Helium and the heavier elements are built from the nuclei of hydrogen by a process, first proposed by Arthur Eddington (1882-1944) in 1920, called nucleosynthesis. The first step in this process took place in the Big Bang when large amounts of hydrogen were converted to helium. From about four minutes after time zero, some twenty-five percent of the mass present ended up as Helium nuclei, almost seventy-five percent as hydrogen nuclei, with tiny traces of deuterium and lithium. This was the raw material for nucleosynthesis.
The essential process is that elements are built up inside stars by the adding together of helium-4 nuclei (=alpha particles) to existing nuclei, which means that the elements are built up in steps of four atomic mass units.
The nucleus made by sticking two helium-4 nuclei together, beryllium-8, is extremely unstable and breaks apart after 10-7 of a second. This problem is overcome by the triple-alpha-process, also known as the Salpeter process, after Edwin Ernest Salpeter (1924-2008), in which three helium-4 nuclei fuse together to make one nucleus of carbon-12. Fusing atomic nuclei together to form heavier ones always results in an overall loss of mass and energy release in line with E=mc2.
Once a star contains carbon-12 further fusion takes place for as long as energy continues to be released. A fourth alpha particle makes oxygen-16 and successive fusions produce neon-20, magnesium-24 and silicon-28. While this is happening, radioactive decay causes particles to be ejected from the nuclei to form other elements and isotopes – but the elements formed from helium-4 nuclei remain the most common. This process ends when pairs of silicon-28 combine to form iron-56 and related elements such as nickel-56 and cobalt-56, and to make heavier elements energy has to be added in order to force the nuclei to fuse together.
This can occur when neutrons penetrate nuclei and stay there. There are always neutrons around as by-products of fusion reactions inside the star. Heavier elements can be built up slowly (s-process) with neutron capture building up elements from iron-56 to bismuth-209. Heavier elements still are produced rapidly (r-process) when there is a flood of neutrons available such as happens during supernova explosions
Leave a Reply