Where is the temperature high enough to allow nuclear reactions? One important answer is: inside stars.
We showed how two light elements (H) could combine to form a heavier element (He). There are many such reactions, which can produce heavier and heavier elements. (“Heavy” meaning having a higher number on the periodic table.)
Above is the triple-alpha process, which is how carbon is produced from 3 helium atoms. All carbon in the universe was likely made this way. Inside stars.
This is called stellar nucleosynthesis. (Stellar = star, nucleo = nucleus, synthesis = making.) Wikipedia has an excellent list of reactions.
