Heavy Metal

Where do the heavy elements on the periodic table come from?  The general answer is from what’s called the r-process of stellar nucleosynthesis.   This translates to ‘rapid neutron capture’ being the method by which most of the elements heavier than Iron are formed on the periodic table.  This process requires immense energy and was originally thought to only occur within core-collapse supernova explosions.

Credit: © alexphoto71 / Fotolia

“Understanding how heavy, r-process elements are formed is one of hardest problems in nuclear physics,” said Anna Frebel, assistant professor in the Department of Physics at the Massachusetts Institute of Technology (MIT) and also a member of the MIT Kavli Institute for Astrophysics and Space Research (MKI). “The production of these really heavy elements takes so much energy that it’s nearly impossible to make them experimentally,” Frebel continued. “The process for making them just doesn’t work on Earth. So we have had to use the stars and the objects in the cosmos as our lab.”

Even though we have powerful particle colliders, we can’t make a lot of these elements on Earth, so what kind of crazy event could produce them in space? Scientists had figured out that extreme space environments would be needed to produce the heavy elements, but what events specifically? Where do these types of events occur?

MKI researchers have answered these questions.  By observing bright stars in the tiny galaxy Reticulum II, approximately 100,000 light years from Earth, they found that these stars contained huge amounts of r-process elements.  The stars couldn’t have made these heavy elements on their own, and so there must have been an event in history that seeded these elements in the galaxy.  With the abundances in the current generation of stars and the small size of the galaxy, the culprit is almost certainly a collision of two neutron stars.

This means that most of the heavy elements in the universe, including those on Earth, formed in the merger of neutron stars and spread across the cosmos.  It’s an elegant answer to a difficult question about the origins of elements, and it’s been a long time coming to fill in a major gap in our understanding of the universe.




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