A planetary nebula is the result of a sun-like star shedding its outer layers at the end of its life. In the center is the compact stellar remnant known as a white dwarf.
NASA; ESA; K. Noll / STScI
By Adam Mann
A thermonuclear bomb may be tapping deep into the cores of some dead stars. A new theoretical study shows how certain stellar corpses known as white dwarfs could collect critical mass of uranium that would trigger a massive supernova explosion.
The findings could provide insight into the destruction habits of white dwarfs, which are responsible for creating heavy elements such as iron and nickel. White dwarf supernovae illuminate their surroundings with the power of 5 billion suns, and astronomers have used them as “standard candles” to measure great distances across the cosmos. But such blasts are still not fully understood, and the new study could explain certain abnormally fuzzy observations of this type of supernovae.
“It’s a nice result,” said astrophysicist Pier-Emmanuel Tremblay of the University of Warwick, who was not involved in the work.
At the end of their life, stars blow up to 10 times as much as our sun and shed their outer layers. This leaves a scorching hot Earth core made up almost entirely of naked atomic nuclei and free electrons.
Certain quantum mechanical properties of the electrons prevent them from compressing further, allowing them to hold the dense entity. This remnant, called a white dwarf, begins to cool and eventually freezes into a giant solid crystal over billions of years.
The heaviest elements freeze first and settle as sediments in the center of the dead star. That left Illinois State University theoretical physicist Matt Caplan and his colleagues wondering: Can uranium, one of the heaviest elements on the periodic table, accumulate in a white dwarf?
Uranium-235, a rare isotope of the element, can split spontaneously, releasing neutrons and energy. If there is a critical mass of the isotope nearby, the neutrons strike other uranium-235 nuclei in a chain reaction that leads to a powerful explosion.
“It’s a crazy idea,” admits Caplan. “It was a bunch of bored theoretical physicists during the pandemic thinking about this weird problem.”
White dwarfs are mainly carbon and oxygen; only one part per trillion is uranium. Still, Caplan and his co-author, nuclear astrophysicist Chuck Horowitz of Indiana University, Bloomington, have calculated that grains of sand containing uranium, thorium and lead can precipitate within the first several hundred million years if a white dwarf cools.
The concentrations of uranium-235 in these crystals are said to be alarmingly high. “Suddenly, instead of being one in a trillion cores, you have one in ten,” says Caplan. “And that means you could have a bomb.”
If the uranium ever reached critical mass, it would explode spontaneously – igniting the white dwarf’s carbon and oxygen reserves, resulting in a cataclysmic supernova explosion. The findings appeared on preprint server arXiv this month and have been accepted for publication in Physical Review Letters
For now, the scenario remains hypothetical. Caplan hopes other researchers can test the theory with powerful computer simulations of supernovae. Such work could also point to astronomers how to recognize such paroxysms.
Still, not much is known about the internal makeup of white dwarfs, so it’s unclear whether they contain enough uranium-235 to trigger an explosion, Tremblay says.
“I find physics very interesting,” he says. “But we have to ask whether this has happened or will happen.”