Where does the Gold come from?

Gold doesn’t come from your local jewelry store, and the Gold rush that occurred in the Yukon territory at the turn of the 20th century is not the source I’m talking about either.  I want to take it further back, to the origins of gold the element.  Similar to the origins of most other elements on the periodic table, it requires an immense amount of energy, such as the nuclear fusion that goes on within a star.  But Gold can not be made by a star’s thermonuclear engine.  Gold requires more energy, as does every other element heavier than Iron.  So where do the heavier elements originate?

Two Neutron Stars Colliding. Credit: Stephan Rosswog, Jacobs University Bremen.

Stars fuse Hydrogen into Helium during their main phase, called the main sequence.  In later stages, Helium is fused in a chain to form larger elements like Carbon, Oxygen, Magnesium, and eventually all the other light elements.  Every fusion chain up to this point results in a net release of energy, making it a self-sustaining process.  Until we get to Iron.

Once Iron is in the core, it requires extra energy to fuse it.  And when it does fuse, you end up with a net loss of energy, meaning it just doesn’t happen.  Iron builds up in the core of a star, eventually fusion runs out of energy in the outer shells, the star collapses, and boom! Goodbye star.

A supernova explosion does produce heavier elements in abundance providing enough energy to make elements heavier than Iron for a short time during the explosion.  But does this include Gold? It’s a major candidate, but we don’t know for sure.

The other major candidate is the merger of two neutron stars.  You might think that this is a rare event in the universe, and you would be completely correct, but even rare events pile up in a universe with 100 Billion Billion (low balling) stars, so you could still end up with a lot of Gold.

So how do we tell which, if either of them, are correct?

Part of the trick is to figure out how to make the heavy elements in the lab.  If we can find the processes by which heavy elements are formed, we could look for signs of these and similar processes in space.  But the real power is in simulation.

Using computer modeling of stars and stellar environments, we can narrow down the amounts of heavy elements produced in each of these scenarios.  The models produced Gold in simulations of both supernovae and neutron star mergers, but they are starting to narrow the view to identify which one produces the larger fraction.

You might be thinking “What is the point if we don’t get the answer?”

In many cases, in science, the answer is deep, difficult, and hidden in mountains of data.  By doing experiments to narrow the view, the next generation of experiments can reach an answer.  It’s literally like finding a needle in a haystack.  If you can narrow it down to which hay bale has the needle, the problem isn’t so daunting.

So for now, the golden needle is still lost in the hay, but we are zeroing in on the right bale.



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