Betelgeuse is the 10e-brightest star in the night sky and second-brightest in the constellation Orion, although the red supergiant has dimmed by about a third of its normal brightness in 2019-2020. This so-called Great Dimming was accidentally captured by a weather satellite called Himawari-8.
Observations of Himawari-8 were presented in an article published in natural astronomy. The results suggest that the perceived dimming was caused by a combination of the star’s cooling and nearby dust condensation.
The paper’s lead author, Daisuke Taniguchi, an astronomer at Japan’s University of Tokyo, explains that Himawari-8 is a weather satellite in geostationary orbit around the Earth, capable of observing almost half of Earth’s surface. the planet. The main purpose of the satellite is “daily weather forecasting, weather study”.
So how come this weather satellite is now telling us about Betelgeuse, about 724 light-years from Earth?
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The researchers realized that while capturing information about Earth’s climate, Himawari-8 could have picked up signals from Betelgeuse in the background. Sure enough, further analysis of footage from 2019-2020 showed that to be the case.
Taniguchi’s team gathered four and a half years of observations from the satellite, including the six-month period in which Betelgeuse faded. The optical and infrared radiation that Himawari-8 normally detects was ideal for studying dimming. Additionally, its orbit above Earth meant it could record infrared light typically blocked by the atmosphere.
In support of previous ground-based observations, the authors suggest that Betelgeuse’s dimming was caused in equal parts by the star’s cooling of around 140°C and by a nearby dust cloud condensing from the hot gas surrounding it.
Taniguchi says “it is now well known that part of the cause of the dimming is a decrease in the surface temperature – as a technical term it is called ‘effective temperature’ – of Betelgeuse. However, it has been debated which is the other part of the cause.
“Some have suggested that dust condensation just above the star’s surface is responsible for this part, but others have pointed out that dust condensation is not necessary to explain the dimming and that, at instead of the dust scenario, an increased unevenness of the stellar surface or a newly formed gas cloud could be the cause.
Other work that has attempted to answer this question of what causes the “other part” of dimming uses “optical and near-infrared data, and therefore inevitably suffers from inaccuracies in theoretical models, which are used to determine the cause,” Taniguchi said.
Using observations from Himawari-8, the team was able to determine the amount of dust around Betelgeuse over time. “That’s because this dust emits mid-infrared light (light with a wavelength of about 10 micrometers), and so it’s relatively easy (and relatively unaffected by model uncertainties) to determine the amount of dust,” says Taniguchi.
“As a result, we found that the amount of dust may have, in fact, increased during the Great Dimming phase, and we therefore conclude that the Great Dimming of Betelgeuse was due to both the decrease in surface temperature and to increased dust extinction.”
In addition to providing answers to the cause of the Great Darkening of Betelgeuse, Taniguchi argues that the book opens new avenues for the study of stars. “We pioneered a new field of ‘time-domain stellar astrophysics using weather satellites’. This concept will provide astronomers with a new way to study the time-varying infrared universe.
He notes that weather satellites also offer advantages for stellar observations over telescopes on Earth, which have to deal with the distorting effects caused by the atmosphere and the Sun. “Ground-based observatories can observe in the mid-infrared, but only in a small number of atmospheric windows,” he says. “Furthermore, ground observations inevitably suffer from the Sun in optics and infrared. For example, Betelgeuse cannot be observed on the ground for several months around August, but we can with Himawari-8.