Most of the antimatter filling the space of our Milky Way galaxy may be the remnants of dead stars, new research says. According to scientists, their work is able to solve the mystery of astrophysics, which has existed for more than 40 years.
Every particle of ordinary matter has an antipode - antimatter, which has the same mass, but at the same time has an opposite charge. For example, the antiparticle of a negatively charged electron will be a positively charged positron. When particles and antiparticles collide, it leads to their destruction (annihilation) and a powerful release of energy. Only one gram of antimatter, colliding with one gram of ordinary matter, is capable of causing an explosion, in which the level of energy release will be twice as high as in the explosion of a bomb dropped on Hiroshima.
More than 40 years ago, scientists first determined that gamma rays emitted during positron annihilation are being released at that moment in all directions of the galaxy. On the basis of this discovery, it was assumed that every second inside the Milky Way, 10 ^ 43 positrons (one with 43 zeros) annihilate. In the same study, it was indicated that the presence of most of these positrons was determined in the galactic center (central bar), and not in the galactic disk itself, despite the fact that the bar itself contains less than half of the entire mass of the Milky Way.
It has been hypothesized that the source of the emission of these positrons is radioactive material synthesized by the stars. However, over the next several decades, scientists were never able to determine the type of stars capable of generating such an amount of antimatter. Later, another assumption was made: the ejection of positrons can be created by rare sources, such as supermassive black holes located in most galactic centers, as well as dark matter particles annihilating with each other.
“The source of these positrons is a mystery with more than 40 years of history. But to explain positrons, you don't need any exotic elements like dark matter,”said lead author of the new study, Australian National University astrophysicist Roland Crocker.
In his opinion, this source may be supernovae - catastrophic explosions of stars capable of generating a huge number of positrons. This, according to the scientist, is confirmed by the fact where these positrons were most often found.
Crocker focused on supernovae similar to the object known as SN 1991bg. This type of object, as it turned out, is more common in other galaxies, but much less often than ordinary supernovae. Unlike most ordinary supernovae, which can eclipse virtually all other stars in galaxies, the type of supernova under investigation does not produce a large amount of visible light and is considered very rare. And that is why, according to the researcher, it was so infrequently found in the Milky Way.
Previous studies have suggested that a similar type of faint supernova could appear when two white dwarfs merge. The latter have a very high density and represent the cores of dead stars (the size of the Earth), left after the stars have completely depleted their thermonuclear fuel and have lost their outer layers. Most stars, including our Sun, will one day become white dwarfs.
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Returning to supernovae of the SN 1991bg type, it should be noted that they appear specifically when two low-mass white dwarfs collide, one of them rich in carbon and oxygen reserves, and the other with helium. Despite being rare among supernovae, this species is capable of generating huge volumes of a radioactive isotope known as titanium-44. And it is he who singles out those positrons that have been discovered by astronomers throughout the Milky Way.
At a time when the majority of supernovae are born from young and massive stars, objects like SN 1991bg are most often found in regions where older stars between 3 and 6 billion years old prevail. This age difference could explain why the previously discovered positrons were observed mainly in the central bar of the Milky Way, which contains a large number of old stars, than in the outer galactic disk.
Crocker also notes here that other sources may be responsible for the appearance of a certain amount of positrons.
“Although this is not necessary, given that objects of the SN1991bg type are able to independently explain the entire phenomenology of positrons. Recent evidence indicates that the positron source is tightly bound to the center of the galaxy. In our model, this is explained by the fact that old stars are mostly scattered within a radius of 200 parsecs (about 650 light years) around the galactic center in the form of a supermassive black hole. Nevertheless, it would be very interesting to consider the black hole itself as an additional source,”concludes Crocker.
NIKOLAY KHIZHNYAK