Astronomers Watch Another Giant Star Dim

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.”