We know that the primary energy source in our sun is a fusion process in which four hydrogen nuclei coalesce to a helium nucleus, thereby releasing a great quantity of energy:



(Recall that the superscript is the atomic mass number, or total number of protons and neutrons in the nucleus.) This fusion process begins at temperatures of around 10 to 20 million degrees.

How did the heavier elements arise? Hydrogen fusion is a deadend process. Helium does not combine with hydrogen to produce lithium, nor does lithium combine with hydrogen to produce beryllium. If the heat from the hydrogen-fusion process is partially confined in the interior of a big star and the temperature increases to 100-200 million degrees, then a second process can begin. Three helium nuclei can fuse to produce a carbon nucleus and release more energy:



In the same temperature zone, carbon nuclei can combine with more helium to make oxygen: