Giant Star Spots Probably Causes Betelgeuse Dimming

Betelgeuse, the bright star in the Orion constellation, has been fascinating to astronomers in recent months due to its unusually sharp decrease in brightness. Scientists have been discussing a series of scenarios trying to explain their behavior. Now, a team led by Thavisha Dharmawardena of the Max Planck Institute for Astronomy has shown that unusually large spots of stars on the Betelgeuse surface have caused the attenuation. Their results rule out the earlier conjecture that it was dust, recently cast by Betelgeuse, that obscured the star.

Giant red stars like Betelgeuse undergo frequent variations in brightness. However, Betelgeuse’s astonishing drop in luminosity to approximately 40% of its normal value between October 2019 and April 2020 was a surprise to astronomers. Scientists have developed various scenarios to explain this change in the star’s brightness, which is visible to the naked eye and almost 500 light years away. Some astronomers even speculated about an impending supernova. An international team of astronomers led by Thavisha Dharmawardena of the Max Planck Institute for Astronomy in Heidelberg has shown that temperature variations in the photosphere – the star’s luminous surface – caused the brightness to drop. The most plausible source for such temperature changes are the gigantic spots of cold stars, similar to sunspots, which, however, cover 50 to 70% of the star’s surface.

“Towards the end of their lives, the stars become red giants,” explains Dharmawardena. “As the fuel supply runs out, the processes by which stars release energy change.” As a result, they swell, become unstable, and pulse with periods of hundreds or even thousands of days, which we see as a fluctuation in brightness. Betelgeuse is a so-called Red Supergiant, a star that, compared to our sun, is approximately 20 more massive and approximately 1000 times larger. If placed at the center of the solar system, it would almost reach Jupiter’s orbit.

Due to its size, the gravitational pull on the star’s surface is less than that of a star of the same mass but with a smaller radius. Thus, pulsations can eject the outer layers of such a star with relative ease. The released gas cools down and turns into compounds that astronomers call dust. This is the reason why red giant stars are a major source of heavy elements in the Universe, from where planets and living organisms eventually evolve. The production of light-absorbing dust has previously been considered by astronomers as the most likely cause of the sharp decrease in brightness.

To test this hypothesis, Thavisha Dharmawardena and her collaborators evaluated new and archived data from the Atacama Pathfinder Experiment (APEX) and the James Clerk Maxwell Telescope (JCMT). These telescopes measure radiation in the spectral range of sub-millimeter waves (terahertz radiation), whose wavelength is a thousand times greater than that of visible light. Invisible to the eye, astronomers have been using them for some time to study interstellar dust. Cold dust, in particular, shines at these wavelengths.

“What surprised us was that Betelgeuse became 20% darker even in the sub-millimeter wave range,” reports Steve Mairs of the East Asia Observatory, who collaborated on the study. Experience shows that such behavior is not compatible with the presence of dust. For a more precise evaluation, she and her collaborators calculated what influence dust would have on measurements in this spectral range. It turned out that, in fact, a reduction in brightness in the submillimeter range cannot be attributed to an increase in dust production. Instead, the star itself must have caused the change in brightness measured by astronomers.

The physical laws tell us that the luminosity of a star depends on its diameter and especially on the temperature of its surface. If only the size of the star decreases, the luminosity decreases equally at all wavelengths. However, changes in temperature affect radiation emitted across the electromagnetic spectrum differently. According to the scientists, the darkening measured in visible light and sub-millimeter waves is therefore evidence of a reduction in the average Betelgeuse surface temperature, which they quantify at 200 K (or 200 ° C).

“However, an asymmetric temperature distribution is more likely,” explains co-author Peter Scicluna of the European Southern Observatory (ESO). “The corresponding high-resolution images from Betelgeuse from December 2019 show areas of variable brightness. Together with our result, this is a clear indication of huge star spots that cover 50-70% of the visible surface and have a temperature lower than the brightest photosphere. ” Star spots are common in giant stars, but not on this scale. Not much is known about their lives. However, the theoretical model calculations seem to be compatible with the duration of the Betelgeuse brightness drop.

We know from the sun that the number of spots increases and decreases in an 11-year cycle. Whether giant stars have a similar mechanism is uncertain. An indication of this could be the previous minimum brightness, which was also much more pronounced than in previous years. “Observations in the coming years will tell us whether Betelgeuse’s sharp decrease in brightness is related to a point cycle. In any case, Betelgeuse will continue to be an exciting subject for future studies,” Dharmawardena concludes.