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It’s a mystery.

“Despite decades of efforts, we still don’t know where, or even exactly how, the heaviest elements were made. It’s a compelling puzzle,” said Rebecca Surman. “I want to figure out where they came from and how they got here.”

The lightest elements, from helium to iron, are made inside stars in very hot, dense conditions. These reactions, combining two lighter elements to form a heavier one (i.e. carbon + helium = oxygen), produce a lot of energy—the kind that makes stars shine. The sun, for instance, is bright because it’s fusing hydrogen into helium.

But the heaviest elements can’t be made this way, fusing them does not release energy, it requires it. So there must be another process to produce the likes of gold, lead, mercury and uranium.

“We’re very confident that neutron capture processes created most of the elements heavier than iron on Earth and in our solar system,” Surman said. “But we don’t know exactly where in the galaxy this synthesis takes place. That is what I’m trying to figure out.”

And she can’t just rig up an experiment in a lab to do so. Heavy elements can’t be manufactured on Earth, at least not without a sophisticated machine (a particle accelerator) that makes very small amounts at great expense.

So she’s looking to space, particularly at massive stars exploding at the ends of their lives (supernovae) and at pairs of stars colliding to make black holes surrounded by disks of leftover material (accretion disks).

“These environments are very hot and very dense, and may have enough neutrons to make the heaviest elements,” Surman said. 
Her work is supported by grants from the Office of Science, U.S. Department of Energy.