Question

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.

Answer 1

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.

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 . 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.