A star lives in the grip of two opposing forces:

• Gravity attracts all the matter in the star toward its center.
• The energy from fusion heats the star and produces the pressure needed to counteract gravity.

For most of its life a star produces energy by fusing hydrogen to make helium. When the hydrogen becomes depleted, the star's core heats up and begins to fuse the accumulated helium to make carbon. This process, as shown in the table, produces heavy elements up to iron. However, unlike the previous reactions, the fusion of iron ABSORBS energy and leads to the catastrophic collapse of the star's interior. In the resulting explosion, fusion reactions produce all elements heavier than iron in the universe. In the beginning of the universe, the big bang explosion created matter that was about 75% hydrogen, 25% helium, and a small amount of lithium. All heavier elements in the solar system were created in an earlier generation of stars and blown off into space by supernova explosions.

The supernova described here, produced in the death of a large star, is called a Type II supernova. Another kind, Type I, can occur in a binary system containing a small collapsed star.

X-ray image of several thousand year old supernova remnant. Courtesy of NASA