HELP on STANDARD ABUNDANCE DISTRIBUTION (SAD) (atoms per 10åç of Hydrogen) (plotted on a logarithmic scale)
This is a plot of standard abundances, the composition of the gas cloud from which the sun and the solar system condensed. Data from Anders & Grevesse (19989). The units are in 'numbers of atoms per 10åç of hydrogen'. Hydrogen and helium are by far the most abundant elements of the gas cloud, making up between them 99% of all atoms. The hydrogen was present from the very start of the Big Bang, the helium was manufactured by hydrogen fusion in the first few minutes of the Big Bang. The heavier elements of the gas cloud were produced by fusion reactions within very massive stars and by the later supernova explosions of these stars very early on in the history of the Universe. The sun was born much later out of this gas cloud, and now comprises 99.9% of the present solar system. There is a very noticeable dip in the plot for lithium, beryllium and boron, which are consumed by nuclear reactions within solar interiors. For most other elements, the composition of the gas cloud closely mirrors that for the carbonaceous chondrites, except for the volatile elements which have boiled off the meteorites at a time when they were hot. Most striking of all is the alternation of abundances, elements with even atomic number being more abundant than those with odd, being due to their nuclear stability. See help on 'Number of stable isotopes'. There is a general decline in abundance with increasing atomic number, but a pronounced peak in abundance around iron, the most stable elements.
