Giant Star Spots Believed To Have Caused The Mysterious Dimming Of Betelgeuse

An artist's impression of the Red Supergiant Betelgeuse. Its surface is covered by large star spots. MPIA graphics department

Starting in October of last year and continuing until a few months ago, the super-red giant star Betelgeuse experienced an unprecedented drop in brightness. It was so severe that it temporarily took it out of the top 20 brightest stars visible to the naked eye. The cause of it is still uncertain but new research strengthens the case that giant star spots were to blame for the dramatic dip in luminosity.

The star is variable, with its brightness waxing and waning in cycles of hundreds of days. Yet, it had never dimmed this much before, becoming 2.5 times less bright than usual. One possible explanation is that the star released a cloud of dust that blocked its light for a while. However, as reported in The Astrophysical Journal Letters, this didn’t match with the observations.

If something was filtering the light, it should affect certain wavelengths more than others, but this was not the case. The dimming was present in both optical light and millimeter-wave observations. For this reason, researchers believe that it was caused by large star spots, blemishes on the surface of the star, which accentuated the regular brightness cycle. 

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

This comparison image shows the star Betelgeuse before and during its unprecedented dimming with the SPHERE instrument on ESO’s Very Large Telescope. ESO/M. Montargès et al.

Beautiful images of the star taken late last year show that changes in its brightness were not everywhere. Combined with the new observations, the researchers suggest that star spots could be covering between 50 and 70 percent of the star.

Spots on the Sun come and go regularly, intensifying during the Solar Maximum, a phase of intense activity that happens every 11 years. It is unknown how star spots work on other stars, especially on evolved stars such as Betelgeuse. Recently though, star spots have been used to explain peculiar features of other aged stars.

Betelgeuse's previous brightness cycle also featured an increased dimness, although not to the level of this one. The primary cycle lasts about 420 to 430 days and researchers are interested in what the next minimum will look like.

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

Located about 700 light-years away, Betelgeuse is 20 times more massive than the Sun and roughly 1,000 times larger. If it was in the Solar System, it's surface would almost reach the orbit of Jupiter.

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