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The Question

(Submitted February 26, 1998)

I am an educated layman with a question. I have often read that supernovae are the prime source of elements heavier than helium in the ISM, but I wonder about formation of planetary nebulae and novae. They may be less efficient at adding to the metal inventory of the ISM, but they are more numerous. How then do astrophysicists identify supernovae as the better source overall? Are statements like "Most of the atoms of carbon, oxygen and nitrogen in our bodies came from supernovae." justified?

The Answer

What a good question! It is absolutely true that virtually all of the elements heavier than iron are from supernovae, because stars do not make these, since the element with the highest binding energy per nucleon is iron.

As far as other elements (Fe and lighter), they are mostly made in stars. The heavier the star, the more different elements it will make, and the shorter its life. Our Sun, on the other hand, will not make much past helium. In addition, it is only the more massive stars that go supernova. So, if a star makes iron, it will likely go supernova.

In short, elements below Fe are mostly made in stars, while heavier elements are made predominately in supernovae. Now, your question is about the distribution mechanism. Supernovae are by far the best mechanism for ejecting these elements very far away from the star.

In addition, most all of the other mechanisms like stellar winds involve the outer layers of a star. Because they are made toward the center of the star, one would expect heavy elements to stay toward the middle of stars where they are produced, while the outer layers blow off lots of hydrogen.

That said, it is also thought that much of the CNO elements come from asymptotic giant stars burning helium, convectively dredging it to the surface, and then blowing it off in a wind.

So, all in all, it is safe to say that supernovae are the primary distribution mechanism for heavy elements in our galaxy, but they are not the only one.

Thanks for the good question.

Jonathan Keohane, Mark Kowitt and David Palmer
for Ask a NASA scientist

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