NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/physics/21631

Internet Message Format | 1992-12-22 | 7.8 KB

Path: sparky!uunet!gatech!destroyer!news.itd.umich.edu!pablo.physics.lsa.umich.edu!metzler
From: [e-mail] (Chris Metzler)
Newsgroups: sci.physics
Subject: Re: When your sun forges iron...
Date: 23 Dec 1992 00:14:28 GMT
Organization: University of Michigan Department of Physics
Lines: 141
Distribution: world
Message-ID: <[e-mail]>
References: <[e-mail]> <[e-mail]> <[e-mail]> <[e-mail]>
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Over the course of several articles, [e-mail] (Jim Carr) writes:
|> In article <[e-mail]> [e-mail] (Lamont Granquist) writes:
|> >
|> >In fact what happens is that you get a buildup of Fe-56 in the core.  After
|> >the pressure gets great enough the Fe-56 begins to start to fission.
|> >
|> >This is bad.
|>
|> No, it is wrong.
|>
|> The iron does not fission.  It just sits there.  Since no more fusion is
|> taking place, no energy is released, and there is no longer any radiation
|> pressure to oppose the gravitational attraction and the star collapses on
|> itself until the nucleons reach a density where they feel the repulsive
|> short-range core in the nucleon-nucleon interaction.  They "bounce" off
|> of this repulsive potential, leading to the explosion of the star.  Along
|> the way, electron capture occurs as well, producing the excess neutrons
|> needed for r-process nucleosynthesis and the characteristic neutrino
|> pulse detected from the most recent nearby supernova.
|>
|> Fe-56 cannot fission any more than it can fuse.  It is at the bottom
|> of the binding energy curve.
|>

This is partially wrong.

It is, of course, true that Fe-56 sits at the top of the iron peak.  But so
what?  All that means is that fission or fusion processes involving iron must
be endoergic.

In Supernovae of Type II, when the core burns to iron, it is no longer possible
to support the mass above by nuclear processes, and the core collapses.
This contraction results in very high temperatures and a lot of energetic
photons.  For a Type II supernova, the iron core is above the Chandrasekhar
limit, meaning that electron degeneracy pressure is not sufficient to
support the mass of the star.  The matter can thus infall until the core is
so hot and the photons so energetic that iron nuclei can photodisintegrate into
He nuclei and neutrons.  This is not, strictly speaking, what we normally
think of when someone says "fission," but the iron nonetheless does break
down into smaller elements.

|> 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.
|>

Single quantum = photon.  See above.

|> >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

Ah!  We now arrive, I think, at the source of all the confusion.  What
Lamont Granquist was referring to (I think) and what I am referring to
is the classical scenario for a Type II supernova -- the death of a
massive star.  This is the "How Supernovae Work" model that is taught
in introductory astronomy classes.  Type I supernovae, which involve
stars of lower mass, are not discussed -- primarily because they're more
complicated, but also because we're not as sure about how they work
as we are about Type II's.  I think Lamont has been talking about
apples and you about oranges -- both fruit, but not the same thing.
The current paradigm of Type I's is that they occur when an existing
white dwarf accretes matter from a binary companion and thus goes over
the Chandrasekhar limit.

|> 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.

You asked about references.  All the below spend some time on supernovae
of Type II and the role of photodisintegration of iron, and eventually,
the alpha particles themselves -- followed by electron-capture and
neutron star formation.

Shu, F.  "The Physical Universe."  University Science Books, Mill Valley, 1982,
    pp. 154-155.  Probably the most widely-used intro astrophysics textbook.
Bowers, R.L., and Deeming, T.  "Astrophysics I:  Stars."  Jones and Bartlett,
    Boston, 1984, pp. 276-282.
Harwit, M.  "Astrophysical Concepts."  Springer-Verlag, New York, 1988, p. 241.
Mihalas, D., and Binney, J.  "Galactic Astronomy."  W.H. Freeman & Co., 1981,
    pp. 142-143.  Probably the most widely-used graduate astronomy text.


        -- Chris

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