The mystery of Betelgeuse's 'Great Dimming' has been solved thanks to a satellite photobomb.


Much of the world was preoccupied with a reddish, fading point of light more than 500 light-years distant in late 2019, just months before the COVID-19 epidemic will overrun the globe. Betelgeuse, the crimson supergiant star that can be seen as the constellation Orion's right "shoulder," has unexpectedly diminished by more than a factor of two. Some scientists believed it was on the edge of bursting as a supernova, which is expected to happen in the next 100,000 years or so. The fading had halted by early February 2020, and the star had recovered to its normal brilliance within weeks, leaving experts with unanswered concerns regarding this strange phenomenon known as the "Great Dimming."

The answers came gradually from a slew of observatories that had been paying close watch to the star. First, experts who used the Hubble Space Telescope to view Betelgeuse before, during, and after the event reported that a tremendous ejection of hot material from the star's surface had formed an obscuring cloud of dust, which caused the apparent dimming. Then, using data from China's Weihai Observatory, a different team discovered that Betelgeuse's temperature had dropped by at least 170 kelvins during the Great Dimming, and the researchers attributed the drop to a very large, relatively cool dark spot that they concluded must have formed briefly on the star's surface, rather than a dust cloud.Finally, observations made with Chile's Very Large Telescope led to the conclusion that both possibilities were right. The formation of a black patch in the star's southern hemisphere had decreased surrounding temperatures and spit forth a bubble of hot gas, according to this hybrid model. As the departing material cooled, a massive, starlight-blocking dust cloud developed, resulting in the Great Dimming.

Now, an unusual telescope—a camera aboard a weather satellite—has joined the fray, bringing with it a new set of data. When three graduate students at the University of Tokyo noticed that Betelgeuse appeared in the field of vision of Japan's Earth-observing satellite Himawari-8 during the Great Dimming, they decided to dig further into archive photographs acquired by the satellite during the Great Dimming. Their findings, which were published in Nature Astronomy, corroborate the two-part concept while also offering the intriguing prospect of using data from additional meteorological satellites for a variety of astronomical studies. The photographs from Himawari-8 have even prompted the National Oceanic and Atmospheric Administration to investigate if one of its own satellites can reproduce the results.

Andrea Dupree, an astronomer at the Harvard-Smithsonian Institute for Astrophysics who is aware with the findings, said, "It's quite ingenious what they've done." "Of course, I adore the end outcome." Dupree was the lead author of a previous research that utilized Hubble data to link the Great Dimming to Betelgeuse ejecting a dust cloud, a finding that she admits was controversial at the time.

Dupree isn't afraid to experiment with different ways when it comes to making difficult observations. From April to August, Earth's orbit around the sun brings Betelgeuse so near to our star in the sky that the accompanying glare makes observations from most ground-based or low-Earth-orbit observatories impossible. A telescope located elsewhere in the solar system or in certain high orbits around Earth may nevertheless have an unobstructed view of the planet. Dupree approached NASA's Goddard Space Flight Center in early 2020, requesting permission to use the agency's STEREO-A spacecraft, which circles the sun rather than Earth, to obtain another look at Betelgeuse during the summer months, spurred on by the Great Dimming. Despite her own ingenuity, Dupree claims she would never have considered using a robot.

A Tweet sparked the idea to use Himawari-8 data. Lead author Daisuke Taniguchi was surfing through Twitter when he came upon a tweet regarding Earth's moon photobombing some of Himawari-8's photographs. He wondered if Betelgeuse might be observed with the weather satellite as well. There were various advantages to the concept that made it appealing. "Ground-based telescopes are necessarily hampered by the Earth's atmosphere, and they are unable to view numerous infrared wavelength ranges," Taniguchi explains. While space-based telescopes do not face this obstacle, obtaining observation time on them is "very difficult."

So Taniguchi reached out to Kazuya Yamazaki, a fellow graduate student and eventual research co-author, to see if they could get around the competition and collect their own data. "I wasn't totally convinced at first," Yamazaki says, "since [in Himawari-8's photographs] the stars are really dim, compared to the moon." Yamazaki, Taniguchi, and a third graduate student, study co-author Shinsuke Uno, decided to give it a shot.

Betelgeuse is not difficult to notice when it comes within Himawari-8's range of view—it appears as a dot hanging exactly at the edge of Earth's disk. It also has the advantage of being brilliant at both optical and infrared wavelengths, which increases the likelihood of it being detected by meteorological satellite detectors, which are not meant for astronomical uses. Finding the star in satellite photos is one thing; utilizing the data to make precise stellar measurements is quite another. The most difficult and time-consuming element of the study, according to Yamazaki, was data wrangling.

Dupree has sought the support of Jon Fulbright, a calibration scientist with NASA and NOAA's Geostationary Operational Environmental Satellite-R (GOES-R) family of weather-monitoring satellites, to determine if the Himawari-8 result can be replicated using those spacecraft. Fulbright is still attempting to glean insights on Betelgeuse from the GOES-R data as of this writing, and is struggling with the time-consuming unit conversions and pixel scaling necessary for the process. According to him, the advantages of employing such an unusual data source may not always exceed the disadvantages.

"I'm undecided about whether this is a one-time affair," adds Fulbright. He and his colleagues, like the Japanese team, believe that in order for this unique technique to attain its full potential, improved methods for bridging the gaps between meteorological and astronomical data sets must be created. However, such potential astronomical synergies may only arise if future generations of Earth-observing satellites are built with them in mind. "Perhaps something like this will get people's thoughts flowing," he says.