These stars could have a sort of atomic bomb at their center

April 13, 2021
Harmless-looking white dwarf stars might be carrying an explosive surprise. (NASA/JPL-Caltech/University of Wisconsin via AP)

Harmless-looking white dwarf stars might be carrying an explosive surprise. (NASA/JPL-Caltech/University of Wisconsin via AP)

Physicists may have discovered the first naturally occurring "bombs" lying dormant within white dwarf stars roaming outer space, and if detonated, the explosive material could force an entire star's destruction.

Published March 29 in Physical Review Letters, the new theory proposes that this eruption — triggered by an isotope of uranium and culminating in the star's subsequent explosion — stems from a fission chain reaction.

Such reactions are synthesized on Earth by militaries making nuclear weapons, such as those dropped on Japan during World War II, study author Matt Caplan, an assistant professor of physics at Illinois State University, told The Academic Times.

"A fission chain reaction is like dominoes — except the dominoes explode," he said. "And when one domino explodes, the fragments of that exploding domino will knock over the other dominoes, which then explode. One becomes three, becomes nine, becomes 27, becomes 81."

These "dominoes" are isotopes of uranium, specifically uranium-235. That version of the element is found in both nuclear power plants and inside white dwarf stars, which represent the last evolutionary stage of other stars, such as the sun, after they run out of nuclear fuel. Once a larger star eventually turns into a white dwarf, it begins a cooling process.

A byproduct of that billion-year-long process, uranium is thought to begin the chain because it's considered an actinide. Actinides are the heaviest elements, and according to Caplan, the heavier an isotope, the greater its likelihood of spontaneous explosion. 

"We were thinking about, 'what are the heaviest things in the star?'" he said, recalling the starting point for the team's hypothesis. 

Once realizing the answer was uranium, they started to wonder if uranium snowflakes, conglomerates of the element, might form within the star, too. Because uranium is relatively unstable, or radioactive, having so much of it together would greatly increase the probability that at least one particle would explode.  

A single explosion in the snowflake — just one domino being tipped over — is all that's needed to potentially begin the fission chain reaction, Caplan says.

"The bomb is the snowflake that we're talking about," he said. "It's maybe the mass of a grain of sand and is so compressed by the high density and pressure in the white dwarf, it's basically microscopic."

He continued, "Once this little snowflake starts to grow and precipitate, it will accumulate more and more nuclei. It'll grow so large, and it'll have so many nuclei in it, that it's just bound to have one of these rare decays that'll set it off."

In turn, the fission chain sets off a process called fusion because it creates enough heat to affect lighter elements, such as carbon. Those then start to assemble into heavier elements, such as nickel. In the white dwarf, fusion isn't stoppable; the whole star eventually explodes in what is called a Type Ia supernova

Caplan noted that while his idea would be the first to offer a fission bomb-like explosion in nature, there was another case of a fission chain reaction on Earth. And it also has ties to uranium-235. 

"There were these rocks a billion years ago … that had a higher fraction of uranium-235 than they do today, because it's constantly decaying away," he said. "The uranium-235 abundance was high enough that this rock was supporting a chain reaction."

In contrast, however, that reaction didn't end with a boom. It was moderated by water that trickled into the rock, which controlled the mechanism before it provoked an explosion.

The researcher also stressed that his proposal for how white dwarf stars explode doesn't negate existing theories. Rather, he suggests that it could explain the outlier white dwarfs that don't seem to supernova in accordance with a textbook Type Ia, which involves a companion star catalyzing the explosion.

"There are a handful of Type Ia supernova that are strangely dim, that don't seem to explode with quite as much mass and energy as a traditional supernova," Caplan said. "Our method could explain these lower-mass, lower-luminosity Type Ias."

Understanding how Type Ia supernovas happen could substantially help scientists study the expansion of the universe. When a supernova is spotted from Earth, its distance from the planet can be mapped based on the brightness it emits.

"If you see a Type Ia supernova anywhere, you can determine how far away it happens because it has the same effective brightness," Caplan said. "It's how the accelerating expansion of the universe was discovered 20 years ago. A Nobel Prize has been given for that."

Caplan also relayed that the materials found in the inner core of the Earth, iron and nickel, actually come from Type Ia supernovas. That means realizing a supernova's source could offer insight into planet and planetesimal formation.

"Type Ia supernova play an incredibly important role in enriching the universe with heavy elements," he said. "It's important to understand when they can happen, how and why, and what sort of things can come out of them."

Regarding the realistic viability of the theory, Caplan said that while it's still in the early stages, he feels confident about its footing.

"Science is a spectrum from 'impossible' to 'definitely happens.' When a physicist has a new idea, we do everything we can to show that it's impossible, and only when we fail at that do we get to say our idea is possible," he said. "That's where we're at — it might be possible for uranium snowflakes to trigger supernova — and that's really exciting."

The study, "Actinide Crystallization and Fission Reactions in Cooling White Dwarf Stars," published March 29 in Physical Review Letters, was authored by C. J. Horowitz, Indiana University; and M. E. Caplan, Illinois State University. 

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