# Origin of elements heavier than Iron (Fe)

In all the discussions about how the heavy elements in the universe are forged in the guts of stars and especially during a stars death, I usually hear that once the star begins fusing lighter atoms to produce Iron (Fe) that's the end of the star's life and the whole system collapses onto itself and based on how massive the star was initially it has different outcomes like a white dwarf, a neutron star or a black hole.

I have rarely heard a detailed explanation of how the elements heavier than Iron are produced. I would appreciate a convincing explanation of this process.

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Elements heavier than iron are only produced during supernovae; in these extreme energetic conditions atoms are bombarded by a very large number of neutrons. Rapid successive neutron capture, followed by beta decay, produces the heavier atoms. See http://en.wikipedia.org/wiki/Supernova_nucleosynthesis.

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Your first sentence is totally incorrect. – Rob Jeffries Oct 13 '14 at 21:08
Elements heavier than iron are also produced in neutron star collisions. It's speculated that most of Earth's gold came from neutron star collisions – Jim Jul 14 '15 at 12:40

Elements heavier than iron are produced mainly by neutron-capture inside stars, although there are other more minor contributors (cosmic ray spallation, radioactive decay).

Neutron capture can occur rapidly (the r-process) and occurs mostly inside supernova explosions (though other mechanisms such as merging neutron stars have been mooted). The free neutrons are created by electron capture in the final moments of core collapse. At the same time this can lead to the build up of neutron-rich nuclei and the decay products of these lead to many of the chemical elements heavier than iron once they are ejected into the interstellar medium during the supernova explosion. The r-process is almost exclusively responsible for elements heavier than lead and contributes to the abundances of many elements between iron and lead.

To my surprise there is still ongoing debate about the site of the primary r-process. My judgement from a brief scan of recent literature is that whilst core-collapse supernovae proponents are in the majority, there is a case to be made that neutron star mergers may become more dominant, particularly for the r-process elements with $A>110$ (e.g. Berger et al. 2013; Tsujimoto & Shigeyama 2014).

However, many (50%) of the chemical elements heavier than iron are also produced by slow neutron capture; the so-called s-process. The free neutrons for these neutron-capture events come from alpha particle reactions with carbon 13 (inside asymptotic giant branch [AGB] stars with masses of 1-8 solar masses) or neon 22 in giant stars above 10 solar masses. After a neutron capture, a neutron in the new nucleus may then beta decay, thus creating a nucleus with a higher mass number and proton number. A chain of such events can produce a range of heavy nuclei, starting with iron-peak nuclei as seeds. Examples of elements produced mainly in this way include Sr, Y, Rb, Ba, Pb and many others. Proof that this mechanism is effective is seen in the massive overabundances of such elements that are seen in the photospheres of AGB stars. A clincher is the presence of Technetium in the photospheres of some AGB stars, which has a short half life and therefore must have been produced in situ.

According to Pignatari et al. (2010), models suggests that the s-process in high mass stars (that will become supernovae) dominates the s-process production of elements with $A<90$, but for everything else up to and including Lead the s-process elements are mainly produced in modest sized AGB stars that never become supernovae. The processed material is simply expelled into the interstellar medium by mass loss during thermal pulsations during the AGB phase.

As a further addition, just to drive home the point that not all heavy elements are produced by supernovae, here is a plot from the epic review by Wallerstein et al. (1997), which shows the fraction of the heavy elements in the solar system that are produced in the r-process (i.e. an upper limit to what is produced in supernovae explosions). Note that this fraction is very small for some elements (where the s-process dominates), but that the r-process produces everything beyond lead.

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Is there any reason to believe that supernovae stopped at element 92, or even 118? I know there are limits to how large a nucleus can get, but I would think that a supernova would be a lot more powerful than any of the reactors we've used to create trans-uranics. – supercat Nov 5 '14 at 0:48
@supercat Sorry for not spotting this earlier. I believe all the stable elements beyond lead are produced almost exclusively in supernova explosions via the r-process. The question about the limits on nuclear size is a different one - possibly already answered on Physics SE - but governed by the properties of the strong, weak and electromagnetic forces. Very heavy and exotic elements may exist briefly in the cores of supernovae before they explode and are probably still present in the crusts of neutron stars. – Rob Jeffries Nov 20 '14 at 12:31
While not beyond lead, gold is produced in neutron star collisions, see this news article. I noticed you gave that process an honorable mention. Might be worth including the gold thing, but if not this is still a thorough answer. +1 – Jim Jul 14 '15 at 12:48
@JimsBond I am aware of the work (or at least the press release, - the accompanying peer-reviewed journal article does not mention gold once!). There is a body of work though that suggests the very heavy r-process elements are prmarily produce din neutron star mergers. I will update a bit. – Rob Jeffries Jul 14 '15 at 13:08
Include the accompanying article instead of the press release. I'm interested to read it. And there's no rush to edit. Your answer is already orders of magnitude more correct and complete than the accepted one – Jim Jul 14 '15 at 13:10

Inside a star there are two primitive force competing with each other. 1st is the gravitational force which attracts the star's mass towards its core and shrinking the star, due to which the temprature and pressure increases and nuclear fusion stars which releases energy applying a outward radiation pressure(IInd force) balancing the gravitation force and saves the star from shrinking and exploding. any star do not have enough pressure and temperature to convert the nucleus of iron to further elements (by nuclear fusion). so the nuclear fusion inside the star stops. so the gravitasional force overcomes the radiation pressure and the star shrinks and explodes known as supernova explosion and that explosion has enough Temp. and Pressure to form all the further nuclei from iron. 90% of the star's masses gets distributed in space(Starting of a new universe) and the remaning 10% mass forms a neutron star (having no charge).

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This is not a detailed enough answer. How are the heavier elements formed at high temp. and pressure? – sarat.kant Sep 19 '15 at 22:26

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