Formation of Elements

During most of their lives, stars fuse hydrogen into helium in their cores, but the fusion process rarely stops at this point; most of the helium in the universe was made during the initial big bang. When the star's core runs out of hydrogen, the star begins to die out. The processes that occur during this period form the heavier elements.

The dying star expands into a red giant star. A typical red giant at the Sun's location would extend to roughly the earth's orbit. The star now begins to manufacture carbon atoms by fusing three helium atoms. Occasionally a fourth helium atom combines to produce oxygen. Stars of about the Sun's mass stop with this helium burning stage and collapse into white dwarfs about the size of the earth, expelling their outer layers in the process. Only the more massive stars play a significant role in manufacturing heavy elements.

Massive stars become much hotter internally than stars like the Sun, and additional reactions occur after all the hydrogen in the core has been converted to helium. At this point, massive stars begin a series of nuclear burning, or reaction, stages: carbon burning, neon burning, oxygen burning, and silicon burning. In the carbon burning stage, carbon undergoes fusion reactions to produce oxygen, neon, sodium, and magnesium. During the neon burning stage, neon fuses into oxygen and magnesium. During the oxygen burning stage, oxygen forms silicon and other elements that lie between magnesium and sulfur in the periodic table. These elements, during the silicon burning stage, then produce elements near iron on the periodic table.

Massive stars produce iron and the lighter elements by the fusion reactions described above, as well as by the subsequent radioactive decay of unstable isotopes. Elements heavier than iron are more difficult to make, however. Unlike nuclear fusion of elements lighter than iron, in which energy is released, nuclear fusion of elements heavier than iron requires energy. Thus, the reactions in a star's core stop once the process reaches the formation of iron.