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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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3 Answers 3

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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 produced mainly by neutron-capture inside stars, although there are other more minor contributors (cosmic ray spallation, radioactive decay or even the collision of neutron stars).

Neutron capture can occur rapidly (the r-process) and occurs mostly inside supernova explosions. 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.

However, many of the chemical elements heavier than iron are produced by slow neutron capture inside relatively low-mass giant stars; the so-called s-process. The free neutrons for these neutron-capture events come from alpha particle reactions with carbon or neon and hence the s-process only occurs in the interiors of stars evolved enough to have produce significant quantities of these elements - mainly asymptotic giant branch (AGB) stars with masses of a few $M_{\odot}$. 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, Eu, 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.

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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

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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