During the process of stellar evolution nuclear fusion reactions take place within a star. These give rise to the formation of the chemical elements. Clues to the processes involved may be deduced from the graph below, which shows the cosmic abundances of elements (y-axis) plotted against atomic number (x-axis). The abundances are measured relative to 1 million silicon atoms and the y-axis is a logarithmic scale. In this case the data are for the solar-system and abundances have been measured from the optical analysis of the solar spectrum.
There are three important observations to be made from this graph:
It is thought that the early Universe consisted almost entirely of the element hydrogen, with a small amount of helium present too. Hydrogen, therefore, is thought to be the starting material from which all other elements have been built. This is consistent with the very high abundance of hydrogen in the solar abundance profile. The process may be thought of as a series of fusion reactions which weld together simple atomic nuclei to build increasing complex atomic nuclei. The manner in which this is done depends upon the internal temperature of the star and on its mass.
Early in star development hydrogen is utilised to manufacture the element helium. As the hydrogen in the star is used up, the star contracts and its temperature rises so that nuclear reactions can take place which permit the synthesis of the elements carbon, nitrogen and oxygen, from helium (see http://chemistry.ewu.edu/breneman/origin.htm). When the helium is almost completely consumed the carbon and oxygen can be transformed into elements with masses up to that of silicon. Increasing nuclear reactions, at higher temperatures lead to the formation of elements with masses up to that of iron (Fe). Beyond this point heavier elements cannot be formed by the process of nuclear fusion because the temperatures required are higher than those found in stars.
Elements heavier than iron are formed by the addition of neutrons which are absorbed by the atomic nucleus. The very high abundance at the iron peak suggests that nuclei of the element iron are formed more readily than they are used in the process of neutron capture (see http://www.phy.anl.gov/ria/scicase/astro/astro_text.html).
The anomalously low concentrations of the elements Li, Be and B indicate that they are by-passed in nuclear fusion reactions and their genesis seems to be explained by the partial decay of heavier nuclei of the elements carbon and oxygen.
Differences in concentration between elements with even and odd atomic numbers can be explained by the greater stability of atomic nuclei with paired neutrons. Thus elements with even atomic numbers have the greater nuclear stability. This allows their abundances to grow.
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