&Bullet; physics 14, 48
White dwarf explosions can be triggered by uranium in the star core, which detonates like a nuclear weapon.
A new proposal suggests that if a white dwarf star explodes as a supernova, the initial trigger could be a stellar version of an atomic bomb  . This scenario differs from the textbook explanation, which includes instability resulting from sucking in the mass of the star of a companion star. The researchers proposing this scenario say that the crystallization of uranium while the star’s core is cooling could lead to out of control nuclear fission. This fission “bomb” could in turn trigger an H-bomb-like (nuclear fusion) explosion of lighter elements to create the supernova. Several key questions about the process remain to be answered, but experts say the theory is worth exploring.
A white dwarf is a very dense star with a mass comparable to that of the Sun but a size similar to that of Earth. These objects are formed from sun-like stars that have condensed under gravity after burning most of their fuel. Some white dwarfs end their lives as type 1a supernovae, which are believed to only occur if the star is part of a binary star system, as a single white dwarf should be stable as it cools.
However, Charles Horowitz of Indiana University and Matt Caplan of Illinois State University point out that heavy elements, including uranium, are among the first to solidify when the interior of a white dwarf cools. This cooling and solidification process separates the complex plasma-like mixture into its components – a process known as phase separation. Even if the initial amounts of uranium and similar elements are very small, “the first solids will accumulate very strongly in these elements,” write Horowitz and Caplan in their work.
Uranium nuclei are occasionally subject to spontaneous fission, breaking into smaller pieces, releasing energy and particles such as neutrons that can break up nearby uranium nuclei. If the uranium mass exceeds a critical amount, the fission process can become a self-sustaining chain reaction that creates a massive release of nuclear energy, similar to the explosion of an atomic bomb.
Horowitz and Caplan performed calculations and computer simulations that showed that a critical mass of uranium can indeed crystallize from a typical mixture of elements found in a cooling white dwarf. If the uranium explodes, the resulting heat and pressure in the star’s core could be high enough to trigger a nuclear fusion of lighter elements, especially carbon and oxygen, and thus a supernova. (Similarly, today’s thermonuclear fusion bombs are detonated by fission bombs.)
Identifying an observable signature of this process “can take a lot of work,” says Horowitz. But he believes simulations of carbon fusion igniting in the only place he and Caplan predict – near the center of the star – have some tell-tale features of the supernova spectrum, or the brightness versus time chart, known as the light curve.
Stan Woosley, an astrophysicist at the University of California at Santa Cruz, says the work is “a whole new idea.” There is probably “more than one way to build a Type Ia supernova,” he says, “and this is certainly the most innovative and creative suggestion I’ve heard in a while.”
However, he adds that much more work is needed to consolidate the conclusions. In particular, he asks whether the crystallized uranium will be pure enough. “We don’t know for sure,” admits Horowitz. “At this point, we think the solid is very enriched in uranium, but we are working on molecular dynamics simulations to refine the composition.”
Paul Bradley, who works on fusion at Los Alamos National Laboratory in New Mexico, believes that there may not be enough of the specific isotope of uranium (uranium-235) that is most easily split, and that conditions inside the star may not support it not a chain reaction. And even if the carbon fusion were to ignite, Bradley isn’t sure if it would be strong enough to blow the star apart.
In the end, Horowitz and Caplan are right to formulate the idea as a hypothesis at this point in time, says astrophysicist Friedrich Roepke from the University of Heidelberg in Germany. But he thinks it’s a hypothesis worth pursuing.
Philip Ball is a freelance science writer based in London. His latest book is How to make a person grow (University of Chicago Press, 2019).
- CJ Horowitz and ME Caplan, “Actinide crystallization and cleavage reactions in cooling white dwarf stars”, Phys. Rev. Lett.126, 131101 (2021).