Elemental facts

Natural History, Nov, 2002 by Frank W. Kenney

Contrary to what Neil deGrasse Tyson wrote in "Cosmos on the Table" (7/02-8/02), iron atoms have the most binding energy per nuclear particle, not the least. Thus iron is the most stable of all the elements. Rather than being the prime cause of a supernova, therefore, is iron not a participant, along with other elements, in the explosion?

Tyson's article raises several other questions: Why is carbon so rare on Earth, compared with the rest of the cosmos, even though Earth is the only place where life (which is, after all, based on carbon) is known? More precisely, if, as Tyson states, carbon is ten times more abundant in the cosmos than silicon is, why is the reverse true on Earth? Finally, how does a dying star churn out carbon, and what else does it produce?

Frank W Kenney
Hartsdale, New York

NEIL DEGRASSE TYSON REPLIES: Indeed, due to a slip of the pen, I should have written that iron has the most, not the least, binding energy per nuclear particle of all the elements. But that changes nothing in the subsequent discussion. The term "binding energy" is somewhat misleading. High binding energy, in fact, refers to a state of high negative energy. When we say that iron has the highest binding energy of all the elements, we mean that iron has the least total energy per nucleon, leaving no energy to give up in a nuclear reaction such as fission or fusion. High-mass stars do indeed fuse iron into more massive elements, but they do so at the expense of their livelihood. Yes, it's a bad day for a star when iron is all that's left in its core.

Because of the circumstances under which our planet formed, carbon is much less abundant on Earth than silicon is, even though carbon wins by a factor of ten for the Sun, the outer planets, and across the universe. But there's still plenty of it on our planet to go around. The latest estimate from all sources and sinks of carbon, on and near Earth's surface, is about [10.sup.17] metric tons of the stuff. The biggest advantage that carbon holds over silicon, however, remains its capacity to form large, complex, strongly bound molecules under a wide variety of conditions--just what life needs.

Dying stars, including the ones that do not explode, are the principal polluters (or enrichers, if you prefer) of the cosmos. Carbon is made within stars from the fusion of three helium nuclei. Depending on a star's mass and composition, powerful convective forces may dredge up freshly made carbon and other intermediate-mass elements from the star's core to its surface, where they are gently released into space during the dying days of the star's red-giant phase.