When Betelgeuse mysteriously darkened in late 2019 by more than one magnitude, astronomers around the world rushed to turn their telescopes toward the red giant star on Orion’s shoulder.
Little did they know they were joined by a Japanese weather satellite named Himawari-8.
From its geostationary orbit, Himawari-8 takes high-resolution images of Earth, capturing clouds, changes in vegetation – and occasionally photobombing astronomical objects, appearing just off the Earth’s limb. Inspired by a tweet from a Himawari-8 image who had captured the Moon by chance, two graduate students from the University of Tokyo, Daisuke Taniguchi and Shinsuke Uno, wondered if they could exploit the records of the weather satellite for astronomical research.
They found that Betelgeuse appeared in images of Himawari-8 approximately once every 1.72 days – including before, during and after its enigmatic dimming period from 2019 to 2020. Additionally, Himawari-8’s camera 8 operates at mid-infrared wavelengths, where it can see temperature differences between clouds and the ground. This range of wavelengths is also good for observing astronomical dust, such as the dust that shrouds young stars – or the dust that some astronomers believe temporarily blocks Betelgeuse’s light and darkens it.
To help analyze data from Himawari-8, Taniguchi and Uno recruited Kazuya Yamazaki, a graduate student in meteorology, and the trio published their results on May 30 in natural astronomy. From the satellite data, they were able to estimate the amount of dust around Betelgeuse. They found that the dimming is most likely due to a combination of dust and also a cooling of the star’s temperature by about 250 degrees Fahrenheit (140 degrees Celsius). This is consistent with one of the more popular theories – that Betelgeuse expelled a hot clump of gas that condensed into dust when exposed to a cold region on the star’s surface.
The team believes weather satellites have a lot of unrealized potential as astronomical telescopes. Mid-infrared radiation is blocked by the Earth’s atmosphere, making it invisible to ground-based telescopes. There is also a shortage of space observatories currently operating in mid-IR. The Spitzer Space Telescope was out of service since January 2020and the SOFIA airborne observatory will also be closed later this year. While the next James Webb Space Telescope will be able to observe in the mid-IR, its time is so precious that it won’t be able to repeatedly look at an object for years like weather satellites can.
Taniguchi says he and his colleagues have already started other projects with the Himawari-8 data, including creating a catalog of dozens of other giant stars and searching for transient infrared objects. “I hope other astronomers around the world will start their own projects using Himawari-8 or other weather satellites,” he said. Astronomy.