When a white dwarf star explodes as a supernova, it could detonate like a nuclear weapon on Earth, a new study finds.
White dwarfs are the faint, fading cores of dead stars the size of the Earth that remain after medium-sized stars have used up their fuel and shed their outer layers. Our sun will one day become a white dwarf, just like more than 90% of the stars in our galaxy.
Previous research has shown that white dwarfs can die in nuclear explosions known as type Ia supernovae. Much is still unknown about what causes these explosions, but previous work suggested they could happen when a white dwarf gets extra fuel from a binary companion, perhaps as a result of an impact. (In contrast, type II supernovae occur when a single death dies and collapses on its own).
Now, researchers have proposed a new way for Type Ia supernovae to happen: a white dwarf can detonate like a nuclear weapon.
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As a white dwarf cools, uranium and other heavy radioactive elements known as actinides crystallize in its core. Occasionally, the atoms of these elements spontaneously undergo nuclear fission and split into smaller fragments. These instances of radioactive decay can release energy and subatomic particles, such as neutrons, that can break down nearby atoms.
If the amount of actinides in a white dwarf’s core exceeds a critical mass, it can trigger an explosive runaway fission chain reaction. This eruption can then cause nuclear fusion, where atomic nuclei fuse to generate enormous amounts of energy. In a similar way, a hydrogen bomb uses a nuclear fission chain reaction to detonate a fusion explosion.
The calculations and computer simulations of the new study showed that a critical mass of uranium can indeed crystallize from the mixture of elements commonly found in a cooling white dwarf. If this uranium explodes as a result of a nuclear fission chain reaction, the scientists found that the resulting heat and pressure in the white dwarf’s core could be high enough to cause fusion of lighter elements, such as carbon and oxygen, resulting in a supernova.
“The conditions to build and detonate an atomic bomb seemed very difficult – I was surprised that these conditions could be met naturally in a very dense white dwarf,” study co-author Charles Horowitz, a nuclear power plant. astrophysicist at Indiana University, told Space.com. “If that’s true, this offers a whole new way to think about thermonuclear supernovae and perhaps other astrophysical explosions.”
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So how many type Ia supernovae could help explain this new mechanism? “Maybe about half,” Horowitz said.
In particular, these new findings could explain the type Ia supernovae that take place within a billion years of the formation of a white dwarf, since not all of their uranium has decayed radioactively. When it comes to older white dwarfs, type Ia supernovae can happen through mergers of two white dwarfs, Horowitz said.
Future research may include performing computer simulations to determine whether and how fission chain reactions in white dwarfs can cause fusion. “There are many different physical processes going on during the explosion, and therefore there are many potential uncertainties,” Horowitz said. Such work could also reveal ways to detect whether or not type Ia supernovae have occurred due to this newly discovered mechanism.
Horowitz and study co-author Matt Caplan, a theoretical physicist at Illinois State University, detailed their findings online March 29 in the journal Physical Review Letters.
Originally published on Space.com.