Betelgeuse is going dim because it’s covered in star spots, research finds

Betelgeuse is normally one of the brightest stars in the night sky, but late last year something strange began happening to it: The star was becoming much less bright than it used to be. In December, astronomers announced that Betelgeuse was dimming dramatically, leading some to think that it could soon explode in an epic supernova.

Although it’s typical for red giant stars like Betelgeuse to vary in brightness over time, in this case, the star’s output dropped so dramatically that scientists figured something unusual must be going on.

Further study of the star in February this year showed that it had dimmed by almost two thirds, going down to just 36% of its usual brightness. That difference was enough that it could be observed with the naked eye, even from 500 light-years away. At the time, researchers theorized that the variations could either be due to a dust cloud being ejected from the star and blocking its light, or that the irregular surface of the star could be creating the brightness variations.

Red Supergiant Betelgeuse
An artist’s impression of the Red Supergiant Betelgeuse. Its surface is covered by large star spots, which reduce its brightness. During their pulsations, such stars regularly release gas into their surroundings, which condenses into dust. MPIA graphics department

New research suggests that this second theory is more likely, and that Betelgeuse’s unusual behavior is due to temperature variations in the lowest layer of the star’s atmosphere caused by star spots. Star spots are similar to sunspots, dark patches that temporarily appear on the surface of the sun due to magnetic fields and give rise to solar flares. These star spots cover between 50% and 70% of Betelgeuse’s entire surface.

“Towards the end of their lives, stars become red giants,” lead author Thavisha Dharmawardena of the Max Planck Institute for Astronomy explained 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.”

Elderly Betelgeuse has bloated to a size so large — 1,000 times the size of our sun, while only being 20 times the mass — that gravitational forces can easily pull off its outer layers. That leaves behind areas of cool dust and gas, with areas of varying temperatures across the surface that reduce the brightness of the star overall.

One open question is whether Betelgeuse will continue to dim or will rebound in brightness. We know that sunspots on our sun follow an 11-year cycle, but scientists aren’t yet sure if the same is true of star spots on giant stars.

“Observations in the coming years will tell us whether the sharp decrease in Betelgeuse’s brightness is related to a spot cycle,” Dharmawardena said. “In any case, Betelgeuse will remain an exciting object for future studies.”

The findings are published in the Astrophysical Journal Letters.

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