Stars across the universe run on nuclear fusion, but nuclear fission—the mechanism at the core of an atomic bomb—is an exceedingly rare process in nature.
Now, two scientists think they may have identified an exotic natural environment where fission not only occurs, but could actually trigger one of the most important types of explosion in the universe: a supernova.
When stars die, they leave behind all kinds of strange remnants, from black holes to neutron stars to white dwarfs, all of which have their own trippy properties. White dwarfs, the necrotic remains of stars like our Sun, can weirdly die for a second time in blowups called Type 1a supernovae, which have such a consistent brightness that scientists use them as “standard candles” to measure the expansion of the universe.
These explosions are thought to occur when white dwarfs become overwhelmed by material tugged from a companion star, but a forthcoming study in Physical Review Letters suggests that an ultra-rare fission reaction may trigger some of these brilliant blasts. A copy of the study is available on the preprint server arXiv.
“People think [white dwarfs] have a companion star because they didn't know how to get the star to explode without a companion,” said Charles Horowitz, a professor of physics at Indiana University who led the study, in a call. The new research suggests that “we may be wrong about companions, for at least some of the Type 1as,” he added.
Horowitz and co-author Matt Caplan, an assistant professor of physics at Illinois State University, suggest that crystallized radioactive elements, which they call “uranium snowflakes,” can make trouble in the bellies of white dwarfs. These snowflakes would appear roughly 100 million years after the formation of the dead stars, allowing the remnant time to cool off enough for heavy elements—including the actinide group to which uranium belongs—to freeze into latticed crystals.
“It would be a very dirty crystal,” said Caplan in a call (“dirty” meaning that it would not be pure uranium). “The snowflake is probably going to include lead, it's probably going to include thorium, and may even have like a special crystal structure that's 50/50 actinides and light elements.”
Within seconds, these bizarre snowflakes could grow to the size of sand grains within a white dwarf. Since uranium is radioactive and unstable, a nuclei in these formations would inevitably decay, shooting a trio of neutrons at its close neighbors and setting off a chain reaction of nuclear fission similar to the detonation of an atomic bomb.
That’s when things get really pyrotechnic. All the heat and energy produced from the fission would cause lighter elements such as carbon and oxygen to fuse together, creating a supercharged state similar to a fusion weapon, such as a hydrogen bomb. As Caplan puts it, this is like setting off the largest nuclear bomb in the universe with the tiniest nuclear bomb in the universe.
“If it was hot enough, in a tight enough spot, and if you release enough energy,” Caplan said, “that can release enough energy to burn all of the carbon and oxygen in the star. Once the carbon and oxygen starts burning, that's the powder keg going up. At that point, you have a supernova” that would “completely obliterate the star.”
In other words, the decay of a single uranium nucleus could initiate one of the rarest and scientifically significant forms of stellar explosion in the universe. It’s a wild idea, but Horowitz and Caplan cautioned that this is simply a new hypothesis to explain Type 1a supernovae and that there is no direct evidence that uranium snowflakes spark these explosions.
“We just started this mechanism and now people have to study, if this is what sets off the white dwarf, exactly how does the rest of the burning of the white dwarf go?” Horowitz said. “What does that look like?”
“I think the most important next step is just getting our friends who do supernova simulations to try and put it in a code and see if these little snowflakes can get hot enough to start a fusion reaction,” added Caplan.
“This is not impossible—that’s basically all that our paper is saying,” he concluded. “There's still a ton of work to be done.”