Heavy Elements "In a sense each of us has been in a star; in a sense each of us has been in the vast empty spaces between the stars; and - if the Universe ever had a beginning - each of us was there! Every molecule in our bodies contains matter that once was subjected to the tremendous temperatures and pressures at the centre of a star. This is where the iron in our red blood cells originated. The oxygen we breathe, the carbon and nitrogen in our tissues, and the calcium in our bones, also were formed through the fusion of smaller atoms at the centre of a star. Terrestrial ores containing uranium, plutonium, lead, and many other massive atoms must have formed in a supernova explosion - the self-destruction of a star in which a sun's mass is hurled into space at huge velocity. In fact, most of the matter on Earth and in our bodies must have gone through such a catastrophic event!" With these words begins Martin Harwit's book "Astrophysical Concepts" (published in 1988 in the 'Astronomy and Astrophysics Library' of the Springer-Verlag). Indeed, we all come from stars While hydrogen, helium and lithium were produced during the Big Bang, all heavier chemical elements result from nuclear reactions in the interiors of stars. When stars die in one of the supernova explosions just mentioned, heavy-element enriched matter is dispersed into surrounding space and will later be incorporated in the next generations of stars. In fact, the gold, which people wear as jewellery, was produced in an exploding star and deposited in the interstellar cloud from which the Sun and its planets were later formed. Thus, the older a star is, the lower is generally its content of heavy elements like iron and other metals. Measurements have shown that old stars that are members of large agglomerations known as globular clusters are normally quite "metal-poor": their metal-content ranges down to about 1/200 of that of the Sun - in which these metals constitute only 2% of the total mass, the rest being still in the form of hydrogen and helium. In fact, observations with ESO's Very Large Telescope have recently uncovered hundreds of stars in our own galaxy with much lower metal content than even the globular clusters, in some cases only 1/10,000 of the solar value. Such stars must have been formed in the very infancy of the Milky Way.

The three principal production sites of nuclei. Carbon and all heavier elements are produced in stars. The Big Bang yields only the lightest species. For species with mixed origin, such as ³He and ⁷Li, the relative proportions change with time and location in our galaxy — and elsewhere. (Source: J. Geiss, and R. von Steiger 1977, "Production of Light Nuclei in the Early Universe" in 'Fundamental Physics in Space', ESA-SP 420, pp. 99-106.)