6 CONCLUSION

Over the last few decades, nuclear astrophysics has grown into a mature field of science, and brought us a greatly improved understanding of the origin of the elements. We are now certain that only a few of the nuelides around us were made within a short period just after the birth of our universe, and that the majority of them were made in the generations of stars that followed. In detail, one can identify the following sites for the production of the nuelides:

1. The lightest elements (mainly hydrogen and helium isotopes) were made in the big bang. Most of this primordial hydrogen has survived, while the amount of helium has been increased a small amount by cycles of stellar nucleosynthesis. Most of the ⁷Li in the oldest stars is also believed to be primordial.

2. Elements like carbon and nitrogen were produced mainly in intermediate mass stars ( M ? 8 12 M ?).

3. The medium-heavy elements ( A ? 16 60) were mainly made in a series of quasistatic burning phases in massive stars, with important contributions from the explosive burning that occurs in supernovae, especially for the elements near the iron abundance peak.

4. The elements above the iron peak were produced in both the r- (rapid) and s- (slow) neutron-capture processes.

A discussion of intermediate mass stars as the site for the production of carbon and nitrogen, and of spallation as one of the probable mechanisms to produce ⁶Li, ⁹