Production of the Lightest Elements
The lightest elements were produced mostly in the big bang that gave birth to the Universe. As we shall see
later,
big bang nucleosynthesis could not proceed beyond helium to heavier elements
because there are no stable
isotopes of mass 5 and 8. Except for trace amounts of elements like lithium, all other elements (and some of
the helium) were produced in stars. Thus, the stars are responsible for all the heavy elements in the
Universe.
As we have seen, the stars convert some of their hydrogen to helium either by the PP chain, which is dominant
in lighter stars, or by the CNO cycle, which is dominant in stars heavier than the Sun. This is as far as the
element production goes while the star is on the main sequence. Further element production must wait until
after the star leaves the main sequence.