[chris (OP)]

Chris has been in touch from Australia to ask:

I recently attended a public lecture on very old stars. One characteristic of such stars, apparently, is that they have very low Fe contents. I thought that very old stars should be, conversely, very rich in Fe.

My logic is that as a star get older, and its H fuel runs out, it starts fusing He, then heavier elements, and so on down to Fe, which is apparently is the heaviest element that can be made by fusion in stars.

So shouldn't very old stars be very RICH in Fe, having had time to convert all, or most, of its fuel to Fe and unable to sustain fusion reactions past Fe? Or am I missing something.

Love the show and never miss it.  I listen to the NS as a podcast while mountain biking when I do my best thinking!

[evan_au]

very old stars.... have very low Fe contents. I thought that very old stars should be, conversely, very rich in Fe.

In the question, "Old" refers to two very different populations of stars, which is why there are two different answers:
a) A star which is old relative to the age of the universe (eg 10+ billion years)
b) A star which is old in its individual lifecycle (which may only be 100 million years if it is a massive star)

To explain further:
a) The early universe was composed almost entirely of Hydrogen and Helium. Stars that formed from this mixture would have been unlike any stars we see today and would have gone supernova quite quickly; we don't expect any of these "Population III" stars still exist.
However, the next generation of stars (so-called "Population II" stars) had some heavy elements, but they are notable for their very high concentrations of Hydrogen & Helium, and their very low concentration of anything else (including Iron). Any that still exist must be very low in mass, or they would have exploded long ago. These stars are too small to produce iron in their core. These stars will continue glowing long after the Sun has burnt all its fuel..
In contrast, "younger" Population 1 stars like the Sun (only 5 billion years old) were formed from the debris of previous massive stars that burned their fuel into iron, and then exploded, spewing their iron-rich interiors into dust clouds. So the Sun has far more iron than the older Population II stars.
So these small Population II stars could be "old" because they are 10 billion years old.
See: https://en.wikipedia.org/wiki/Stellar_population

b) A massive star, (much bigger than the Sun) is needed to achieve the temperatures and pressures needed to burn their fuel to iron. These stars burn through their fuel very quickly - perhaps only 10 million years for the largest of them.

As these stars age, they do start to produce their own iron in the core, and it does increase over time. Sometimes there will be multiple concentric shells of different fuels burning inside a star. Once the fuel has been used, the star is unable to hold itself against its huge gravity, and it explodes in a supernova, to a black hole or a neutron star.
These large Population I stars could be "old" because they are 0.1 Billion years old. It's all relative!
See: https://en.wikipedia.org/wiki/Stellar_nucleosynthesis

PS: If the numbering scheme seems backwards, it's because astronomers thought the Sun was "normal", until they found some "abnormal" stars (which they named Population II). Cosmologists needed something before Population II, so they became Population III. They are named in order of discovery/deduction.

[puppypower]

There is a consideration, not used in existing theory, that suggests that first generation stars can form most of the elements. This consideration has to do with the atomic density of ionized elements. If you were to take an iron atom and ionize its first eight electrons, the ionized iron atoms will have a lower atomic density, than the ionized oxygen atoms with eight ionized electrons. 

Density is mass/volume. Although iron is more massive than oxygen, oxygen becomes denser at these ionized conditions. The reason is the iron remains fluffier due to the 18 remaining electrons in its lower orbitals. The ionized oxygen becomes denser because density; mass/volume, is based on oxygen being an atomic nucleus, with no electron cloud to give it extra volume like iron.

The affect is similar to a large steel ship floating on water. Although iron and steel is denser than water, the iron ship can float on water and not sink, because there is extra volume, built into the hull. The inner electrons, of the ionized iron  adds hull volume to iron atoms relative to the ocean of oxygen nuclei.

In terms of our sun, say we assume our modern sun is a second generation star. The iron it gathered, will become ionized and float above the smaller denser atomic nuclei, like a steel ship on water. The inner electrons of the iron is the hull of the steel ship. Under these conditions the iron will form a shell, floating above the fusion core.This shell is used to   control run-away fusion.

If the core fusion gets too hot, more electrons are ionized from the iron shell. This will cause the shell to get denser; less hull, and sink, somewhat, sealing off the core from excessive fuel diffusion. As the fusion core cools, due to less fuel, the iron shell gains electrons, gets fluffier, floats higher, allowing more fuel to enter; solar flare.

Picture the scenario where the core cools, due to less fuel diffusion, such that the iron shells starts to expand as it gains electrons; sun spot. The new higher diffusion rate, will induce a fusion surge; backdraft, that will create an affect I like to call fusion hammer. The fusion surge, will pounds the iron shell with radiation, heat and atomic nuclei creating higher atoms.

In terms of first generation stars, the same atomic density consideration applies with the shell composed of the highest atoms, with fusion hammer building up higher and higher atoms and changing the composition of the shell over time. As the shell gets thicker and thicker, eventually the fusion become diffusion limited. The cooling of the core, expands shell, and the resultant high level fuel surge can create fusion hammer conditions that can blow out the shell. I tend to believe that our solar system formed from our sun, during a massive fusion hammer; cleans the pipes. The sun reformed for a second life, and the planets formed from the shell debris and other entrained matter.

[jeffreyH]

III, II, I Just like a cosmic count down to heat death.