Betelgeuse isn’t the only giant star to undergo a “Great Dimming.”

CHARA image of RW Cephei, before and after
The CHARA array took two false-color images of RW Cephei, one in December 2022 (left), when the star had dimmed, and one in July 2023 (right), when the star had recovered its usual brightness. 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, resolvable only by an interfereometer.
CHARA / Georgia State University

Remember the Great Dimming of Betelgeuse? In late 2019, this red supergiant in Orion became 1.2 magnitudes fainter than normal. Detailed observations of the star with the European Very Large Telescope in Chile, enabled by Betelgeuse’s relatively short distance of 640 light-years, revealed that the southern hemisphere of the star’s disk had darkened. The light was temporarily blocked by a huge dust cloud, which had probably condensed out of a massive ejection of stellar material. As the cloud slowly expanded and dispersed, the star regained its original brightness.

Well, Betelgeuse is no longer alone. In late 2022, the well-known variable star RW Cephei — a yellow hypergiant and one of the largest stars in the Milky Way Galaxy — experienced a similar Great Dimming caused by a similar event. According to Narsireddy Anugu (Georgia State Unversity), supergiants and hypergiants may experience huge surface mass ejections once every century or so, “but they need to occur more or less in our direction to cause a significant dimming of the star.”

Several astronomers, including members of the American Association of Variable Star Observers, noted that RW Cep had faded to about one-third of its normal brightness. Finding out what happened isn’t easy — it’s impossible for a single telescope to resolve its disk. That’s especially true given its extreme distance, tens of times farther away from Earth than Betelgeuse. However, an interferometer, consisting of several telescopes linked together to hugely increase the spatial resolution, can do the trick.

Anugu led an international team of astronomers that used a six-telescope interferometer at Mount Wilson, the Center for High Angular Resolution Astronomy (CHARA) Array, to image RW Cephei in December 2022, and again in July 2023, when the star had almost returned to its normal brightness. The team’s images, presented at the 243rd meeting of the American Astronomical Society in New Orleans, and published in The Astronomical Journal, resemble the Very Large Telescope’s images of Betelgeuse about four years ago. In the case of RW Cephei, the western part of the star’s disk appear to be blocked by dust. (The box-like shape of the star is an imaging artifact, due to the spacing of the CHARA telescopes.)

Additional spectroscopic observations of the star, obtained with the 3.5-meter telescope at Apache Point Observatory, revealed that the dimming of RW Cephei was much less pronounced at near-infrared wavelengths than at visible wavelengths. This supports the dust cloud scenario: Dust particles absorb visual light much more efficiently than infrared light, which passes through mostly unhindered.

So if this is the second Great Dimming of a giant star observed in just a few years, can we expect more? Probably, says Anugu, because astronomers are now paying much more attention. In particular, the relatively cool hypergiant VY Canis Majoris might show similar behavior that would warrant interferometric follow-up observations. “In fact,” Anugu says, “the huge brightness drop of the star Rho Cassiopeia that was observed in 1946 may have had a similar origin.”




Image of Anthony-Mallama


January 12, 2024 at 5:11 pm

A very informative article about an interesting star that can be observed with binoculars.

The dimming can be seen using the AAVSO light curve tool at by typing in ‘RW Cep’ and clicking ‘Send’. I think that the V-band measurements (green squares) illustrate the changes best. The minimum near Julian Date 2459945 corresponds to December 31, 2022. The star was near magnitude 7.0 before that, faded to about 7.5 or 8.0 and then brightened to 6.5 during 2023.

You must be logged in to post a comment.

Image of robin_astro


January 12, 2024 at 7:50 pm

It was also possible to follow changes in the spectrum using amateur equipment. The paper cites my observations reported in the BAA Variable Star Section Circular.
(The spectrum has now returned to the pre dimming condition)

You must be logged in to post a comment.

Image of William Forrest

William Forrest

January 19, 2024 at 2:56 pm

Why are there no measurements of the thermal emission from those putative newly formed dust grains? If the year were 1972, I would have rushed off to my local Mt. Lemmon observatory and measured RW Cep in the J,H,K,L,M,N and Q bands to guage the amount and type of dust formed, and the fraction of the solid angle covered by dust.

In 50 years, we've seemed to lose our IR capabilities! Such a shame.

Granted IR observations are hard, especially from the ground, but they're crucial for situations like this! See the analysis of the patchy dust clouds produced by R CrB.

You must be logged in to post a comment.

You must be logged in to post a comment.