Known as white dwarfs, these dense cores are packed with heavy radioactive elements called actinides that can spontaneously undergo nuclear fission – the splitting of atoms. Depending on certain conditions, these cores can eventually undergo uncontrolled fission, culminating in a massive stellar explosion known as a supernova.
"The conditions to build and set off an atomic bomb seemed very difficult. I was surprised that these conditions might be satisfied in a natural way inside a very dense white dwarf," Charles Horowitz, a nuclear astrophysicist from Indiana University Bloomington and one of the study's researchers, told Space.
"If true, this provides a very new way to think about thermonuclear supernovae, and perhaps other astrophysical explosions," he added.
White dwarfs are the dim, Earth-size cores of dead stars. They form when average-sized stars have exhausted their fuel and shed their outer layers. The sun will one day become a white dwarf, as will more than 90 percent of the stars in the Milky Way galaxy.
Past studies show that white dwarfs can die in type Ia supernovae, a type of stellar explosion. Much remains unknown about what triggers type Ia supernovae, but prior research suggests that they can happen when a white dwarf absorbs material from another star. These two celestial objects orbit each other in an arrangement called a binary star system.
In their study, Horowitz and co-author Matt Caplan, a theoretical physicist from Illinois State University, proposed that type Ia supernovae might also occur when a white dwarf undergoes the processes behind the explosion of a hydrogen bomb.
As a white dwarf cools, actinides such as uranium crystallize within its core. The atoms of these elements can spontaneously undergo nuclear fission, which releases energy and neutrons. Neutrons can collide with other atoms and break them up, repeating the process.
If the amount of actinides exceeds a critical mass, these elements can set off an explosive runaway nuclear fission chain reaction. This, in turn, can trigger nuclear fusion, where atomic nuclei fuse with each other and generate enormous amounts of energy in the process. (Related: Gold and elements heavier than iron were formed on Earth after neutron stars collided billions of years ago: Study.)
The pair's calculations and computer simulations showed that a critical mass of uranium could indeed crystallize from the mixture of elements in a white dwarf. If this heavy uranium were to explode due to a nuclear chain reaction, the white dwarf would become so hot and pressurized as to trigger the fusion of lighter elements, resulting in a supernova. A hydrogen bomb also works the same way – a nuclear chain reaction is initiated to set off a nuclear fusion explosion.
Horowitz said that this mechanism could be responsible for around half of all type Ia supernovae in the cosmos. These stellar explosions should occur within a billion years of a white dwarf's formation since uranium takes a very long time to decay.
The pair recommended running more computer simulations to definitively answer whether fission chain reactions in white dwarfs could indeed trigger nuclear fusion. Though the study was compelling, Horowitz admitted that there were plenty of physical processes that occur during a supernova, which meant there were many potential uncertainties.
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