NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/physics/21541

Internet Message Format | 1992-12-22 | 4.4 KB

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From: [e-mail] (Jim Carr)
Newsgroups: sci.physics
Subject: Re: When your sun forges iron...
Message-ID: <[e-mail]>
Date: 21 Dec 92 18:44:41 GMT
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Reply-To: [e-mail] (Jim Carr)
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In article <[e-mail]> [e-mail] (Lamont Granquist) writes:
>[e-mail] (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
[e-mail]                           |  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)