G299 is an elegant remnant of a type Ia supernova that exploded around 2,500 BC Credit: NASA / CXC / U.Texas.
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When a star similar to our Sun burns, it usually ends up as a white dwarf, a star embellishment that gradually cools. But some white dwarfs have a much more explosive fate. They explode like a supernova called a type Ia supernova. These monumental explosions usually occur in close binary stars, in which a white dwarf steals the mass of a stellar partner. When theft increases its mass above a certain limit, it ignites like a supernova.
Classical model of a type Ia supernova. Credit: NASA, ESA and A. Feild (STScI), chris.
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This limit is still the same. White dwarfs explode like a type Ia supernova, always with the same brightness, which astronomers enthusiastically use to measure distances in space. Type Ia supernovae play the role of so-called standard candles. However, the universe is full of surprises. Gradually, it turned out that some supernovae of this type are less bright than they should be. Which doesn’t make sense.
In their new study, Chuck Horowitz of Indiana University and Matt Caplan of Illinois State University suggest that some white dwarfs could explode like a supernova without having a stellar partner to steal matter. They would be single dwarf supernovae.
White dwarfs are actually compressed nuclei of burnt stars, whose mass originally did not exceed 10 Suns. It consists mainly of carbon and oxygen, between which a few other chemical elements are mixed, including uranium. As the white dwarfs cool, the heaviest atoms in their material fall first into the dwarf.
Matt Caplan. Kredit: Matt Caplan, Illinois State University.
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According to Horowitz and Caplan, the smallest dwarf in space may be the smallest dwarf in space. These are tiny flakes of radioactive uranium, which are formed by the blinding of individual atoms of this element. When a randomly flying neutron hits such an explosive flake shortly after its formation, it can trigger a nuclear fission reaction, which can then trigger an avalanche of such reactions as in a nuclear bomb explosion. A nuclear explosion of uranium flakes can ignite an entire dwarf and it will detonate like a supernova. As Caplan says, like a spark ignites a barrel of gunpowder.
There would have to be a lot of uranium-235 in order to do this. This means that this mechanism would only work for white dwarfs, which formed from the largest stars for which it is still possible. Such stars live shorter times than, for example, the Sun, and there would be enough uranium left for the white dwarf.
Other astrophysicists have been fascinated by this, although they are rather skeptical. Ryan Foley from the American University of California, Santa Cruz does not see this very promisingly. But he would like such a mechanism to work, because it is fascinating. The Horowitz and Caplan models seem to be physically possible. But it is not clear whether this is really happening in space. Researchers intend to model it thoroughly, and perhaps other observations will suggest something.
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