A team of scientists from the Max Planck Institute for Astronomy believe that unusually large, cool surface spots are the most likely explanation for the recent dimming of the red giant star Betelgeuse that raised fears it was going supernova.
Betelgeuse, also known as α Orionjis, is located in the constellation of Orion about 700 light-years from the Sun and is one of the largest stars visible to the naked eye. In the last stage of its life as it burns the remnants of its hydrogen and helium, the ancient red giant is 20 times more massive than the Sun and if it was positioned where the Sun is, its diameter would reach out to the orbit of Jupiter. In 2019, it caused a bit of a stir when its brightness dropped to 40 percent of normal, sparking speculation that it was about to go supernova and explode with energy exceeding that of an entire galaxy.
By April 2020, the star had returned to its normal range of brightness, which varies between 0 and +1.6 magnitude. However, the question remained as to why it did dim so dramatically. One leading hypothesis was that it was due to dust being ejected from the star, but an international team led by Thavisha Dharmawardena has concluded that the culprit was cool regions or star spots on the surface of the star. These are similar to sunspots but cover 50 to 70 percent of Betelgeuse's surface.
The evidence to support this idea came from new and archival data collected from the Atacama Pathfinder Experiment (APEX) and the James Clerk Maxwell Telescope (JCMT), which measured radiation in the terahertz or submillimeter band of the spectrum. The reason for this is that dust glows in terahertz wavelengths, but Beletelgeuse became 20 percent darker even in this range, which surprised the scientists as such behavior isn't compatible with the presence of dust.
This suggested that it was the brightness of the star itself that was to blame. According to the team, the luminosity of a star depends on its diameter and its surface temperature. If a star becomes smaller – as it would before going supernova – it becomes dimmer uniformly across the entire spectrum, but if it becomes cooler, the spectrum darkens unevenly. The question remains, is this an anomaly or related to Betgelgeuse's natural variability?
"Observations in the coming years will tell us whether the sharp decrease in Betelgeuse's brightness is related to a spot cycle," says Dharmawardena. "In any case, Betelgeuse will remain an exciting object for future studies."
The team's research is published in The Astrophysical Journal Letters.
Source: Max Planck Institute