does not give off energy, it absorbs it. Fusion as an energy source, whether for us or for the stars, will work only with elements lighter than iron. Synthesis of elements beyond probably was a slower process of reactions involving neutron capture.

Now we have a set of primary reactions that will produce the elements at least through iron. But how did these reactions occur during the evolution of the universe? These elements are synthesized in the interiors of stars. The first generation of stars began as hydrogen, and generated helium and the heavier elements by their fusion processes. Our own sun is a second-generation star; it was formed from the debris of the breakup of earlier stars. Hence it and the planets around it are enriched in the heavier elements.

The probable life history of a star, considered as an element-factory, is told over the course of pages 23, 24, 25 and 26. Depending on the size of the star, it can go successively through several of the element-generating reactions described previously. For each reaction, burning begins at the center of the star, and spreads slowly to the surface as fuel in the core is depleted. If the star is massive enough, the heat generated by one reaction may be enough to trigger the next. The minimum size for helium burning appears to be 0.7 times the mass of our sun, and for carbon and oxygen burning, 5 solar masses.