From: jac@ds8.scri.fsu.edu (Jim Carr)
Newsgroups: sci.physics
Subject: Re: When your sun forges iron...
Date: 21 Dec 92 18:44:41 GMT
Reply-To: jac@ds8.scri.fsu.edu (Jim Carr)
Organization: SCRI, Florida State University

In article <1992Dec18.231931.24746@u.washington.edu> lamontg@stein.u.washington.edu (Lamont Granquist) writes:
>jac@ds8.scri.fsu.edu (Jim Carr) writes:
>>No, it is wrong.
>
>[...]
>
>>Fe-56 cannot fission any more than it can fuse.  It is at the bottom 
>>of the binding energy curve.   
>
>Oh sure it can fission, it just takes positive energy to do it.

There is more than just energy involved.  The energy must be delivered 
in a single quantum (or a few on a 10^{-20} second time scale) to 
cause such a reaction.  For example, a 100 MeV proton would cause 
a variety of reactions, some of which would leave you with various 
nuclear fragments.  The temperatures in a stellar core will not give
you many events like this.  The thermal energies are just enough to 
get you over the Si+Si coulomb barrier and fuse to Fe, but this is 
not nearly enough to excite Fe to the point where it would come apart. 

The point is that we are talking about thermal fission (and thermal 
fusion) if we are in a stellar environment.  The reactions that are 
most probable at these energies are not fission or spallation to 
smaller fragments, but a variety of neutron and electron capture 
processes that procede until the nova occurs. 

>My ASTR prof explicitly said that the Fe-56 in the core begins to break
>apart as the pressure increases and not only does not produce any
>energy, but begins to absorb it.  If you think he's wrong, I'd like more
>info than just a statement of "thats wrong".

Well, there was quite a bit of info in the text following what you quoted
above, but perhaps I should elaborate.  If you would like a more detailed 
description, I suggest you read Bethe's article in Ann. Rev. Nucl. Part. 
Sci. 38, 1 (1988).  His first chapter outlines the sequence of events 
and later chapters cover the details in review fashion.  

If you have reference to the role of Fe fission, please let me know 
what it is, since this is the first time I have ever heard it mentioned
in this context. 

The key point is that nuclear fusion proceeds until you have burned Silicon 
to Iron.  At that time, it is not possible to generate any more energy at 
the given temperature and pressure.  If the star is massive enough, the 
pressure of the degenerate electron gas will not support it, and the 
core collapses under its own gravity.  This happens fast, in about a second. 
During this collapse, the density of the star core exceeds the density of 
normal nuclear matter.  Thus it no longer even makes sense to speak of 
nuclei at all -- the nuclei are pressed together into a giant mess with 
large clusters (A ~ 1000) nucleons and, eventually, into a blob of 
nuclear matter.  The physics then becomes the physics of the thermodynamics 
and equation of state of dense nuclear matter, which is dictated by the 
properties of the short-range part of the nucleon-nucleon interaction. 

What is fundamentally wrong with your prof's emphasis on fission of Fe-56 
is that it makes it hard to see how you get a neutron star (essentially 
a single giant nucleus) in the low-mass case or how you produce all the 
heavy nuclei in a supernova.  Electron capture and neutron capture rates 
in thermally excited nuclei are the main emphasis of research on the 
dynamics of supernovae and the related questions of nucleosynthesis. 
The other main error is that fission of Fe is basically a 'high' energy 
process and cannot be important (by phase space limitations) relative 
to the low energy process that I mentioned above. 

--
J. A. Carr                                    |  "The New Frontier of which I  
jac@gw.scri.fsu.edu                           |  speak is not a set of promises
Florida State University  B-186               |  -- it is a set of challenges."
Supercomputer Computations Research Institute |   John F. Kennedy (15 July 60)