both of Margaret’s parents were chemists (in fact her mother had met her father when she was taking a chemistry course of which he, some 17 years her elder, was the teacher), and in particular her mother had to fight for her education against her family’s wishes that she settle for a more traditional trajectory. As such, it is not surprising to learn that young Margaret had access to essentially whatever scientific material piqued her curiosity. She was, from a young age, fascinated with abnormally large numbers, and used her father’s binoculars, guided by advice in the periodical The Children’s Newspaper, which her parents bought her a subscription to, to scan the night sky for objects of interest.


She was bought in turn a microscope, a chemistry set and, at around the age of 12, she was provided with the writings of a distant maternal relative, James Jeans (1877–1946), who was both a distinguished astronomer and an author of popular books on astronomy. Likely, the book pressed into Margaret’s hands was his 1929 The Universe Around Us, which excited her with its accounts of the unimaginably great distances between the stars. Years later, she recalled the thrill she felt reading that the nearest star was 26,000,000,000,000 miles away – suddenly, her fascination with huge numbers clicked in with actual physical objects those numbers could meaningfully be applied to. She formed at that moment a youthful conviction that some day, she would make the calculation of interstellar distances her profession.



While most of her friends at the Francis Holland School for Girls were interested in working towards attending Oxford, Margaret, on the advice of her mother, was destined for University College, London, where she matriculated in 1936. It was a good fit – UCL provided a rigorous slate of undergraduate science courses, and offered the option of a major in astronomy with a minor in mathematics that was tailor made to Margaret’s strengths and interests. At UCL, she learned all the practical skills necessary for making astronomical observations, maintaining and adjusting the instruments, determining errors in calibration, and working from raw data to orbital computation that would stand her in good stead not only in her first post-college job, but in the variety of astronomical work that she performed across the long stretch of her diverse career.


Graduating in 1939 under the decidedly non-festive pall of coming war with Germany, Margaret first occupied herself as a raid warden, preparing London for the expected all-out air attack that failed to materialise during the period of World War II known as the ‘Phony War.’’ She soon found work more suited to her talents when the University of London Observatory’s director and primary technician were both called up for war service. Somebody needed to be found with the technical skills to maintain the observatory’s equipment, organise repairs in case of a bomb strike, and continue research with the 24 inch Wilson reflector telescope (the lenses on the ULO’s refractor were deemed too valuable to risk keeping in operation during wartime, and were subsequently removed and separately stored). Margaret, with three years of technical training under her belt, and with no chance of being called off to serve in the war, seemed an ideal person to take on the job, and for five years she had constant access to a telescope, and the in-hindsight wonderful experience of carrying out astronomy in an open observatory, physically next to the device she was operating, something which she lamented later generations, huddled off in front of monitors stuffed into rooms far from the telescopes they controlled, rarely got to experience.


Following the war, Margaret attended graduate level classes at UCL beginning in 1947, where she met fellow student Geoffrey Burbridge, whose interest at the time was physics rather than astronomy. The two were married in 1948, and thereby began their long career navigating the complex reality awaiting academic couples who worked in related fields. After their marriage, Geoff (as he was usually known) switched his field to astronomy, and together they decided to pursue a career in spectral analysis, which would require them to find a better telescope, in a less perpetually cloud bedecked city, than they had available to them in London. They applied for money to study at the Haute Provence Observatory, but were told that, if they wanted British money to look into a telescope, it needed to be a British one. So, the couple decided to self-finance their trip to Haute Provence, where they arrived in 1949 and fatefully happened to meet another visiting British astronomer, Fred Hoyle.


UCSD Special Collections & Archive


The Burbridges’ next move was to take some advice given them by the astronomical living legend of his age, Otto Struve, to apply for fellowships to study at American observatories. Margaret had previously applied for a Carnegie Fellowship to study at Mt. Wilson Observatory, only to be told that Mt. Wilson on no account accepted women researchers at its facilities, but fortunately both she and Geoff managed to find financing that gave them access to the Yerkes Observatory attached to the University of Chicago, where Otto Struve had gathered a team that included future Hubble Space Telescope guiding force Nancy Grace Roman and future Nobel Prize winner Subrahmanyan Chandrasekhar. Once in the United States, the Burbridges took every opportunity they had to study at different observatories and learn from far-flung experts who between them were producing the pieces that would lead to a radically new view of How the Universe Got Its Stuff. From Harvard’s Cecilia Payne-Gaposchkin, Margaret learned about elemental abundances in different stars. From Chandrasekhar the couple learned new ways of relating spectral data to the underlying atmospheric physics of the star in question. From Maria Goeppert-Mayer’s (more about whom in the physics volume of this series) theories about nuclear stability and a possible polyneutron origin of the cosmos, they began thinking about the role neutrons might play in progressions of element formation that might happen within stellar cores.


