Weather satellite accidentally captures Betelgeuse’s Great Dimming

Betelgeuse is the 10th-brightest star in the night sky and the second-brightest in the constellation Orion, though the red supergiant dimmed to approximately one third its normal brightness in 2019–20. This so-called Great Dimming was accidentally captured by a weather satellite called Himawari-8.

Himawari-8’s observations have been presented in a paper published in Nature Astronomy. The results suggest that the perceived dimming was caused by a combination of the cooling of the star and condensation of nearby dust.

Lead author of the paper, Daisuke Taniguchi, an astronomer at Japan’s University of Tokyo, says Himawari-8 is a meteorological satellite in geostationary orbit around Earth, able to observe almost half the planet’s surface. The satellite’s main purpose is “daily weather forecasting, investigation of the weather”.

So, how is it that this weather satellite is now informing us about Betelgeuse, approximately 724 light years from Earth?

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Researchers realised that, in capturing information about Earth’s climate, Himawari-8 might have picked up signals from Betelgeuse in the background. Sure enough, further analysis of the images from 2019–20 showed this to be the case.

Taniguchi’s team pieced together four-and-a-half years of observations from the satellite, including the six-month period when Betelgeuse dimmed. The optical and infrared radiation that Himawari-8 normally detects were ideal for studying the dimming. Additionally, its orbit above Earth meant that it could record infrared light usually blocked by the atmosphere.

Betelgeuse observed with the Himawari-8 meteorological satellite near the edge of the Earth’s disk. (a) False-colour image taken by the Himawari-8 satellite on 19 January 2020. Betelgeuse is located at the top right corner of the image. (b),(c), Zoomed-in images. Credit: D. Taniguchi et al.

Supporting earlier ground-based observations, the authors suggest that Betelgeuse’s dimming was caused in equal parts by the star cooling by approximately 140°C, and by a neighbouring dust cloud condensing from warm gas around it.

Taniguchi says “it is now well known that a part of the cause of the dimming is a decrease in the surface temperature – as a technical term, it’s called “effective temperature” – of Betelgeuse. However, it has been under debate what is the other part of the cause.

“Some suggested that the dust condensation just above the surface of the star is responsible for this part, but others pointed out that the dust condensation is not required to explain the dimming and that, instead of the dust scenario, increased patchiness of the stellar surface or a newly formed gas cloud might be the cause.”

Other works that have tried to answer this question about the cause of the “other part” of the dimming use “optical and near-infrared data, and thus inevitably suffer from inaccuracy in theoretical models, which are used to determine the cause,” Taniguchi says.

Thanks to Himawari-8’s observations, the team was able to determine the amount of dust around Betelgeuse over time. “This is because that dust emits mid-infrared light (around 10 micrometre wavelength light), and thus it is relatively easy (and relatively unaffected by model uncertainties) to determine the amount of dust,” Taniguchi says.

“As a result, we found that the amount of dust may have, in fact, increased during the Great Dimming phase, and thus we conclude that the Great Dimming of Betelgeuse was due to both the decreased surface temperature and the increased dust extinction.”

In addition to providing answers to the cause of the Great Dimming of Betelgeuse, Taniguchi argues that the work provides new avenues for the study of stars. “We pioneered in a new area of ‘time-domain stellar astrophysics making use of weather satellites’. This concept will provide astronomers a new way to investigate the time-variable infrared universe.”

He notes that weather satellites also provide advantages for stellar observations compared to telescopes on Earth, which have to contend with distortion effects caused by the atmosphere and the Sun. “Ground-based observatories can observe in the mid-infrared, but only within a small number of atmospheric windows,” he says. “Moreover, ground-based observations inevitably suffer from the Sun in the optical and infrared. For example, Betelgeuse cannot be observed on the ground for several months around August, but we can with Himawari-8.”

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