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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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