Nuclear Friday: Stellar Chemistry

No nuclear weapons today since everything I think about nuclear policy may soon be wrong. Instead, we’ll take a look at star stuff. When Carl Sagan said “We are made of Star Stuff” he meant that the common chemical elements of life, carbon, nitrogen, oxygen, and all heavier trace elements, were forged in the cores of long dead stars. Life was made possible in the cosmos through the death of earlier generations of stars. Astronomers had no idea that such processes were even necessary until Cecilia Payne showed the cosmos was made up almost entirely out of hydrogen and helium in her 1295 doctoral project.

Subsequent research showed that stars were powered by various nuclear fusion processes. Our sun is fairly simple in that it’s powered right now by just two processes for the most part, proton burning and deuterium fusion. The Sun will run out of deuterium in a few billion years and contract to such densities that it will begin to fuze helium into mostly carbon and a few other elements. Carbon is as far our sun will go, but other stars have enough mass to contract to higher densities and temperatures where more exotic forms of fusion can take them up to iron. That’s the end of the line as no fusion reaction involving iron or heavier elements generates energy. The star then suffers a collapse followed by a huge supernova explosion that disperses its elements to space to become part of the next generation of star systems. The core of the supernova will become a neutron star or even a black hole.

That’s basic scientific literacy about the synthesis of the elements in one paragraph. But how does this happen? And when you think about it, a supernova’s outer layers hardly seem like a good place fur synthesizing heavy elements by fusion. There’s plenty of heat and pressure, but supernovas are over in hours. That’s not much time to get fusion done. As it turns out most of the heavy elements released by supernova explosions aren’t made  by fusion. Additionally, research has shown that many elements can’t be made by supernova explosions. Where do they come from?

I’ll let Mike Merrifield from the University of Nottingham explain how it works:

This video may presume that the viewer is familiar with beta decay. This process happens when a nucleus of an atom has too many neutrons. A neutron will transform itself into a proton while giving off a high energy electron. Net electrical charge stays the same during the reaction as a neutral particle is transformed into a positive and negative particle. Before physicists understood the properties of of the electron really were and when this type emission was poorly understood, it was just called beta rays. It is possible to safely witness large amounts of beta decay and its beautiful effects. Since beta decay can make elements with new protons, the two types of neutron capture described in this video are capable of making new elements.

Something like the “r-process” happens to a limited extent in nuclear detonations and is part of why ground bursts make so much fallout while an air burst of an efficient weapon makes almost none. There were proposals to weaponize a few specific types of neutron capture to make a “salted bomb” such as a cobalt bomb. The “s-process” never happens in nuclear explosions.

To paraphrase Shakespeare: My mistress’ bombs are nothing like the sun.



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