Between late 2019 and early 2020, the red supergiant star Betelgeuse experienced the so-called "Great Dimming". The star's brightness dropped suddenly and remarkably. It became so faint, that it dropped off the top 20 brightest stars in the night sky list. It was so dramatic that even in cities, people could see that Orion was missing the star that makes his shoulder.

The leading explanation for the extreme dimming event was a combination of the star's periodical variation combined with a sudden release of dust. New observations, published in Nature Astronomy, confirm that scenario. And they come from a very unlikely source. A weather satellite.

Himawari-8 is a Japanese weather satellite that has been in geostationary orbit since 2014. Lead author of the new paper Daisuke Taniguchi and colleagues discovered that Betelegeuse photobombed several weather images taken by the satellite as it goes around Earth. The satellite has instruments that see our planet and the rest of the cosmos in mid-infrared and the fact that it takes so many photos at regular intervals gave the team an idea.

There must be many images and data in the archive related to Betelgeuse. The team was able to collect observations of Betelgeuse between 2017 and 2021 and it provided some very insightful results. According to the data collected by Himawari-8, Betelgeuse’s temperature dropped by about 140 °C (252 F°). But that was only responsible for half of the change.

The other half came from the dust. The Japanese satellite has a high sensitivity for particular wavelength bands where signals from water molecules are strong. It is designed to study water in the Earth’s upper atmosphere but it turns out to be so good that it was able to detect water around Betelgeuse too. And the team tracked this over the years.

Other molecules, such as silicon oxide, could muddle up the signal that the team saw as an absorption signal from the water molecules, but the astronomers believe that even with limitations, the drop related to the dimming is clearly there.

Water molecules and the many other dusty components around Betelgeuse contributed to the star appearing fainter. And according to this work, the drop in temperature and the dust contributed to the Great Dimming in equal measure.

The team was able also to collect data on four other stars. Their work shows that weather satellites, due to their high-precision instruments and location, can be used for much more than tracking clouds and sunshine. They can also be used for astronomical observations without any special adroitness, getting more out of the standard data they collect.