----- The following copyright 1991 by Dirk Terrell
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 Lecture #9  "Burning the Ashes"

   Although I detect the usual eagerness to jump to the 'flashy' (no pun 
intended, well, maybe a little bit) subjects of supernovae, white dwarfs, 
black holes, etc., I thought it might be good to go into a little detail 
about how elements are formed first, since I detect a little 
misunderstanding about it. To do this, let's look at what happens to a star 
of 15 solar masses (that is, starts out with 15 times as much mass as the 
sun) once all of the hydrogen in the core is consumed. At this point the 
core of the star consists of helium "ash" left over from the fusion of 
hydrogen. Since there is no longer enough pressure support to balance the 
gravitational force (i.e. the weight of the overlying layers) the core 
collapses as we said last time. This collapse releases a lot of energy, 
enough, in fact, to cause the hydrogen in the layers above the helium core 
to become hot enough to begin fusing hydrogen in a thin shell around the 
core. We are very original and call this a hydrogen shell source. As the 
core collapses, the temperature rises and eventually becomes high enough 
that helium nuclei can start fusing together to form beryllium (Be). The Be 
nucleus is unstable and will break back down into two He unless another He 
nucleus combines with it to form carbon (C). The He nuclei contain two 
protons and two neutrons. The Be nucleus, therefore, contains four protons 
and four neutrons. The C atom has six protons and six neutrons. The net 
result of these reactions is that three helium nuclei combine to form a 
carbon nucleus (notice that we are forming NUCLEI, not atoms. The 
temperature and pressure in the core of a star are much to high for the 
binding of electrons to the nuclei.) This process of helium fusion is known 
as the triple alpha process because alpha particles that were discovered in 
the late nineteenth century in radiactive decays turned out to be helium 
nuclei. Now, these C nuclei can combine with He nuclei to form oxygen, with 
8 protons and eight neutrons. There is also a small amount of neon and 
magnesium produced.

   When the helium in the core is exhausted, what is left is carbon and 
oxygen. The core again contracts, this time causing a helium shell source to 
be created. Now we have a star with a carbon-oxygen core, a helium shell 
source just above the core, and a hydrogen shell source above that. Carbon 
fusion will occur when the temperature in the core reaches about 800,000,000 
Kelvins (same as centigrade at this point). Soon the carbon is used up, the 
core contracts, and a carbon shell source is set up. Our star will continue 
to go through the same cycle with neon fusion, oxygen fusion, and finally 
silicon fusion. Nope, that isn't a typo. Neon which is atomic weight 20, 
fuses before oxygen, which is atomic weight 16. How does that happen, you 
ask? At these high temperatures, a process known as photodisintegration 
becomes important. Basically what happens is that photons have energies high 
enough to break apart these nuclei. It turns out that a neon nucleus is held 
together a little less tightly than an oxygen nucleus. Thus photons can 
break the neon apart more easily than the oxygen. When the neon is broken 
apart it splits into an oxygen and a helium nucleus. The helium nucleus can 
then combine with another neon nucleus to form magnesium. Oxygen burning's 
products are silicon and sulfur. Photodisintegrations continue and helium 
and hydrogen nuclei combine with silicon to form heavier elements. When 
silicon burning is completed, the core of the star contains primarily 
iron-group nuclei like chromium, cobalt, nickel, manganese, and iron.

   At first you might expect the iron core to contract, and proceed with 
iron burning. Well, the core does contract, but iron fusion does not take 
place. Iron nuclei (atomic weight 56) are the most tightly bound nuclei 
there are. If the iron nuclei were to combine with, say, helium nuclei, 
energy would have to be absorbed, rather than released. We say that the 
reaction is endothermic (takes energy) rather than exothermic (releases 
energy). The star finds itself in quite a predicament. Up till now it has 
managed to stave off gravity by 'burning' nuclear fuels. Now, there is no 
more fuel. What happens now? Tune in next time!

 Dirk