A Giant Star is Fading Away. But First, it Had an Enormous Eruption

About 16,000 light-years away, a massive star experienced an unusual dimming event. This can happen in binary stars when one star passes in front of the other. It can also be due to intrinsic reasons like innate variability. But this star dimmed by as much as one-third, a huge amount.

What happened?

The star is named RW Cephei. It’s one of the largest stars we know of. Its radius is almost 1,000 times as large as the Sun’s. Put another way, it’s almost as large as Jupiter’s orbit.

Many stars, maybe all of them, exhibit some variability in their luminosity, though it’s often very small. RW Cephei is considered a semiregular variable star, which means its variation is definite on occasion but is otherwise irregular. But this dimming episode was too pronounced to be attributed to intrinsic variability, and RW Cephei has no binary companion.

Astronomers working with Georgia State University’s CHARA Array spotted the dimming event last year. CHARA is an array of six 1-metre telescopes that work in conjunction as an interferometer. The team behind the new research presented their findings at the 243 AAS Conference, and it’s also published in The Astronomical Journal. Its title is “The Great Dimming of the Hypergiant Star RW Cephei: CHARA Array Images and Spectral Analysis.” The lead author is Narsireddy Anugu, an astronomer and optical systems scientist at the CHARA Array.

CHARA consists of six one-meter telescopes and related facilities. Image Credit: CHARA/GSU

“We made our first CHARA observations in December 2022, just before the winter weather closure, but the results were so remarkable we decided to pursue additional observations once the star was accessible again,” said Anugu.  Anugu led an international team of scientists in a quest to make the first close-up pictures of RW Cephei to determine the source of the fading.

Astronomers Wolfgang Vollmann and Costantino Sigismondi announced in 2022 that RW Cephei had faded dramatically over the previous few years. The pair reported their findings in The Astronomers Telegram. “The star is significantly dimming instead of rebrightening,” the pair wrote. “This phenomenon undergoing on RW Cep might be similar to the one that occurred on the red supergiant Betelgeuse at the end of 2019.”

Astronomers figured out that Betelgeuse’s dimming was likely caused by an ejection of gas that cooled into dust and blocked some of the star’s light. Could a similar mechanism be behind RW Cephei’s dimming? Maybe, but Betelgeuse didn’t dim by one-third, whereas RW Cephei did.

CHARA stands for Center for High Angular Resolution Astronomy. The high angular resolution allowed astronomers to get a pretty detailed look at the star, even though it’s about 16,000 light-years away. RW Cephei is also a huge hypergiant, which helps. CHARA can see things smaller than a human on the Moon.

CHARA Array false-color images of RW Cephei from December 2022 (left) and Jul 2023 (right). The patchy appearance results from dust created by a huge ejection from the star. The star is huge, but it is so far away that it appears about one million times smaller than the full moon in the sky. By July 2023, the star was brightening again. Image Credit: CHARA/Anugu et al. 2023

CHARA images showed that the star wasn’t round, a very unusual finding. But scientists couldn’t be sure if that was correct. They used specialized computer programs to refine the image.

“The spacing of the CHARA telescopes induces a level of uncertainty in the exact details of the pictures, so we need intelligent algorithms to recover the whole image,” said Fabien Baron. Baron is a co-author of the paper and also wrote the computer algorithms.

These two images from the research show RW Cephei in two separate bands. The H band is on the left, and the K band is on the right. The images show an asymmetry between the star’s brighter left-hand side and the dimmer right-hand side. Image Credit: Anugu et al. 2023

The images show that the star’s surface is undergoing convulsions, altering its round appearance. There are also brighter and darker patches on its surface, and its overall appearance changed during the ten-month period of observations that covered its dimming and re-brightening. “The most striking features in the reconstructed images are the large variations in brightness across the visible hemisphere of the star,” the researchers write in their paper.

Another of the study’s co-authors, Katherine Shepard of the Apache Point Observatory in New Mexico, decided to try a different tactic. She used a camera to record both visual and infrared light coming from RW Cephei. Her observations showed that the dimming was much more pronounced in visible light than in infrared.

Those observations were a telltale sign that dust was blocking the light. The researchers concluded that the same thing that happened to Betelgeuse happened to RW Cephei. The star shed some of its gas in a massive outburst, and the gas cooled into dust that blocked one-third of the star’s visible light.

“The Great Dimming of RW Cep may be the latest in a series of mass ejections over the last century.”

From “The Great Dimming of the Hypergiant Star RW Cephei: CHARA Array Images and Spectral Analysis.”

Both Betelgeuse’s dimming and RW Cephei’s dimming are due to mass loss.

This comparison image shows the star Betelgeuse before and after its unprecedented dimming. The observations, taken with the SPHERE instrument on the ESO’s Very Large Telescope in January and December 2019, show how much the star has faded and how its apparent shape has changed. Image Credit: ESO/VLT/SPHERE

Aging stars lose mass as they burn their nuclear fuel. This weakens their gravity, and the aging star is unable to hold onto all of the material in its outer layers. Though there are many unanswered questions about the details of the process, both Betelgeuse and RW Cephei show that mass loss can occur in periodic violent outbursts. These outbursts then block the star’s light for a period of time before being dissipated. As the dust dissipates, the star begins to return to its normal brightness.

“The Great Dimming of RW Cep may be the latest in a series of mass ejections over the last century,” the authors write in their paper. “Thus, the current fading may be the latest of continuing mass ejection and dust formation episodes, and the newly formed dust now partially obscures the visible hemisphere.”

“We suggest that the maximum light time may have corresponded to a particularly energetic convective upwelling of hot gas that launched a surface mass ejection event,” the authors explain. “This gas is now cooling to the point of dust formation, and the part of the ejected cloud seen in projection against the photosphere causes the darker appearance of the western side of the star.”

The researchers also point out that the duration of these dimming events is related to the size of the star and the dust cloud it ejects. Betelgeuse is smaller than RW Cephei, and its event lasted about one year, while RW Cephei’s event may last several years. Even more massive stars like Canis Majoris, with a radius over 14 times that of the Sun, could experience episodes that last decades.

RW Cephei’s current eruption is likely one of several massive eruptions the star has experienced in the last century. Similar mass loss events will probably plague the star as it evolves toward its demise.

“This one was special because the cloud was ejected in the direction of Earth,” said CHARA Director Douglas Gies, “so we were in the right place to witness the full effects of the cataclysm.

Evan Gough

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