The sudden – huge – decrease in brightness is probably the result of gigantic sunspots.

In recent months, astronomers have been captivated by one of the brightest stars in the night sky: the nearby red supergiant Betelgeuse. Although this famous, red-colored star normally shines particularly brightly, the brightness had diminished very quickly since October. Several statements have already been reviewed. Because maybe the star was about to explode? Or is it just a bit dusty at the moment? Neither, it says new research. No, the dimming of the star is the result of… sunspots.

What is going on?
The Betelgeuse star is about 600 light-years away from Earth. Nevertheless, the star is easy to see on clear evenings; it is quite bright. In addition, Betelgeuse clearly distinguishes itself from other stars by the red color it has. In recent months, the star was discredited because its brightness decreased rapidly. Although red giants regularly show variations in brightness, the striking decrease in brightness of Betelgeuse really came as a surprise to astronomers. On December 7, the brightness had decreased to a magnitude of +1.12. And in mid-February, Betelgeuse even had only 36% of its normal brightness. A huge decrease that even catches the eye. Never before had Betelgeuse had such limited brightness in the more than 25 years that astronomers monitored the star. This phenomenon was also beautifully captured on unique images of the Very Large Telescope.–

For a moment, expectations were high and astronomers hoped that the star would explode soon. Although the star is still young (the star is about 8 million years old, our sun is about 4.5 billion years old), it is no longer long-lived and will die ‘soon’. That is because he is running out of fuel at a rapid pace. Without that fuel – or source of energy – the star can no longer compete with gravity and part of its own mass flows towards the core. That core gets heavier and heavier and is ultimately doomed to collapse. The result is a massive explosion: a supernova. It is expected to be visible from Earth even in broad daylight. When is this about to happen? Somewhere in the next 100,000 years. So it could take tens of thousands of years, but it could be tomorrow, so to speak.

Dusty
While many would have liked to see the star explode, the clarity of Betelgeuse suddenly increased again at the end of February. It meant that there probably had to be another cause for the mysterious decrease in brightness. In early March, astronomers formulated a credible hypothesis: the star had produced dust that hung between us and the star and blocked part of the starlight, making it seem as if the brightness of Betelgeuse was diminishing. “We see it all the time in red supergiants and it’s a normal part of their life cycle,” said researcher Emily Levesque at the time. But this theory is also sweeping a new research team off the table.

Wavelengths
The researchers studied Betelgeuse using the Atacama Pathfinder Experiment (APEX) and the James Clerk Maxwell Telescope (JCMT). These telescopes measure radiation from the spectral range of submillimeter waves (terahertz radiation). These wavelengths are a thousand times longer than that of visible light. Astronomers use this method to study interstellar dust. Because especially cool dust glows at these wavelengths. “What surprised us was that Betelgeuse darkened 20 percent even in submillimeter wavelengths,” said researcher Steve Mairs. And experience shows that such behavior does not correspond to the presence of dust. For a more thorough evaluation, the researchers calculated what the influence of dust would be on measurements within this spectral range. And this shows that a decrease in brightness in submillimeter wavelengths cannot be attributed to an increase in dust. And so the researchers rule out that dust was the cause of the star’s dimming.

Sunspots
What caused the decrease in brightness? Possible temperature fluctuations in the photosphere. And such temperature changes are the result of gigantic sunspots. Physical laws tell us that a star’s brightness depends on its diameter, but mainly on its surface temperature. Therefore, as the size of a star decreases, the brightness in all wavelengths also decreases proportionately. Temperature changes, however, affect the radiation emitted along the electromagnetic spectrum. According to the researchers, the measured decrease in brightness of Betelgeuse is therefore evidence of a decrease in the average surface temperature. “A disproportionate temperature distribution is most likely,” explains researcher Peter Scicluna. Pictures of Betelgeuse from December 2019 show areas of varying brightness. This is a clear indication of huge sunspots that cover between 50 and 70 percent of the visible surface and have a lower temperature than the brighter photosphere. ”

These images from Betelgeuse show the distribution of brightness in visible light on the surface before and during the decrease in brightness. Due to the disproportionality, the authors conclude that there may be massive sunspots on Betelgeuse. Image: ESO / M. Montargès et al.

–

Sunspots are common in giant stars, although the observed scale on Betelgeuse is unprecedented. What we know is that our sun has an eleven-year cycle characterized by a solar minimum – a period when the sun is very quiet and generates few sunspots and flares – and a solar maximum, in which the star is much more active. Whether giant stars have a similar mechanism is unknown. “Observations in the coming years will tell us whether the sharp decrease in Betelgeuse’s brightness is related to such a cycle,” says Dharmawardena. “Betelgeuse will therefore remain an exciting object.”