RE: Article: "Sun is Mostly Iron, Not Hydrogen"

From: Reason (
Date: Thu Jan 10 2002 - 09:31:37 MST

> > Regarding a cooling/collapse problem resulting from an
> > iron core, or a core with a substantial iron fraction:
> > it is currently accepted that iron lies at the energy
> > minimum for nuclear binding energy based on nucleon
> > number. Thus, as Anders states, moving away from the
> > energy minimum by transmuting iron into either heavier
> > or lighter elements would be an energy absorbing
> > process. However, as an alternative, might not this
> > process tend toward a steady reversibilty, with an
> > equilibrium condition established where there was no
> > ongoing loss of energy to nucleosynthesis? Wouldn't
> > it likely be the case that the energy would not be
> > systematically lost, but only stored temporarily in
> > elements with a distribution of nucleon numbers
> > centered around iron, and a correlated distribution of
> > binding energies?
> The binding energy curve has a quadratic minimum around iron, so energy
> storage is unlikely. I haven't done the math (and I'm neither an
> astrophysicist or skilled in statistical mechanics) but the chemical
> equilibrium seems heavily iron-tilted. I think this is studied in most
> textbooks in stellar astrophysics.

The energy you get from fusing elements or fissioning elements is the
difference in the binding energy between the reactants and the products.
Binding energy is a measure of how stable an element is -- more binding
energy is required to bind less stable atomic nuclei. Iron has the most
stable atomic nucleus (ignoring neutron stars, that is, but I digress), and
therefore the least binding energy. Anything you do with iron, fission or
fusion, means you put have to put energy into the process.

Anything lighter than iron has methods of fusion that lead to a release of
energy; fission requires energy. Anything heaver than iron has methods of
fission that lead to a release of energy; fusion requires energy.

(And I'm ignoring isotopes, but never mind; the rule still holds for them).

> > Wouldn't the current empirical evidence of a stable
> > sun suggest just such a condition of stable
> > equilibrium, with equal numbers of nuclei transmuting
> > away from and then back to iron? I base this
> > supposition on the (thermodynamic?) principle that all
> > systems tend toward their energy minimums.
> But the sun isn't an equilibrium system, it is very much an open system
> - fuel is consumed and heat radiated away.

It's a fascinating process, the aging of stars. You start off fusing
hydrogen to helium in the core. Then you run out of hydrogen, and the fusing
moves to an expanding shell. The inner helium core grows and collapses
without the sustaining radiation pressure of fusion reactions until it gets
hot and dense enough to ignite. Which it does pretty much all at once. Bang!
Helium fusion generates much more energy, so the star expands to a new
equilibrium. Now you have a helium fusing core surrounded by a hydrogen
fusing shell.

Later on, you start to run out of helium, and build up a carbon core. So the
start have a shell of helium fusion and a shell of hydrogen fusion outside.
As the core gets larger, it collapses until it ignites with carbon fusion.

The process of getting from a helium core ignition to a carbon core ignition
is somewhat faster than getting a helium core ignition.

Cute reaction pics:

After this, you can probably guess how it goes, with cores of increasingly
heavy elements igniting. The process speeds up and the ignition event
becomes more energetic for each new core element, ending in iron and a

This of course is a horrible simplification, ignoring the effects of mass.
Some stars will never be massive enough to ignite cores of a given
material -- it takes increasing heat and pressure to get elements to fuse as
they get heavier.

This is a good life-cycle description with reference to what happens inside
a star at each point in time:

> I think everybody should read a bit of geophysics, astrophysics or
> similar sciences to get a whiff of just how marvellous our world is.

Couldn't agree more, but then I'm biased. I think that if most people
understood just how complex and awe-inspiring the natural universe is,
there'd be less of this magical thinking and attempts to generate imaginary


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