Betelgeuse’s brightness gradations recorded a few months ago are clearly not a sign of an impending explosion. According to a recent study, they could be due to the presence of gigantic spots lining the surface of the star.
A giant at the end of life
Located 650 light-years from Earth in the constellation Orion, Betelgeuse is a red supergiant. To get an idea of its proportions, tell yourself that the star is 11 times more massive and about 2,700 times larger than the Sun. It’s simple: if you put it in the center of our solar system, its surface would extend to the orbit of Jupiter.
That said, who says giant red says star at the end of its life. Betelgeuse could indeed explode in a supernova tomorrow as in 100,000 years. From a cosmological point of view, his death is therefore imminent. Hence the interest of researchers when, a few months ago, the star presented “signs of instability”.
Between January and March last year, telescopes in fact recorded sudden and very significant drops in its brightness, sometimes even up to 40%.
To explain it, several hypotheses have been put forward. The first, obviously, evokes its impending explosion. However, if that had been the case, we would have observed a steady drop in the energy emitted by the star. However, it seems that Betelgeuse has since March regained some of its lost luminosity.
An expulsion of matter?
A second hypothesis for its part advances the idea that the star, which constantly emits strong stellar winds, recently expelled a big puff of matter then transformed into dust. If this had been the case, it could then have absorbed part of its luminosity before evaporating completely.
This idea has been the subject of a recent study. Generally speaking, if Betelgeuse had effectively expelled part of its outer layers, as many red supergiants tend to do, this process would have led to a slight drop in its surface temperature.
However, according to the study, we know that on February 14 it was around 3325 degrees Celsius, only 50 to 100 degrees Celsius cooler than the usual temperature of the star. In other words, on paper, this hypothesis clearly held water. But is another explanation possible?
The sunspot hypothesis
In a recent article published in The Astrophysical Journal Letters, a team of researchers led by Thavisha Dharmawardena, of the Max Planck Institute for Astronomy, links this dramatic light falls not to a puff of expelled material, but to the presence of gigantic developed spots in its photosphere.
To support their idea, the researchers said they used old and new data collected by the Atacama Pathfinder Experiment (APEX) and the James Clerk Maxwell Telescope (JCMT), two instruments sensitive to sub-millimeter waves.
They were then surprised to find that Betelgeuse had become 20% darker even in this wavelength range. Behavior, they explain, is not compatible with the presence of dust.
We know that the brightness of a star depends on its diameter and its surface temperature. Thus, the darkening observed in both visible light and in millimeter wavelengths suggests that the star has experienced a reduction in its surface temperature.
If the idea agrees with the observations of the previous study, the researchers mention here however an “asymmetric temperature distribution”.
Combining all these data (darkening in sub-millimeter wavelengths + asymmetric temperature distribution), scientists thus evoke the presence of these enormous spots. These could cover 50% to 70% of the visible surface of the star, at temperatures lower than the very bright photosphere.
However, the presence of these stellar spots has yet to be confirmed. But if it turns out that they are indeed there, they would represent an entirely new class of stellar phenomenon. Future research will then need to determine the potential causes and identify their different cycles.