Number 82 (Story #4), June 2, 1992 by Phillip F. Schewe and Ben Stein OXYGEN AND IRON , two of the most important elements of life on Earth, were not much present in the early phase of our galaxy: the metallicity (proportion of elements heavier than helium) for old stars is only 0.02%, while that of younger stars like our Sun (4.6 billion years) is 2%. Supernovas, the furnace for producing metals, spew heavy elements out into the interstellar medium, where they can be taken up in future generations of stars. A star's metallicity is usually pegged to its iron content because iron's complex spectrum is easier to detect than oxygen's. (Astronomy, July 1992.)