Introduction

We do not know the total set of conditions necessary for the development of sentient life, but it is a pretty safe bet that chemically based life, at least, requires both a wide range of chemical elements and a good deal of time. Other chapters in this volume attempt the difficult task of estimating how many planets might have had long-lived, stable supplies of water, carbon dioxide, etc., and temperate climates. This chapter addresses the much simpler issue of the numbers, ages and locations in our Galaxy of stars with adequate supplies of heavy elements to make terrestrial planets, in principle, possible.

Carbon, oxygen, phosphorus and the other substances needed by terrestrial living creatures have not been here since the beginning of the universe. Rather, they are the products of a long series of nuclear reactions that occur in the centers of (mostly massive) stars and that have timescales of millions to billions of years (Burbidge et al., 1957; Trimble, 1991). In some of the very oldest stars in our Milky Way Galaxy, only one atom in 100 000 is not hydrogen or helium (Edmunds & Terlevich, 1992; Spite, 1992). Enrichment is a continuous process. Even as you read this, massive stars like Betelgeuse and Antares are synthesizing new heavy elements out of H and He, and supernovae like 1987A are spewing out the products to be raw materials for future generations of stars and planets.