Thursday, January 30, 2014

The origin of carbon

Last summer I was asked to write an article on the origin of carbon for The Geographer, which is the quarterly newsletter of the Royal Scottish Geographical Society. The original article can be found here (see page 8), but I've been given permission to reproduce it here (any comments/corrections are welcome):

Carbon is the fourth most abundant element in the Universe (after hydrogen, helium and oxygen) and is the sixth lightest element. To understand it's origins and relative abundance we first have to go back to the origin of the Universe itself.

By the mid-20th Century Edwin Hubble's observations of an expanding Universe suggested that it had started out from an extremely dense and hot initial state: a "cosmic fireball" produced by the Big Bang. However, a question for the Big Bang model was how it produced the known elements in their currently observed abundances (called Big Bang nucleosynthesis). In 1948 a PhD student called Ralph Alpher, working with the renowned physicist George Gamow, published a paper called "The Origin of Chemical Elements" claiming to solve this problem. But, the title slightly overstated the outcome of their work. It was ground-breaking and correctly predicted that in this "comsic fireball" the three lightest elements (hydrogen, helium and lithium) would be made in the abundances that are observed today. However, their work couldn't produce any heavier elements and it was in fact the problem of making carbon that was the stumbling block. The basic process of forming elements is that you take nucleons (protons and neutrons) and fuse them together to create heavier atomic nuclei. You can then fuse further nucleons, or atomic nuclei, together to produce heavier and heavier elements. This is complicated by several facts: the rates that fusion reactions take place can differ enormously for different nuclei; the rates depend very strongly on temperature and density; and, certain nuclei are unstable to radioactive decay and are very short-lived. To create carbon you require six protons and six neutrons, so it can be made by fusing two helium nuclei (two protons and two neutrons) to give a beryllium nucleus and then sticking on another helium nucleus to give carbon. However, Alpher and Gamow found that because the beryllium nuclei only has a lifetime of ~10-16 seconds there wasn't enough time during the hot and dense early stages of the Universe for it to fuse with another helium nucleus and produce carbon. They were therefore left with a Universe containing only the three lightest elements, which was contrary to all observational evidence!

This problem with Big Bang nucleosynthesis was jumped upon by opponents of the Big Bang as a failure of the model. One such person was Sir Fred Hoyle, a forthright theoretical astrophysicist at Cambridge, who, along with others, put forward Steady State models of the Universe (i.e. an infinite Universe with no beginning). However, his models still required that there was some way that elements could be produced, so the problem of creating carbon from lighter nuclei still needed to be solved. In the calculations for trying to fuse three helium nuclei (called the triple alpha process, since helium nuclei are also known as alpha particles) he still found that only insignificant amounts of normal carbon could not be produced during the short life of beryllium, but the production rate would dramatically increase if carbon nuclei were created in an "excited" state i.e. a nucleus with additional potential energy in it. There was no theoretical reason why such an "excited" state should exist (in fact it is still unknown [sorry for the non-open access article link] why this state exists!), but Hoyle argued that because we exist and we require carbon for our existence, then if this is the only way significant amounts of carbon can be produced then this state must be possible. His calculations gave him a precise number for the amount of energy in this state, but he had to convince someone to run an experiment to see if it was true. While visiting the California Institute of Technology in 1953 he persuaded the nuclear experimental groups led by Willy Fowler and Ward Whaling to look for this excited state and soon after it was confirmed that it did indeed exist1.

This didn't mean that Big Bang nucleosynthesis could now produce carbon and the heavier elements as the process was still far too slow given the expansion of the Universe, but there were other environments where it could take place - the cores of massive stars. Hoyle and Fowler, along with the married couple of Margaret and Geoffrey Burbidge, were able to show how all the elements from beryllium up to iron were synthesised in the cores of stars (called stellar nucleosynthesis). In these massive stellar cores there is a high enough temperature and density of helium nuclei so that even though the beryllium produced from fusing two helium nuclei is extremely short-lived there is enough of it that some will fuse with another helium nuclei to form the excited state of carbon. Since carbon was required as the starting point for production of all the heavier elements this allows the large variety we see today. The deaths of these massive stars in supernova explosions has since seeded the Universe we the huge quantities of carbon we see today.

The evidence now shows that the lightest elements were indeed produced during the Big Bang and the Universe has had enough time to produce all other elements (including Carbon) in their observed abundances, via processing in stars.

1A more detailed account of this and the many other people actually involved in the work can be found in H. Kragh, (2010) When is a prediction anthropic? Fred Hoyle and the 7.65 MeV carbon resonance.

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