Supernova surprise creates elemental mystery

Cassiopeia A is a supernova remnant in the constellation Cassiopeia. Credit: NASA/CXC/SAO

Michigan State University (MSU) researchers have discovered that one of the most important reactions in the universe can get a huge and unexpected boost inside exploding stars known as supernovae.

This finding also challenges ideas behind how some of the Earth’s heavy elements are made. In particular, it upends a theory explaining the planet’s unusually high amounts of some forms, or isotopes, of the elements ruthenium and molybdenum.

“It’s surprising,” said Luke Roberts, an assistant professor at the Facility for Rare Isotope Beams, FRIB, and the Department of Physics and Astronomy, at MSU. Roberts implemented the computer code that the team used to model the environment inside a supernova. “We certainly spent a lot of time making sure the results were correct.”

The results, published online on Dec. 2 in the journal Nature, show that the innermost regions of supernovae can forge carbon atoms over 10 times faster than previously thought. This carbon creation happens through a reaction known as the triple-alpha process.

“The triple-alpha reaction is, in many ways, the most important reaction. It defines our existence,” said Hendrik Schatz, one of Roberts’s collaborators. Schatz is a University Distinguished Professor in the Department of Physics and Astronomy and at the Facility for Rare Isotope Beams and the director of the Joint Institute for Nuclear Astrophysics—Center for the Evolution of the Elements, or JINA-CEE.

Nearly all of the atoms that make up the Earth and everything on it, people included, were forged in the stars. Fans of the late author and scientist Carl Sagan may remember his famous quote, “We’re all made of star stuff.” Perhaps no star stuff is more important to life on Earth than the carbon made in the cosmos by the triple-alpha process.

The process starts with alpha particles, which are the cores of helium atoms, or nuclei. Each alpha particle is made up of two protons and two neutrons.

In the triple-alpha process, stars fuse together three alpha particles, creating a new particle with six protons and six neutrons. This is the universe’s most common form of carbon. There are other isotopes made by other nuclear processes, but those make up just over 1% of Earth’s carbon atoms.

Still, fusing three alpha particles together is usually an inefficient process, Roberts said, unless there’s something helping it along. The Spartan team revealed that the innermost regions of supernovae can have such helpers floating around: excess protons. Thus, a supernova rich in protons can speed up the triple-alpha reaction.

But accelerating the triple-alpha reaction also puts the brakes on the supernova’s ability to make heavier elements on the periodic table, Roberts said. This is important because scientists have long believed that proton-rich supernovae created Earth’s surprising abundance of certain ruthenium and molybdenum isotopes, which contain closer to 100 protons and neutrons.

Supernova surprise creates elemental mystery
In the triple-alpha process, stars fuse three helium nuclei, also called alpha particles together (left) to create a single carbon atom with a
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