Evening Meeting Wednesday 14th February 2018 - "The Origins of Atoms" - Dr. Geoff Steel.

We all have seen and been taught about the Periodic table of elements but how many understand how these elements came into being. Geoff's talk went a long way in helping in this understanding.

The first lighter elements were formed by Big Bang nucleosynthesis. The Big Bang being the cosmological model that best explains the origin of the universe. During the first second following the Big Bang, a wide range of events were occurring including cosmic inflation, elemental particle formation, the formation of baryonic matter, collisions between elementary particles, decreases in particle energies and temperatures. Between one second and three minutes after the Big Bang temperatures cooled to about 109K at which Big Bang nucleosynthesis could occur. This led to the production of protium (1H), deuterium (2H), tritium (3He), helium (4He), and some Lithium (7Li) but no heavy elements were formed. The heavier elements had to wait until later to be formed by Stellar Nucleosynthesis.

Most stars produce energy by fusing hydrogen to form helium, with the release of energy. The outward flow of energy (thermal pressure) released balances the collapsing force of gravity, and this stabilizes the star's size. As helium increases in the core it reduces the rate of hydrogen fusion which in turn decreases thermal pressure and the star starts to contract. Depending on the initial mass of the star Helium can fuse to form carbon nuclei which leads to the formation of a carbon core via a process known as the Triple Alpha Process. This increases thermal pressure of the core to a point where it will overcome gravity and the size of the star increases giving rise to a red giant. If a star had sufficient mass, though, eventually enough carbon would accumulate so that the temperature and density reach a point where carbon nuclei could be fused into Neon nuclei. This carbon burning core would be surrounded by two outer shells, the innermost burning Helium, and the outermost burning hydrogen. This pattern of the central core collapsing and increasing temperature continues until a further round of fusion occurs and more shells form. How many shells are eventually formed is dependent on the initial mass. Eventually iron (56Fe) will be formed but the average binding energy per nucleon has now reached maximum so there can be no more energy-generating reactions, all reactions are required to be endothermic. The star therefore undergoes gravitational collapse. The star then enters a runaway phase leading to the development of a supernova explosion which in turn can lead to the production of even higher elements via a process involving neutron capture.

Finally Geoff explained that the abundance of lithium, beryllium and boron in the universe is thought to be due to the process of Cosmic Ray Spallation. This refers to the process where interstellar collisions between very energetic cosmic rays and interstellar nuclei causes the heavier nuclei to break up forming smaller nuclei. During the collision there is expulsion of a large number of protons and neutrons from the object hit. Cosmic Ray Spallation not only occurs in deep space but also in the earth's upper atmosphere and crustal surface.

The next meeting will be on March 14th when Bob Loveridge will give a talk entitled: “Cretaceous Fossils of the Araripe Basin, NE Brazil”