All of these influences, combined with their own measurements of atypical elemental distribution in the spectra of a series of Apm stars they observed, and with their 1954 meeting of Willy Fowler, coalesced into the project that would consume their mid 1950s and change the face of astronomy. Together with Fowler and Hoyle, Margaret and Geoff worked on establishing eight stellar processes that were important in the creation of new elements (nucleosynthesis): Hydrogen burning, Helium burning, the alpha process that occurs when Helium nuclei fuse and which can produce carbon, Hoyle’s e-process that explained the relative abundance of iron as against other heavy elements through thermal equilibrium processes, the s-process of slow neutron capture that took place in the cores of red giants, the r-process of rapid neutron capture that occurs during supernova events, the x-process governing lithium, boron, beryllium, and deuterium, and the p-process which sought to explain the production of lower abundance heavy elements. The results of their work, and the detailing of these processes, were contained in their landmark 1957 paper, ‘Synthesis of the Elements in Stars’ or the B2FH paper as it is more commonly known.


That paper was a sensation that placed Margaret and Geoff squarely at the centre of the astronomical community’s attention, and revived for a short while the hopes of the steady state universe advocates (of which Margaret was one) that their model, of progressive and sustained creation over time as opposed to a single origin event, might prove correct in the end. (That hope, by the way, was all but crushed by the 1964 discovery of the cosmic microwave background (CMB), the existence of which had been predicted by the Big Bang Theory but which did not fit in well with a Steady State model). In the following years, Margaret devoted herself primarily to the fields of spiral galaxies and quasars. In 1962, Geoff and Margaret joined the faculty at UCSD, which did not have the nepotism restrictions on husbands and wives working together in the same department that had so complicated their careers up to that point (and which similarly attracted the Goeppert-Mayers). From here, she carried out studies on the rotational velocities and mass to light ratios of spiral galaxies (the type of work that would later lead Vera Rubin to hypothesise the existence of dark matter), and her spectral studies of quasars, whose extreme redshift (and therefore, by the Big Bang account under Hubble’s interpretation, extreme distance from the Earth) posed a problem for her Steady State view of the cosmos which she attempted to rectify by proposing in 1967 a model for redshift that did not rely on an expanding universe.


Over the next decades, Margaret was dragged into the realm of academic administration on a number of occasions in spite of her determined protestations that she was not temperamentally suited for the work. She became the first woman to direct the Royal Greenwich Observatory, but found the politicking so to her distaste that she left as soon as she reasonably could. She was also tapped for the role of President of the American Astronomical Society, and almost immediately ran into the political kerfuffle surrounding the Equal Rights Amendment whereby some sections of the AAS proposed that no meetings should be held in states that refused to ratify it, while others vehemently opposed the mixing of science and politics. In addition, she was placed in the anxious position of having to refuse the Annie Jump Cannon Award in 1971, because she felt that it was prejudicial to have awards that were only awarded to women, and tended towards the under-valuing of their work.


Though much of her later career was spent in the deep brambles of administrative politics, Margaret Burbridge did have one major last , though ill-fated, contribution, and that was to a project overseen in its early stages by her old Yerkes colleague Nancy Grace Roman, the Hubble Space Telescope. The original plan for the telescope involved the inclusion of four instrumental systems, one of which was the Faint Object Spectrograph. In recognition of her years of spectral analysis work, Margaret was asked by NASA to join the FOS team. It was an important instrument that managed some key observations in spite of some terrible luck, first the famous aberration of the main mirror that rendered a number of the smaller apertures on the device useless, and then issues of insufficient magnetic shielding that created smeared pixels and effectively removed the 115–150 nm part of the spectrum the FOS hoped to survey, and which meant it was unable to detect the important Lyman-alpha line associated with Hydrogen electrons dropping from the n=2 to n=1 energy level. Ultimately, the FOS was replaced in 1997 by the Space Telescope Imaging Spectrograph, which is still in operation.


A hundred years from now, however, I doubt if anybody mentioning the name Margaret Burbridge will do so to reference her time as the Greenwich Observatory director, to wonder about her extended devotion to the Steady State Model, or to gripe about design flaws in the Faint Object Spectrograph. What they will be doing is talking about the miracle year, 1957, when she was a member of that legendary quartet who rewrote the rules for why we have the elements we do in the amounts we do, and thereby brought us all to the realisation that we are, fundamentally, the children of the stars.


FURTHER READING:


You know the tune by now, say it with me! There is no full length biographical treatment of Margaret Burbridge currently available. Your best source for her life is a 37 page memoir she wrote in 1994 for the Annual Review of Astronomy and Astrophysics and which, amazingly in the world of online periodicals, you can access for free. There were also a series of articles in astronomy and physics journals and popular science magazines celebrating her 100th birthday in 2019 which contain the highlights without the spectral details.


And if you'd like to read more about women astronomers like Jill Tarter, check out my History of Women in Astronomy and Space Exploration, which you can order from Amazon, or from Pen and Sword US or UK.





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