Astronomers have been able to spy on how one of the Milky Way's brightest stars, VY Canis Majoris, is growing old. SPbU Professor Nikolay Voshchinnikov was a member of an international research group working on this project.

As a result of the interpretation of the polarisation maps obtained at the Very Large Telescope in Chile, the scientists found out that the dust grains surrounding the star are unusually large in size. Because of this, the star entering the final stage of its existence is suffering a rapid mass-loss. The scientists managed to estimate the size of the dust grains with the help of model calculations based on the light scattering programme developed by SPbU Professor Nikolay Voshchinnikov. This software is available in the Database of Optical Properties of Cosmic Dust Grains developed over 10 years ago under the supervision of SPbU Professor Vladimir Borisovich Ilyin.

VY Canis Majoris is an extreme red supergiant and is one of the most massive and brightest stars in the Galaxy. The diameter of the star is about two billion kilometers. If VY Canis Majoris is placed in the centre of the Solar system, the surface of the star will be located between Jupiter and Saturn. At the same time the mass of the star makes up only 30–40 solar masses, that meaning its constituent substances have quite low density. Every year, this hypergiant "loses weight" by 30 Earth masses, and its stardust expands into the surrounding space. Until now, it has been a mystery which substance of the upper layers of the giant stars' atmospheres is ejected into space. The pressure of light of stellar radiation has always been thought the most plausible mechanism of this process. Yet, because this pressure is very low, it can impact only the large-size dust grains with a sufficient surface area. The scientists were able to obtain detailed images of the hypergiant that have shown that the average radius of the dust grains lost by VY Canis Majoris is 0.5 microns. It is 50 times bigger than the size of ordinary grains of cosmic dust.

"The life of massive stars is very short. When they face the final stages of their existence, they lose much substance. Earlier on, we could only imagine in theory how it happens. Now, the new data obtained via the SPHERE tool has shown that the hypergiant is surrounded by a cloud of very large dust grains. They are large enough to be ejected into space by the powerful radiation of the star. This explains the rapid mass-loss of the star," says lead author Peter Scicluna from the Academia Sinica Institute of Astronomy and Astrophysics.

The discovery of large dust grains in the vicinity of the star means that such a dust cloud must effectively scatter its visible radiation and expand itself under the influence of light pressure. In addition, such large grains should, most probably, survive the inevitable future explosion of VY Canis Majoris as a supernova. This dust will be part of the interstellar matter which will form the stars and planets of the next generation.


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