A planetary nebula is the result of a sunny star that sheds its outer layers at the end of its life. In the center is the compact galaxy, known as a white dwarf.
NASA; ESA; K. Noll / STScI
By Adam Mann
A thermonuclear bomb can strike deep into the core of some dead stars. A new theoretical study finds how certain stellar bodies known as white dwarfs could accumulate a critical mass of uranium that would cause a massive supernova explosion.
The findings could provide insights into the destructive 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 large distances across the cosmos. But such explosions are not yet fully understood, and the new study may take into account certain, deviant observations of these types of supernovae.
“It’s a fun 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 ten times more massively than our sun and shed their outer layers. This leaves a hot hot, earth-sized nucleus that consists almost entirely of naked atomic nuclei and free electrons.
Certain quantum mechanical properties of the electrons prevent them from being further compressed so that they can hold the dense entity. This remaining object, called a white dwarf, begins to cool and eventually freezes over billions of years in a giant solid crystal.
The heaviest elements first freeze out, and sit like sediments in the center of the dead star. This left theoretical physicist Matt Caplan of Illinois State University and his colleagues wondering: can uranium, one of the heaviest elements in the periodic table, accumulate in a white dwarf?
Uranium-235, a rare isotope of the element, can split spontaneously and release neutrons and energy. When there is a critical mass of the isotope nearby, the neutrons hit other uranium-235 nuclei in a chain reaction leading to a powerful explosion.
“It’s a crazy idea,” Caplan admits. “It was a bunch of bored theoretical physicists during the pandemic who thought about this strange problem.”
White dwarfs are mainly carbon and oxygen; only one part per trillion is uranium. Yet Caplan and his co-author, nuclear astrophysicist Chuck Horowitz of Indiana University in Bloomington, calculated that sand grain flakes containing uranium, thorium and lead could precipitate within the first few hundred million years if a white dwarf had cooled.
The concentration of uranium-235 within these crystals will be alarmingly high. “Instead of being one in a trillion cores, you suddenly have one in 10,” Caplan says. “And that means you can have a bomb.”
If the uranium were ever to reach a critical mass, it would inflate spontaneously – igniting the white dwarf’s carbon and oxygen reserves, which would result in a catastrophic supernova explosion. The findings appeared this month on the preprint server arXiv and were accepted for publication in Physical overview letters.
For now, the scenario remains hypothetical. Caplan hopes that other researchers will be able to test the theory with powerful computer simulations of supernovae. Such work can also make astronomers wonder how to detect such paroxysms.
Yet little is known about the internal composition of white dwarfs, so it is unclear whether they contain enough uranium-235 to cause an explosion, Tremblay said.
“I think physics is very interesting,” he says. “But we have to ask ourselves if this has happened or is going to happen.”