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My understanding is that elements heavier than iron and nickel are not formed in a star but, can heavy elements such as lead and others be present/found in a star's core ?

I ask because the following document

http://arxiv.org/abs/astro-ph/0410628

gives the impression that lead and other heavy elements may be found in a star's core and I thought that was not possible.

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    $\begingroup$ Throw a lump of lead into a star. Now it is there. Replace you throwing by nearby supernovae for added realism. $\endgroup$
    – PlasmaHH
    Commented Jun 18, 2016 at 21:41
  • $\begingroup$ I only skimmed the article and the star's wiki entry, and I couldn't find a mass or even spectral type. It's possible that the star is a K dwarf, in which case it can be as old as the universe but not so cool as to be fully convective so the abundances we see from the surface aren't necessarily the same as in the core. If the star really is M or smaller, though, this comment doesn't apply. $\endgroup$
    – user10851
    Commented Jun 18, 2016 at 21:44
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    $\begingroup$ @ChrisWhite it is a Red Giant $\endgroup$
    – Federico
    Commented Jun 20, 2016 at 7:06
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    $\begingroup$ @Federico It is a "first ascent" red giant. That is significant to the paper because it means that the heavy elements should not have been produced by the s-process inside this star. The fact that lead (an s-process element) is seen, can then be interpreted as mainly due to the decay of U and Th. $\endgroup$
    – ProfRob
    Commented Jun 20, 2016 at 16:43

5 Answers 5

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It is a myth that heavier elements than iron are not produced in stars, slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in large stars. For details of what elements are produced and about the process itself, see s-process.

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    $\begingroup$ This requires that lots of neutrons and lots of heavy nuclei (e.g., iron 56) are already present in the star. Iron 56 is the key isotope that enables the production of heavy elements in a star via the S-process. Neither the CNO cycle nor the S-process were available to the first generation of stars. Nonetheless, +1. $\endgroup$
    – David Hammen
    Commented Jun 19, 2016 at 1:12
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    $\begingroup$ @ProfRob The question only asks if heavier elements can be present, not if they were produced there. $\endgroup$
    – user151841
    Commented Jun 19, 2016 at 16:55
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    $\begingroup$ @ProfRob that was my interpretation too $\endgroup$
    – user65081
    Commented Jun 19, 2016 at 16:57
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    $\begingroup$ Then what is your answer? You just talk about the s-process, which actually is irrelevant for the vast majority of stars since it is not occuring in their cores or anywhere else. A non-knowledgeable reader would assume you are suggesting that the heavy elements are made inside the star since you don't offer any other explanation. Or are you saying that they can be present in some stars because of the s-process but not others (which would just be incorrect). $\endgroup$
    – ProfRob
    Commented Jun 19, 2016 at 17:08
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    $\begingroup$ You have not read the paper you are citing. If you had you would realise that the s-process only takes place during the short-lived AGB phase when there is a strong neutron source present. It is the s-process in previous generations of stars that explains any s-process elements that are seen in a dwarf star (or a first ascent red giant), they are not produced in the star itself - which is what you your answer currently implies. $\endgroup$
    – ProfRob
    Commented Jun 19, 2016 at 17:40
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The heavier-than-iron elements are not formed during stellar fusion, but they are formed during supernovae. Then the oldest stars cannot have these heavier elements, but new generations, formed from 'recycled' material of other stars that went supernova can.

See Stellar populations .

There are heavier that iron elements on Earth, the Earth was formed from the same stuff clumping together that the sun was, so some fraction of such elements should also be present in the sun.

EDIT: Sorry, I forgot about the s-process that Wolphram jonny mentioned, thanks for that. The process in supernovae that I mentioned is the r-process.

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The star that is studied in the paper that you refer to is a very old, very metal-poor "Uranium giant". This is an evolved star with a very deep convective envelope.

The Uranium and Thorium that are seen in the atmosphere of the star were not produced in the star. They would have been produced, via the r-process neutron capture mechanism, in the supernova explosion of an earlier, massive star. These elements are notable because elements heavier than lead must have been produced in supernovae.

The U and Th in the supernova ejecta (along with full range of other heavy elements in various proportions) were mixed into the material that formed the star we are talking about here. Those elements would have been present in the core and throughout the whole of the star.

The paper you reference attempts to find the lead content of the star. Lead can be produced by the s-process inside evolved giant stars, by slow neutron capture onto existing iron-peak nuclei. However, in a very metal-poor star, this is likely to be ineffective, and/or has not had time to occur in previous stellar generations and the paper argues that the small amount of lead that is found is consistent with the radioactive decay of (some of) the r-process Uranium and Thorium already in the star when it formed. The lead would also be found all the way through to the core of the star.

As an aside, the effectiveness of the s-process in stars that already have some iron in them, means that about half the heavy element abundance of the solar system was created inside stars and not in supernova explosions. The Sun does contain most of the iron-peak and stable heavier elements in its core and envelope, but these were not produced in the Sun; they were in the material it was born from.

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I'm guessing (and I guess breaking the rules, but I'm interested whether my intuition is at all accurate): I've always assumed that although stellar fusion stops at iron, there is plentiful energy from the prior fusion processes available to create the elements heavier than iron, albeit at a net loss of energy. In decreasing and exponentially smaller amounts the further up the periodic table you climb, obviously, but in a star "exponentially smaller" is seriously relative. Not being a physicist, I have no idea if these further products would be broken down...i.e. would they survive? That they are created within a star (whether during a nova event or otherwise) is not, as far as I am aware, debatable. A tiny loss of energy in a tiny part of a star...it's a little like life -- a suspension of entropy (I really hesitate to say reversal) -- because we have an open system with a humungous fusion generator allowing "unlikely" local phenomena to persist.

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  • $\begingroup$ perhaps you have to move that to questions, after refining. Exponent may be small enough even for star even for universe, exponent is very fast. $\endgroup$
    – MolbOrg
    Commented Jun 19, 2016 at 12:50
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    $\begingroup$ You are correct that the creation of elements heavier than iron is an endothermic, non-equilibrium process. $\endgroup$
    – ProfRob
    Commented Jun 19, 2016 at 15:56
  • $\begingroup$ Actually it does answer the question - or rather, I mean, it meets the standard not to be deletable as "not an answer". That standard says nothing about correctness, completeness, or confidence in the post. $\endgroup$
    – David Z
    Commented Jun 20, 2016 at 20:26
  • $\begingroup$ Wouldn't the actual answer to the initial question be that only elements up to lead (iron to lead in s processes) are produced in stellar interiors, but any element in the "primordial cloud" from which the star formed could be present in the star? (In general/ not in regards to star in article) $\endgroup$
    – Jack R. Woods
    Commented Jun 26, 2016 at 17:25
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Yes. Not only supernovae produce elements heavier than iron, many even heavier elements are produced in dying low mas stars. Here is the table of nucleosynthesis from Wikipedia Commons:

enter image description here

Looks like dying low mass stars even produce lead and other very heavy elements.

Synthesis of elements as heavy as Amercium once has been suspected during explosion of the supernova of the Crab nebula, but later these results were not confirmed.

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