Expert: Steve Nelson - 2/22/2010

Question
QUESTION: Hi Steve, I'm a high school student working on a video about the origin of the elements that cannot be created by fusion within stars. That is, elements heavier than iron that require more energy to create than the process releases. I know that these heavier elements like uranium are produced in supernova explosions, but I've been unable to find out HOW WE KNOW that this occurs. Do we look at the line spectra of the explosion and use them to determine the presence of uranium? Are there other ways we know this occurs? Any help you can offer would be much appreciated.

ANSWER: These processes may lower the *average* binding energy per nucleon, but the *total* binding energy still goes up.  Look up the r-process and s-process.  
http://hyperphysics.phy-astr.gsu.edu/HBASE/astro/nucsyn.html
Wikipedia also has some articles on these processes.

---------- FOLLOW-UP ----------

QUESTION: Forgive me, I'm trying to explain this in terms that a 14-year-old could understand with very little knowledge of the subject myself. So is it that essentially what's happening is that protons and electrons combine to form neutrons in the original atoms, whose nuclei then absorb other neutrons from similar occurrences elsewhere in the star, and then the neutrinos released from this process cause the excess neutrons in these isotopes to transmutate back into protons and electrons, resulting in a heavier element?

Answer
The heavier elements than iron have a maximum binding energy per nucleon (a nucleon is a general term for protons and neutrons, the stuff in the nucleus).  But if you keep adding neutrons from neutron capture you still get more total binding energy in the total nucleus itself because you have to multiply the binding energy per nucleon by the total number of nucleons to get the total binding energy of the nucleus.  Imagine the nucleus is a big bag of magnetic marbles.  Adding more than the optimum number of magnetic marbles causes too much chaos and there's not as much sticking power per marble but the marbles will still stick.  After a while the whole thing will be built of so many marbles with magnetic fields pointing in different directions that it'll become unstable (like uranium).