A study indicates that large spots are covering 50% to 70% of the star’s visible surface, at a lower average temperature than normal.
Between October 2019 and April 2020, the brightness of the red supergiant Betelgeuse, located 642 light-years from Earth, in the Orion constellation, decreased by up to 40%. Such an oscillation did not go unnoticed by the eyes of astronomers, who have already created several hypotheses to explain this unusual event.
The first theory to gain strength was that the star, which is one of the brightest in the night sky, had exploded into a supernova. Later, scientists at the University of Washington, in the United States, debunked the idea, suggesting that the variation was due to the existence of a dust cloud around Betelgeuse. Now, astronomers led by the Max Planck Institute for Astronomy in Germany have come up with a new hypothesis.
According to the team, led by astronomer Thavisha Dharmawardena, star spots on the surface of Betelgeuse are the most likely cause for the decrease in brightness observed in the star. In an article published in The Astrophysical Journal Letters on Monday (29), scientists demonstrate how the variation in the temperature of the star’s photosphere (the visible surface of an astronomical object) caused its brightness to decrease. The best explanation for this temperature fluctuation is the existence of giant cool spots on the red supergiant that can cover 50% to 70% of its surface.
“Towards the end of their lives, stars become red giants,” explains Dharmawardena, in a statement. “As their fuel supply runs out, the processes change by which the stars release energy. As a result, they bloat, become unstable and pulsate with periods of hundreds or even thousands of days, which we see as a fluctuation in brightness.”
When studying data from the Atacama Pathfinder Experiment (APEX) and the James Clerk Maxwell telescope (JCMT), the researchers noted that there was something that did not fit the hypothesis that the star’s brightness was decreasing because of dust around it – as had been suggested in previous studies.
Checking the submillimeter waves, which mainly serve to study the behavior of cosmic dust, they concluded that the behavior of the star is not compatible with the presence of dust. “What surprised us was that Betelgeuse turned 20% darker even in the submillimetre wave range,” said Steve Mairs, a researcher at the East Asian Observatory, who collaborated on the study.
To understand why Betelgeuse was dimming, they turned to physics. According to them, the luminosity of a star depends on its diameter and, mainly, on the temperature of its surface. If only its size decreases, the luminosity will decrease equally at all wavelengths. But, if it is the temperature that changes, the radiation emitted by the star will be distinct throughout the electromagnetic spectrum.
And what measurements in visible light and submillimeter waves from Betelgeuse show is the reduction in the average temperature of the star’s surface. “An asymmetric temperature distribution is more likely,” explains Peter Scicluna, co-author of the study and researcher at the European Southern Observatory (ESO). “Corresponding high-resolution images of Betelgeuse from December 2019 show areas of varying brightness. Together with our result, this is a clear indication of huge star spots covering between 50 and 70% of the visible surface and having a lower temperature than the brighter photosphere.”