A mysterious darkening of the bright star Betelgeuse can be caused by sunspots and temperature shift

The mysterious, strange dimmer seen last year in the enormous Betelgeuse star may be the result of giant sunspots and temperature fluctuations.

A new study suggests that a "large dark spot" led to a decrease in the surface temperature of Betelgeuse, which in turn contributed to a temporary obscuration of the luminosity of the red giant or its inherent luminosity.

The sudden disappearance made headlines around the world in 2020, as speculation mounted on what was going on. Science is still not clear, but astronomers blame everything, from gas cloud emissions to dust, through stellar fluctuations before Betelgeuse explodes into a supernova.

The new study, led by the Chinese Academy of Sciences, looked at molecules in the spectra—or light signature—of Betelgeuse to try to figure out what was going on. To do so, they used Weihai Observatory (based at Shandong University in Jinan) four times in 2020 across the dimming and brightening period: on Jan. 31, March 19, April 4, and April 6.

To estimate the temperature of the star, astronomers have studied molecules of titanium oxide and cyanide that tend to form more readily in cooler stellar environments.

"The cooler a star is, the more these molecules can form and survive in its atmosphere — and the molecular lines are stronger in the stellar spectrum," lead author Sofya Alexeeva said in a statement released Thursday (Aug. 5). "In a warmer atmosphere, these molecules dissociate easily and do not survive,' said Dr. Alexeeva.

When Betelgeuse was at its dimmest on Jan. 31, 2020, its effective temperature — meaning, the temperature calculated from its emitted radiation — was measured at 3,476 degrees Kelvin (about 5,800 degrees Fahrenheit or 3,200 degrees Celsius.)

But once the star returned to normal light, the measurements indicated a temperature increase of almost 5% to 3,646 Kelvin (approximately 6,100 F or 3,370 degrees C.)

Considering that Betelgeuse could still be several hundred years away from going supernova, astronomers said in their research paper, which was published on Thursday (August,5) in Nature Communications, they concluded that the entire surface is unlikely to be temporarily cooled by that quantity. They said it must be a sunspot—or rather a "star spot"—that prevented some of the Betelgeuse radiation from escaping.

Star blotches, such as sunspots that appear on the sun, are common on giant red stars such as Betelgeuse. Spots appear when disturbances in the star's magnetic field flow to the photosphere or visible surface of the star. Sunspots tend to be large clusters of magnetic activity and can lead to flares or emissions of particles known as coronal mass ejections — the source of the solar wind in our solar system.

While this temperature study was focused on Betelgeuse's dimming, the team said future research in this field could better inform our study of all red giants — the primary source of heavy elements in the universe, due to the stars' tendency to explode and spew material.