The physical processes behind a hypergiant’s “great dimming” may have been unlocked by high-resolution imagery and a century’s worth of AAVSO observations. By Tim Lyster
With a radius of more than a thousand times that of the Sun, and weighing more than 25 Solar masses, RW Cephei is a giant among supergiants—a so-called “hypergiant.” It’s also a well-known variable star, typically fluctuating between magnitude 6.1 and 7.6 in the V band, with a semi-regular periodicity of 350-800 days. Large, massive stars tend to excite the interest of the astronomical community, and RW Cep did just that by acting out of character in late 2022 by fading to about one-third of its normal brightness.
This type of extraordinary dimming is thought to happen when massive stars eject material, creating expansive clouds of gas and dust that can block light coming from the star itself. Betelgeuse, the fiery orange star that marks Orion’s shoulder, underwent a similar dimming episode in 2019. RW Cep’s recent drop in magnitude was particularly extreme, resulting in the star’s brightness falling to a level fainter than ever recorded. The star had a re-brightening phase in 2023, during which it gradually regained its original luminosity.
A recent research paper authored by a team led by Narsireddy Anugu, a staff scientist at Georgia State University’s CHARA Array, offers up an explanation behind the fluctuation in luminosity. In “Time Evolution Images of the Hypergiant RW Cephei during the Rebrightening Phase Following the Great Dimming" [PDF], the authors describe how they combined high-resolution imaging from CHARA, a series of six telescopes acting in concert as one large instrument, with historical observations from the AAVSO and Harvard’s DASCH.
Understanding the "Great Dimming" with High-Resolution Imaging
During the dimming and rebrightening phases, astronomers used CHARA to document the physical changes in the star's appearance. Images taken in the dimming phase showed that RW Cep looked asymmetrical, with a large, dark area on one side. This dark region was likely caused by a dense cloud of dust that had formed close to the star, obscuring part of its surface and reducing its apparent size. As the star brightened, this darkened section began to clear, and the star appeared larger as if the dust cloud was dispersing or moving out of the line of sight.
Through careful image comparisons, the study team observed that RW Cep’s diameter appeared to expand by about 8% during the brightening. This suggested that the dark area on the star during the dimming phase was, indeed, a result of something blocking part of it, rather than an actual shrinking of the star itself. This finding pointed to a “surface mass ejection”—a large amount of material that was expelled from RW Cep’s surface, which then condensed into dust, creating a temporary “veil” over part of the star.
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AAVSO Observers’ Crucial Role in Providing an Historical Baseline
AAVSO observers have been capturing brightness measurements of RW Cephei for more than a century. This long-term dataset is invaluable, because it allowed Dr. Anugu’s team to compare the star’s current behavior with previous events. The AAVSO and DASCH’s datasets indicate that RW Cep has suffered several dimming episodes over the past century, occurring in cycles, but none have been as intense as the 2022 event (see figure XX). The historical context provided by these observations was essential in recognizing that the Great Dimming was part of a recurring process—and not simply an isolated occurrence.
For this study, AAVSO’s records were fundamental in distinguishing regular pulsations—natural brightness changes that occur over shorter periods—from more significant dimming events, like the Great Dimming. By pinpointing six major dimming episodes since 1900, AAVSO observations provided the evidence that RW Cep undergoes episodic mass ejections, each time likely forming dust clouds that temporarily obscure the star. This consistency in observation highlights the commitment and skill of AAVSO members, whose observations form the backbone of time-domain variable star astronomy.
Insights and Future Implications
The combined efforts of high-resolution imaging from the CHARA Array and long-term monitoring by AAVSO observers reveal that mass ejections and dust formation play a significant role in the life cycles of massive stars like RW Cephei. Understanding these processes helps astronomers predict the eventual fate of these stars, whether they end as supernovae, neutron stars, or black holes, and their impact on the surrounding environment.
Significantly, the study exemplifies the role that AAVSO observers can play in expanding our knowledge of stellar evolution. Their careful documentation over long time periods allowed astronomers to recognize the Great Dimming as part of a larger pattern, contributing to new insights about RW Cephei and massive star behavior. Together, AAVSO data and CHARA imaging have provided a clearer picture of the dramatic changes in RW Cephei, uncovering a fascinating story of a hypergiant star navigating through cycles of mass loss and re-emergence.
Dr. Anugu observes, “major outbursts causing dimming events are usually rare in massive evolved stars, often occurring just once a century. However, hypergiants like RW Cep provide more frequent opportunities for study, experiencing outbursts a few times per century. We seized one of these opportunities and conducted this research using AAVSO’s historical data and CHARA’s high-resolution imaging. The star continuously surprises us, and we are committed to long-term monitoring.” 