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The build-up of heavy elements: 

Astronomers and physicists denote the build-up of heavier elements from lighter ones as "nucleosynthesis".  Only the very lightest elements (Hydrogen, Helium and Lithium [2]) were created at the time of the Big Bang and therefore present in the early universe. 

All the other heavier elements we now see around us were produced at a later time by nucleosynthesis inside stars. In those "element factories", nuclei of the lighter elements are smashed together whereby they become the nuclei of heavier ones - this process is known as nuclear fusion. 

In our Sun and similar stars, Hydrogen is being fused into Helium. The diagram below illustrates the fusing process that produces Helium from Hydrogen at the atomic level. At some stage, fusion will create heavier elements like Carbon, then Oxygen, etc.
 

Hydrogen (H) nuclei, which are just protons, 
collide to produce heavier nuclei. When protons collide one of them (at least) gives off some 
energy in the form of a positron (e⁺), i.e. a 
particle equivalent to an electron except that it 
has a positive charge instead of a negative charge. Since protons are positively charged, the emission of a positive charge makes the proton change into 
a neutron (so a neutron is a proton that has lost 
its charge and vice-versa). More collisions, at 
least the ones that stick, cause heavier elements such as D, ³H and He to form. Massive stars are capable of fusing even heavier nuclei such as 
Carbon, Oxygen, etc.  

Image and Copy obtained from Department of
Geosciences, Idaho State University

The highest elemental mass that can be created in a star depends on the mass of the star itself.  Larger stars can produce elements approaching that of Iron.  Smaller stars cannot climb as high on the periodic table.  The process of nuclear fusion relies upon very high temperatures to start; typically, around 10 million Kelvin with higher temperatures required for heavier elements.  The process starts with gravity which draws the gases (hydrogen and helium) and heavier elements closer together.  As the space between the gas atoms decreases the mass, pressure and gravitational forces increase. As the pressure increases so does the temperature.  This cycle of increasing mass, gravity, pressure and temperature eventually creates a gaseous sphere which ignites and begins the nuclear fusion process which starts nucleosynthesis. A star is born.


Jupiter, Saturn, Neptune and Uranus are gas giants, which were on their way to becoming stars when our Sun ignited and blew away the gas which was needed for these stars to get big enough to star their nuclear engines.  

The fusion process requires positively charged nuclei to move very close to each other before they can unite. But with increasing atomic mass and hence, increasing positive charge of the nuclei, the electric repulsion between the nuclei becomes stronger and stronger. In fact, the fusion process only works up to a certain mass limit, corresponding to the element Iron. All elements that are heavier than Iron cannot be produced via this path. 

But then, how were those heavy elements, we now find on the Earth, produced in the first place? From where comes the Zirconium in artificial diamonds, the Barium that colors fireworks, the Tungsten in the filaments in electric bulbs? Which process made the Lead in your car battery? Beyond iron the production of elements heavier than Iron takes place by adding neutrons to the atomic nuclei. These neutral particles do not feel any electrical repulsion from the charged nuclei. They can therefore easily approach them and thereby create heavier nuclei. This is indeed the way the heaviest chemical elements are built up. 

There are actually two different stellar environments where this process of "neutron capture" can happen. One place where this process occurs is inside very massive stars when they explode as supernovae. In such a dramatic event, the build-up proceeds very rapidly, via the so-called "r-process" ("r" for rapid).  The AGB stars.  But not all heavy elements are created in such an explosive way. A second possibility follows a more "peaceful" road. It takes place in rather normal stars, when they burn their Helium towards the end of their lives. In the so-called "s-process" ("s" for slow), heavier elements are then produced by a rather gentle addition of neutral neutrons to atomic nuclei. In fact, roughly half of all the elements heavier than Iron are believed to be synthesized by this process during the late evolutionary phases of stars. 

This process takes place during a specific stage of stellar evolution, known as the "AGB" phase [3]. It occurs just before an old star expels its gaseous envelope into the surrounding interstellar space and sometime thereafter dies as a burnt-out, dim "white dwarf". Stars with masses between 0.8 and 8 times that of the Sun are believed to evolve to AGB-stars and to end their lives in this particular way. At the same time, they produce beautiful nebulae like the "Dumbbell Nebula". Our Sun will also end its active life this way, probably some 7 billion years from now. 

Low-metallicity stars The detailed understanding of the "s-process" and, in particular, where it takes place inside an AGB-star, has been an area of active research for many years. Current state-of-the-art computer-based stellar models predict that the s-process should be particularly efficient in stars with a comparatively low content of metals ("metal-poor" or "low-metallicity" stars). In such stars - which were born at an early epoch in our Galaxy and are therefore quite old - the "s-process" is expected to effectively produce atomic nuclei all the way up to the most heavy, stable ones, like Lead (atomic number 82 [2]) and Bismuth (atomic number 83) - since more neutrons are available per Iron-seed nucleus when there are fewer such nuclei (as compared to the solar composition). Once these elements have been produced, the addition of more s-process neutrons to those nuclei will only produce unstable elements that decay back to Lead. Hence, when the s-process is sufficiently efficient, atomic nuclei with atomic numbers around 82, that is, the Lead region, just continue to pile up. 

As a result, when compared to stars with "normal" abundances of the metals (like our Sun), those low-metallicity stars should thus exhibit a significant "over-abundance" of those very heavy elements with  respect to Iron, in particular of Lead. Looking for Lead Direct observational support for this theoretical prediction would be the discovery of some low-metallicity stars with a high abundance of Lead. 

At the same time, the measured amounts of all the heavy elements and their relative abundances would provide very valuable information and strongly reinforce our current understanding of heavy element nucleosynthesis. But detecting the element Lead is not easy - the expected spectral lines of Lead in stellar spectra are relatively weak, and they are blended with many nearby absorption lines of other elements. Moreover, bona-fide, low-metallicity AGB stars appear to be extremely rare in the solar neighborhood. But if the necessary observations are so difficult, how is it then possible to probe nucleosynthesis in low-metallicity AGB stars?