Unlocking Stellar Secrets: How Physicists Discover the Origins of Heavy Atoms in Stars

In a fascinating setup, isotopes are funneled through pipes to a fragment separator. Here, they’re sorted, eventually reaching a detector known as the SuN. This cylindrical device, 16 inches wide, has metal spokes that give it a sun-like appearance. “It looks like the sun, which is pretty cool,” says Ellie Ronning, a graduate student at MSU.

As the isotopes enter, they start to decay, releasing electrons and flashes of gamma rays. These gamma rays help researchers understand the intricate processes of the i-process. “Until now, no one has been able to observe these specific reactions,” explains Sean Liddick, a nuclear chemist from the FRIB.

By tracking gamma-ray emissions, scientists can determine how quickly isotopes capture neutrons. For instance, when barium-139 gains a neutron, it becomes barium-140. This reaction rate goes into a simulation of the i-process, letting researchers predict how various heavy elements will form. They can then compare these predictions to the elements found in different stars.

Current findings are aligning perfectly with earlier predictions. The amounts of lanthanum, barium, and europium observed match what has been seen in carbon-rich, metal-poor stars, a mystery for astrophysicists since the early 2000s. “We’ve gone from a lot of uncertainty to seeing the i-process fit right where we expected,” Spyrou notes.

The i-process takes place in stars that eventually contributed to those metal-poor stars. Research is still ongoing to determine whether these processes occurred in white dwarfs or red giants. To find out, scientists will analyze more isotopes and build advanced models of the plasma inside these stars.

For decades, astronomers believed that gold, silver, and platinum originated from the r-process, but its specific origins are still debated. “Experiments for the r-process are practically non-existent,” Cowan explains, as it’s tough to replicate conditions like those during neutron-star collisions on Earth.

A notable 2017 observation discovered traces of gold and other r-process elements in remnants of a neutron-star collision, backing this theory. Recently, an exciting finding linked the r-process to a huge flare from a highly magnetic star.

After mastering the i-process, Michigan researchers aim to tackle the r-process. Isolating its isotopes is trickier; it’s as if they’re trying to capture just a single window from a shattered plate. Yet, Spyrou remains hopeful. “We’re close to accessing the important nuclei,” she says.

“With the i-process, we can get results now,” she adds, estimating her lab will nail down key reactions within the next five to ten years. “Just ten years ago, I didn’t even know the i-process existed.” This evolving understanding highlights how rapidly the field of nuclear chemistry is advancing.

This story was supported in part by the Council for the Advancement of Science Writing and The Brinson Foundation.

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