http://ultraman.ssl.berkeley.edu/nucleosynthesis.html

....One of the fundamental problems of astrophysics is the problem of the origin of chemical elements. It has been known for quite some time that elements such as hydrogen, helium, and some lithium would have been formed just moments after the Big Bang....

As galaxies formed, stars began processing primordial matter into heavier elements within the nuclear furnaces of their cores. In this manner, elements up to Carbon, Nitrogen and Oxygen in the periodic table are formed. The injection of these materials to the interstellar medium can occur through stellar winds or in more dramatic fashion as a result of explosive events like novae and supernovae.