Synthesizing the Chemical Elements

• In the Beginning there was H and He. The Early Universe, just after the Big Bang came up against the little Berylium problem - He + He Be requires energy and it not the sort of reaction that nature likes.

  In stars (HB stars) the usual path is

  

  then, before the Be can decay back into two He atoms,

  

  In the early Universe, the density and temperature are dropping rapidly and there is only an instant when everything is right for the triple-alpha reaction. This is too short to produce any Carbon or heavier elements.

• The Universe started with mostly H and a little bit of He.
  

  Yes! Although we have not identified any stars made of ONLY H and He, the oldest stars in the Galaxy are all very deficient in the elements heavier than He with the current record holder down by a factor of around 30,000 (!).

• The Sun and Solar System abundances are the result of MANY cycles of element production and dispersal in stars. We are literally made of star dust.

• We already have a way to mix in ``new'' Helium, Carbon and Oxygen. Main-sequence, RGB, HB and AGB stars produce these elements, deep convection mixes some into the envelopes of the AGB stars and then Planetary Nebulae carry these elements into the interstellar medium to be mixed into the next generations of stars.

  

• To make the heavier elements up to Iron requires nucleosynthesis in massive stars and delivery via stellar winds or, more spectacularly, Supernova explosions.

  SN are like a production and delivery system for the elements.

• What about those elements more massive than Fe? Supposedly equilibrium reactions don't work to produce elements on the other side of the binding energy curve beyond Fe. It turns out that in the excitement of SN explosions there are many non-equilibrium reactions that build up very massive elements. In some cases these elements are stable, in many cases they are not and the process of radioactive decay of heavy elements is just Nature's way of getting back into equilibrium.

• The two principal paths to building ``trans-Fe'' elements are the S-process and the R-process.
  1. S-process is the Slow addition of neutrons to nuclei with the neutron subsequently undergoing a -decay (ejection of an ) to change into a . This way atoms can slowly slowly walk their way up the Periodic table. It is much easier to add the chargeless neutron to a nucleus than it is a .

     

     

     This works up to around Bismuth at atomic #83 and is though to occur in SNI and also in AGB stars during the thermal pulse stage.

     There is some direct evidence for the S-process occuring in some AGB stars. Technetium with atomic #43 is an S-process element that has a radioactive half-life of years. It has been detected in AGB stars that are MUCH older than that! The only thing that could be going on is the production of Tc in the star and then mixing of this to the surface via convection.

  2. R-process is the Rapid addition of neutrons to existing nuclei. The idea is that you add a bunch of neutrons which then start to decay into protons via -decay in the nucleus. This increases the atomic number and is the way to produce the really heavy stuff.

     The R-process occurs only (we think) in SN and mostly in SNII. The evidence for R-process occuring is less direct. First, we see elements like Gold which are thought to only be produced via the R-process. It is also true that if we look at the oldest stars in the Galaxy, which were formed after only one or two SNII (these come from massive stars that have very short lives) had enriched the interstellar medium, the abundance of Iron is very low, but the abundaces of R-process elements are only moderately low.

     If we look at the Crab nebula which is the expanding remnant of the 1054 A.D. explosion we see processed material from deep in the star that went SN, but mostly this is the result of the equilibrium fusion in the ``onion skin''.