Observational Aspects Of Stellar Nucleosynthesis

Summary

Introduction

The origins of the chemical elements must rank highly in any intelligent citizen's list of questions about the natural world. Thanks to the efforts of observers and theoreticians over the last half-century, the citizen may now be provided with answers to ‘Where, when, and how were the elements made?’ This remarkable achievement of astrophysics provides one focus for this set of lectures. It is impossible to tell in the available space the complete story of nucleosynthesis from hydrogen to uranium (and beyond) with full justice to the observational and theoretical puzzles that had to be addressed.

Nucleosynthesis began with the Big Bang (see Steigman's contribution to this volume). According to the standard model of this event, nucleosynthesis completed in the first few minutes of the Universe's life resulted in gas composed of ¹H, and ⁴He with ¹H/⁴He ≃ 0.08 by number of atoms, and trace amounts of ²H, ³He, and ⁷Li. The inability of the rapidly cooling low density Big Bang to synthesise nuclides beyond mass number 7 is due to the fact that all nuclides of mass number 5 and 8 (i.e., potential products from ¹H + ⁴He and ⁴He + ⁴He) are highly unstable.

Ashes of the Big Bang cooled. The photons of the cosmic microwave background radiation were set free to roam the Universe. Then came what is known as ‘The Dark Ages’ before galaxies were formed.