In studying how the rich diversity of elements in the universe came to exist, University of Michigan research has put a number on how heavy an atom nature can produce—and it happens under extreme conditions in specific types of stars and supernovae.
The universe’s heaviest elements are produced in spectacular fashion: in the hot centers of certain stars. By studying the patterns of which elements are present in many different stars, and in what amounts, astronomers can learn how elements are made.
“During their lifetimes, stars fuse lighter elements into heavier ones, and that fusion process is why stars shine,” said lead author Ian Roederer, who completed much of the work as a U-M research professor of astronomy. “When a star dies, it recycles those heavier elements back into space, where they can reform into the next generation of stars.”
In rare types of supernovae and in the collisions of neutron stars—the collapsed cores of massive supergiant stars—something called the “rapid neutron-capture process” takes place. During this process, a rapid burst of neutrons grows lighter elements into heavier ones in less than a second. In fact, before our solar system was born, the r-process in ancient stars produced about half of the heaviest elements found on Earth today, Roederer said.
Now, Roederer and a team of researchers conducted a meta-analysis based on studies from the last three decades that have detailed the concentrations of heavy elements in ancient stars in the Milky Way. The team focused on 42 ancient stars where r-process elements are present in a range of concentrations.
By examining these stars collectively, rather than individually as is more common, they were able to identify previously unrecognized patterns. They found that elements fell into two unexpectedly related groups: a lighter group that includes ruthenium, palladium and silver, and a heavier group that includes gadolinium, dysprosium and erbium. The most likely explanation to explain this relationship is that these groups of elements were produced in part as the fragments of much heavier atoms, through a process called fission. In fission, an extremely massive atomic nucleus splits into two lighter nuclei.
The team published its findings in the journal Science.
“The nuclei that are fissioning are all radioactive. They are likely heavier than any elements found in nature, and heavier than uranium, so we call them ‘transuranic elements,’” said Roederer, now a professor at North Carolina State University. “If our explanation is correct, then we are finding the fragments of the fission process among these lighter elements listed near the middle of the periodic table.”
The researchers’ findings also identified the lower bounds of how heavy an atom that nature can produce. The heaviest atom of uranium that occurs naturally on Earth has 238 protons and neutrons in its nucleus. This study finds that stars can produce an atom that has at least 260 protons and neutrons in its nucleus.
“This is higher than the heaviest atom that was recovered from nuclear weapons tests conducted in the latter half of the 20th century,” Roederer said. “We cannot yet say whether so-called ‘super-heavy’ atoms were present—ones with at least 284 protons plus neutrons—but our results show that answering this question is potentially on the table going forward.”
Another potential answer on the table is to the question of how fission works.
“You cannot reproduce the cosmic conditions of the r-process in a laboratory, or even in a nuclear weapon, to study it up close. The conditions are just too extreme, not to mention dangerous,” Roederer said. “We have some ideas about how fission works, but we have not had reliable evidence from astronomical observations to tell us which of those ideas, if any, are correct. Now we have our first clues about where and how to look to the stars for those answers.”
Roederer hopes astronomers will soon be able to directly detect these transuranic elements in space. To do this, astronomers would need to observe a colliding pair of neutron stars—and now, researchers may be able to do this with the James Webb Space Telescope, launched in 2021. These colliding stars and other extreme sites could provide opportunities for scientists to study fission.
“In the 20th century, scientists used nuclear weapons as physics experiments. It has been several decades since those experiments were conducted in the U.S., and there were many complex ethical questions about those tests, both then and now,” Roederer said. “I hope this study can inspire us to think about how to use observations of the cosmos to gather some of that same information, and potentially even pursue previously unanswerable questions.”