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## The Origin of the Elements – part 1

One of the outstanding problems in astrophysics in the 1940s was how were the elements created. In the 1920s it was realised by Cecilia Payne-Gaposchkin in her PhD work that the Sun was mainly composed of hydrogen. Then, spectral analysis of others stars and the gases of the interstellar medium led astrophycisits to realise that the Universe was composed mainly of hydrogen (about $75\%$), with the remainder being helium (about $24\%$), and only $1\%$ or so being all the other elements (oxygen, nitrogen, carbon etc.)

In the mid 1940s Russian-American physicist George Gamow started thinking about how the elements originated, and he developed a theory with his student Ralph Alpher that they were all created in the early Universe when, he argued, it would have been hotter and denser than it currently is. He published his famous paper “The Origin of Chemical Elements” in Physical Review in 1948, adding renowned physicist Hans Bethe’s name to the author list as a joke so that the paper would have the author list Alpher, Bethe, Gamow (alpha,beta, gamma, geddit? There is also a story that he tried to get his post-doctoral researcher Bob Herman to change his last name to “Delta” 😛 )

The famous Alpher, Bethe, Gamow paper, “The Origin of Chemical Elements”, which appeared in Physical Review in 1948.

In fact, Bethe played no part in writing the paper, but he was happy for Gamow to include his name for Gamow’s little joke. In this paper, Gamow and Alpher argued that all the elements were created in the early Universe. However, when others went through the details it was realised that the numbers did not add up, the Universe expanded and cooled too quickly for all the elements to be created in this way. Although it was possible for hydrogen and helium to be created in the first few minutes of the Universe, by the time the Universe was a few minutes old it had become too cool and the density too low to form the heavier elements beyond helium. Part of the reason for heavier elements not being built up in these first few minutes was due to something called the deuterium bottleneck, which I will explain in a future blog.

In the 1950s an alternative theory for the origin of the elements was put forward by Fred Hoyle and his collaborators Willy Fowler and Geoffrey and Margaret Burbidge. In a series of papers they argued that the elements had been built up in the interior of stars, the most famous of this series of papers was a 1957 paper entitled “Synthesis of the Elements in Stars” which appeared in Reviews of Modern Physics. Hoyle was the main advocate of a theory called the Steady State Theory which he had first proposed in 1948. This was a competing theory to the hot big bang theory, and so of course Hoyle did not believe any elements had been formed in a hot, dense early Universe as he did not believe such a Universe ever existed.

The first page of Burbidge teal.’s famous paper “Synthesis of the Elements in Stars” published in Reviews of Modern Physics in 1957

Again, as with Alpher and Gamow’s theory, detailed calculations found flaws in the Burbidge etal. theory. Although it could explain the creation of elements beyond helium, it was not possible to create enough helium in stars to account for the approximately $25\%$ found to be present in the Universe today. In part 2 of this blog, I will explain what our current understanding is of the origin of the elements in the proportions we observe in the Universe, and what the deuterium bottleneck is and why it is important.