19.06.2022

Images from Japan’s Himawari-8 spacecraft shed light on the red supergiant star’s remarkable fading

This artist's impression was generated using an image of Betelgeuse from late 2019 taken with the SPHERE instrument on the European Southern Observatory's Very Large Telescope. Credit: ESO, ESA/Hubble, M. Kornmesser

In late 2019, mere months before the COVID-19 pandemic would engulf the globe, much of the world was instead concerned with a ruddy, fading point of light more than 500 light-years away. Betelgeuse, the red supergiant star easily recognizable as the right “shoulder” of the constellation Orion, had suddenly and mysteriously dimmed by more than a factor of two. Some astronomers speculated that it was on the verge of exploding as a supernova—an event otherwise predicted to occur within the next 100,000 years or so. By early February of 2020, however, the fading had stopped, and within weeks the star had returned to its regular brightness, which left researchers with lingering questions about this bizarre episode they called the “Great Dimming.”

The answers emerged gradually from a host of observatories lavishing the star with attention. First, a team of researchers who had used the Hubble Space Telescope to observe Betelgeuse before, during and after the event reported that a massive ejection of hot material from the star’s surface had created an obscuring cloud of dust that led to the apparent fading. Then a different team using data from the Weihai Observatory in China found that Betelgeuse’s temperature had plummeted during the Great Dimming by at least 170 kelvins, and the researchers attributed the plunge not to a dust cloud but rather to a very large, relatively cool dark spot they concluded must have briefly formed on the star’s surface. Finally, yet another team used observations with the Very Large Telescope in Chile to conclude that both scenarios were correct. In this hybrid model, the emergence of a dark spot in the star’s southern hemisphere had lowered surrounding temperatures and spat out a bubble of hot gas. An enormous, starlight-blocking dust cloud then formed from this escaping material as it cooled, creating the Great Dimming.

Now an unconventional telescope—a camera on a weather satellite—has entered the mix with another novel suite of observations. After realizing that Betelgeuse appears in the field of view of Japan’s Earth-observing satellite Himawari-8, three graduate students at the University of Tokyo decided to take a closer look at archival images captured by the satellite during the Great Dimming. Their results, published in Nature Astronomy, support the twofold hypothesis while also raising the exciting possibility that data from other meteorological satellites may be repurposed for a broad range of astronomical observations. The study of Himawari-8’s images has even inspired National Oceanic and Atmospheric Administration to explore whether one of its own satellites can replicate the findings.

“It’s very clever what they’ve done,” says Andrea Dupree, an astrophysicist at the Harvard-Smithsonian Institute for Astrophysics, who is familiar with the research. “And of course, I love the result.” Dupree led the earlier study that used Hubble data to link the Great Dimming to Betelgeuse burping out a dust cloud—a conclusion that she notes was initially met with much debate.

Dupree is no stranger to using unconventional methods to make tricky observations. From April to August, Earth’s orbit around the sun brings Betelgeuse so close in the sky to our star that the resulting glare scuttles observations from most telescopes on the ground or in low-Earth orbit. A telescope stationed elsewhere in the solar system or in certain special high orbits around Earth could still have an unimpeded view. Spurred by the Great Dimming, in early 2020 Dupree contacted officials at NASA’s Goddard Space Flight Center to ask to use the agency’s STEREO-A spacecraft, which orbits the sun rather than Earth, to get another look at Betelgeuse during the summer months. But despite her own creativity, Dupree says she would’ve never thought to use a meteorological satellite.

The idea to employ Himawari-8 data started with a Tweet. While scrolling Twitter, lead author Daisuke Taniguchi saw a post about Earth’s moon photobombing some of Himawari-8’s images. He wondered if the weather satellite could be used to observe Betelgeuse, too. There were several benefits that made the idea intriguing. “Ground-based telescopes inevitably suffer from the Earth’s atmosphere and cannot observe many parts of the infrared wavelength ranges,” Taniguchi says. And while space-based telescopes do not have that barrier, the competition to obtain observation time on them is “very severe.”

So Taniguchi got in touch with fellow graduate student and eventual study co-author Kazuya Yamazaki to see if they could circumvent the competition and make their own observations. At first, Yamazaki recalls, “I wasn’t fully confident because [in Himawari-8’s images] the stars are very dark, compared to the moon.” But together with Taniguchi and a third graduate student, study co-author Shinsuke Uno, Yamazaki decided to try.

When it falls within Himawari-8’s field of view, Betelgeuse is not actually that hard to see—it appears as a dot hovering right at the edge of Earth’s disk. It also benefits from being bright at both optical and infrared wavelengths, boosting its chances of registering in meteorological satellite detectors, which are not designed for astronomical applications. But simply finding the star in satellite images is one thing—using the data to perform actual high-precision stellar measurements is another. Data-wrangling, Yamazaki says, was the most arduous, time-consuming part of the study.

Inspired by the Himawari-8 result, Dupree has enlisted the aid of Jon Fulbright, a calibration scientist at the product quality team for NASA and NOAA’s Geostationary Operational Environmental Satellite-R (GOES-R) series of weather-monitoring satellites, to see if those spacecraft could help replicate it. As of this writing, Fulbright is still trying to extract insights on Betelgeuse from the GOES-R data and is grappling with burdensome unit conversions and pixel resizing required for the task. The benefits of using such an unconventional data source, he says, may not always outweigh the drawbacks.

“I go back and forth on whether this is a one-time thing,” Fulbright says. Just like the Japanese team, he and his colleagues suspect that for this novel approach to reach its full potential, better methods must be developed to bridge the gaps between meteorological and astronomical data sets. But those possible synergies with astronomy may only emerge if new generations of Earth-observing satellites are designed with them in mind. “Maybe,” he says, “something like this will get people’s ideas going."

Quelle; SCIENTIFIC AMERICAN