Fusion Beyond Carbon

Astronomers think of 5 to 10 solar masses as the dividing line between low- and high-mass stars. That is, stars about this much more massive than the sun die in a way very different from stars of lesser mass. The major difference in their evolution is that high-mass stars are able not only to fuse hydrogen, helium, carbon, and oxygen, but heavier elements as well. As core burning of one element ends, it begins burning in a shell above the core.

Hydrogen fusion produces helium ash that settles to the star's core. Then helium fusion produces carbon ash, which again settles to the star's core. As the fusion of each heavier element proceeds, the core layers progressively contract, producing higher and higher temperatures. Unlike low-mass stars, in a high mass star core temperatures are high enough to fuse carbon into oxygen, oxygen into neon, neon into magnesium, and magnesium into silicon. The end of the road is iron. When a massive star has iron building up in its core, the final curtain is near. The reason is that for every element up until iron, energy was released when nuclei were fused. But to fuse iron into heavier elements absorbs energy. In terms of fusion, iron is a dead end.

The evolution of the high-mass star is rapid. Its hydrogen burns for 1 to 10 million years, its helium for less than 1 million years, its core of carbon for a mere 1,000 years, oxygen for no more than a year, and fusion of silicon consumes only a week. An iron core grows as a result, but for less than a day. Just before its spectacular death, a massive star consists of nested shells of heavier elements within lighter elements, all the way down to its iron core.

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