Betelgeuse's 'Great Dimming' had an unlikely observer: a Japanese weather satellite

An artist’s illustration of the red supergiant Betelgeuse. Its surface in this view is covered by large starspots, which reduce its brightness. During their pulsations, such stars regularly release gas into their surroundings, which condenses into dust. (Image credit: Graphics Department/MPIA)

Astronomers have a new way to study stars: take advantage of meteorological satellites orbiting Earth.

That's the conclusion of a new paper that presents new data from a Japanese weather satellite that just so happened to observe the red supergiant star Betelgeuse during a period of inexplicable dimming. The serendipitous observations could mean a new tool for astronomers trying to understand how a red supergiant star loses mass and ultimately explode as a supernova.

The second brightest star in the constellation Orion, Betelgeuse (pronounced "bet-orl-gerz" or "beetlejuice") is the 10th-brightest star in the night sky. But from October 2019 through February 2020, it dramatically dimmed to about two-thirds its normal brightness. This so-called "great dimming" event led to speculation that it was about to explode as what scientists call a Type IIP supernova, which it will certainly do within the next 100,000 years.

Scientists analyzing the event used predominantly data from ground-based optical telescopes. Astronomers mostly concluded that Betelgeuse's dimming was the result of either its surface cooling, a new band of dust forming around it or both.

Ground-based telescopes cannot see through dust and gas in the cosmos, which requires infrared vision. That's because Earth's atmosphere blocks infrared radiation as well as X-rays, gamma rays and the majority of ultraviolet rays. So only space-based observatories can see infrared light — and that includes weather satellites like Himawari-8, one of the Japan Meteorological Agency's geostationary weather satellites.

And Himawari-8's astrophysics debut began in an unlikely place: Twitter.

"We saw a tweet stating that the moon was in its images," Daisuke Taniguchi, a Ph.D. student in astronomy at the University of Tokyo and first author of the paper, told Space.com. "I chatted with [third author] Shinsuke Uno on the usage of meteorological satellites for astronomy, found Betelgeuse is in the field of view of Himawari-8 and realized that maybe the Great Dimming of Betelgeuse could be investigated."

Himawari-8 has been positioned 22,236 miles (35,786 kilometers) above Earth's equator since 2015 to study weather and natural disasters (including the eruption of the Hunga Tonga-Hunga Haʻapai volcano on Jan. 15). Although the satellite is up there to image Earth every 10 minutes, the edges of its images include stars.

Taniguchi and his colleagues were able to see Betelgeuse in images taken throughout Himawari-8's lifetime and measured its brightness roughly every 1.7 days between January 2017 and June 2021. And the satellite's Advanced Himawari Imager (AHI) studied Betelgeuse in two ways.

"In the optical and near-infrared wavelength ranges, the circumstellar dust veils the light from the stellar surface," Taniguchi said, explaining that the researchers — like the astronomers limited to using ground-based telescopes — were able to estimate the amount of circumstellar dust around Betelgeuse.

However, circumstellar dust emits only mid-infrared light. "By the observation of such mid-infrared light we can see the dust itself and we can directly measure the time-series of the amount of dust around Betelgeuse," Taniguchi said. The team concluded that the "great dimming" in 2019 and 2020 was caused by two factors in almost equal proportion: the star's temperature fell by approximately 250 degrees Fahrenheit (140 degrees Celsius) and dust condensed from warm gas around the star.

Crucially, this theory is in broad agreement with what astronomers using ground-based telescopes concluded. For example, a study led by the Chinese Academy of Science cited giant sunspots and temperature fluctuations while results from the European Southern Observatory's Very Large Telescope in Chile and from the Hubble Space Telescope suggested Betelgeuse ejected a huge cloud of gas that cooled and condensed into dust.

The scientists' new findings suggest that meteorological satellites could be used as space telescopes for astronomy. "It enables us to obtain high-cadence time series of mid-infrared images, which are hard to acquire with the usual astronomical instruments," reads the paper. As well as not being able to record near-infrared data, ground-based telescopes lose sight of some stars for a few months as the sun drifts in front of them.

"This is a possibility I haven't seen explored much previously," Emily Levesque, author of "The Last Stargazers" and an astronomer focused on red supergiant stars at the University of Washington who was not involved in the new research, told Space.com.

"It certainly depends in part on serendipity, but observations like these could prove to be a fabulous resource for bright, nearby red supergiants," she said. "Particularly since they could complement the upcoming capabilities of the James Webb Space Telescope, which is well-suited to observations of dimmer targets."

Observing stars in the mid-infrared is the best means of directly observing emission from dust around them, Levesque noted, because it can help create a multi-wavelength picture of massive stars and their evolution. After all, mass loss and dust production plays a key role in a star's red supergiant stage.

"The mid-infrared has also historically been difficult to observe," she said, adding that NASA's soon-to-be decommissioned SOFIA airborne observatory has filled a gap while JWST will soon become an invaluable mid-infrared resource. "Combined with creative solutions like the ones presented in this paper we'll hopefully continue to build a much clearer view of red supergiants in this wavelength range in the coming years."

The authors have already begun using Himawari-8's data for other stellar projects. "I believe that our concept of using a meteorological satellite as a space telescope is useful for several kinds of topics in astronomy, especially in time-domain stellar astrophysics," Taniguchi said, referring to the emerging area focused on how astronomical objects change over time. His group is now using Himawari-8 data to make a catalog of how older stars vary in infrared brightness over time and also to search for fleeting infrared signals.

At about 548 light-years away, Betelgeuse is the closest red supergiant star to the solar system. It's about 15 to 20 times the mass of the sun and about 900 times larger. If the giant was at the center of our solar system, then Mercury, Venus, Earth, Mars and the asteroid belt would all be inside Betelgeuse.

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