While it is quite clear that the heavy elements are produced by neutron capture, this process can also happen more slowly in stars that have passed the red-giant stage and evolve onto the so-called asymptotic giant branch during which they can produce slow pulses of neutrons. This process happens over millions of years and produces about half of the heavy elements, including gold. It is still believed that supernova trigger star formation so that any s-process gold in the interstellar medium would be melded into Earth’s veins along with r-process gold. Interested in the r-process? Read this article that involves NASA’s Swift telescope.

Supernova only happen for heavy stars about ten times heavier than the sun. Heavier stars, like larger bonfires, burn more quickly. In lighter, more typical stars (like our sun, which is a so-called main sequence star), fusion of the light elements takes billions of years but there is not enough material to synthesize elements beyond carbon and oxygen. Such stars end their lives as white dwarfs. The process of nucleosynthesis is described in the classic text book Cauldrons in the Cosmos, by C.E. Rolfs and W.S. Rodney (University of Chicago Press). Additionally, an excellent book introducing the relevant concepts for nucleosynthesis both generally and for gold in particular, is The Elements: A Very Short Introduction, by Philip Ball (Oxford University Press).

Wikipedia has good entries for the s-process and the r-process. The landmark paper describing the various processes of stellar nucleosynthesis is referred to as B2FH (after the authors): E. M. Burbidge; G. R. Burbidge; W. A. Fowler & F. Hoyle (1957), "Synthesis of the Elements in Stars" Reviews of Modern Physics 29 (4): 547.

It is important to remember that we still have no direct proof that the rapid neutron-capture process happens in supernova. However, the Hubble space telescope brought tantalizing evidence for a so-called kilonova (brighter than a nova but fainter than a supernova) that would signal the creation of heavy elements from neutron-star mergers.

The deposit of gold on Earth is also debated in the scientific literature. As with iron, the heavy metals may have sunk into the Earth’s core. A plausible explanation for the presence of gold in the mantle would be a prolonged bombardment of metal-rich meteorites. This hypothesis is strengthened after measurements with old rocks, and is discussed in this article titled "Where does all the gold come from?" Another excellent source on gold, how it is mined, and the world’s reserves can be found at this site.

Interested in studying the different elements? Visit the Photographic Periodic Table of the Elements. Compare the different elemental abundances here. Want to watch videos instead? A superb collection of videos (selectable from a periodic table) from the University of Nottingham provides a different view of each element!

For more TED-Ed lessons that address this topic, watch What is the universe made of? and How to detect a supernova.

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