Clarification of the surface temperature of Betelgeuse during and after its tarnishing in the fall of 2019 made astronomers doubt that the decrease in the star's brightness caused a cloud of dust. The research results were published in the scientific journal Nature Communications.
"Our data show that at the moment of maximum decrease in brightness, the star's surface temperature dropped to 3476 Kelvin. In April last year, it increased to 3646 Kelvin. This is enough to explain the dimming of Betelgeuse," said Sofya Alekseeva from the National Astronomical Observatory of China.
Betelgeuse is one of the largest and brightest stars in the sky. Astronomers believe that it is in the last stage of stellar evolution - the stage of a red supergiant. This is how scientists call old stars, which have almost completely exhausted their hydrogen reserves, expanded dramatically and began to dump the material of their outer shells into open space.
At the end of 2019, the brightness of Betelgeuse began to decline rapidly, decreasing by 63% by January last year. In mid-spring, the star returned to its original luminosity. The exact reasons for its tarnishing are not yet known, but most scientists assume that it was generated by the ejection of a powerful cloud of dust. Later, this large star from the constellation Orion experienced another similar episode, the exact reasons for the beginning of which also became the subject of controversy among astronomers.
Alekseeva and her colleagues tried to get an exact answer to this question. Scientists have measured in detail the spectrum of a dying star at the time of its maximum dimming and after its brightness has been restored.
"Our observation technique was based on measuring the lines of titanium oxide and cyano-radical in the spectrum of Betelgeuse. The colder the star, the more of these molecules can appear and survive in its atmosphere, since these substances quickly decay at high temperatures and are not formed anew." explained Alekseeva.
Using a similar technique, scientists calculated the exact surface temperatures of Betelgeuse at the end of January last year and in the first two months of spring. It turned out that the increase in the brightness of the star was accompanied by an increase in the surface temperature of the red giant by at least 170 kelvin.
Such an increase, according to scientists, indicates that the dimming of the star was caused by the formation of a huge spot on Betelgeuse, whose appearance is usually accompanied by a sharp decrease in the surface temperature of the star. This, Alekseeva and her colleagues point out, is sufficient to explain the tarnishing without involving additional processes such as the formation of a dust cloud.
Scientists hope that their theory will test and confirm observations of further fluctuations in the brightness of Betelgeuse and other red giants. These measurements will help to understand what internal processes in the bowels of large aged stars give rise to these spots.