21 Jan A Phoenix Rises: SALT Reveals Massive Star WOH G64 is Still a Red Supergiant
A Phoenix Rises: SALT Reveals Massive Star WOH G64 is Still a Red Supergiant
Astronomers using the Southern African Large Telescope (SALT) have solved a cosmic mystery surrounding one of the most extreme stars known, WOH G64. While recent dramatic fading led many to believe the star had evolved into a new phase or was nearing its end, new SALT observations confirm the “beast” of the Large Magellanic Cloud is still very much alive as a red supergiant.
WOH G64 is a system composed of a red supergiant (at left, in orange) and a smaller hot star (at right, in blue). New observations with SALT suggest that the hot star may be causing the atmosphere of the red supergiant to spread out. Some of it may fuel a disk of gas around the hot star, giving rise to emission lines in the SALT spectrum. This is an artist’s impression of what that may look like (not to scale), created by Jacco van Loon using Copilot.
WOH G64 has long been known as the most luminous, coolest, and one of the dustiest red supergiant stars in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way. Such massive stars are destined to end their lives in core-collapse supernova explosions. However, in the last decade, the star’s behaviour changed radically. It faded significantly, its pulsations suppressed, and its spectrum became dominated by emission lines from ionised gas rather than the cool absorption features typical of red supergiants.
This strange behaviour, coupled with the 2024 discovery of a fresh dust cloud veiling the star, led astronomers to speculate that the red supergiant phase had ended. Theories suggested it might have evolved into a yellow hypergiant—a brief, unstable phase preceding a supernova—revealing a hot companion star previously hidden by the supergiant’s glare.
To determine the star’s true fate, a team led by Dr Jacco van Loon (Keele University, UK) and Dr Keiichi Ohnaka (Universidad Andrés Bello, Chile) turned to SALT. Using the Robert Stobie Spectrograph (RSS) between November 2024 and December 2025, they obtained deep optical spectra of the fading system.
The red portion of the SALT spectrum of WOH G64, plotted here, shows emission lines (in blue) from atoms caused by the hot star but also absorption dips (in red) from TiO molecules in the extended atmosphere of the red supergiant. The system is enveloped in a shroud of dust that formed in the red supergiant wind and that makes it more difficult to see the stars themselves. The absorption dip (in green) from oxygen molecules is caused by the Earth’s atmosphere through which we look out into space.
“WOH G64 has been claimed to have turned into a yellow hypergiant, which could signal a pre-supernova post-red supergiant evolution,” explains Dr. van Loon. “However, our new spectra obtained with SALT show the hot companion’s presence but also clear molecular absorption bands from Titanium Oxide (TiO). This implies that WOH G64 is currently a red supergiant, and may never have ceased to be.”
The detection of these molecules is the “smoking gun” that confirms the primary star is still cool enough to be a red supergiant. The study proposes a new scenario to explain the star’s bizarre behaviour: binary interaction.
The researchers suggest that a fainter, hot companion star (component B) is interacting with the red supergiant (component A). As the companion approaches, its gravity may be stretching the supergiant’s extended atmosphere. This interaction likely triggered the heavy mass loss and the formation of the new dust cloud that obscured the star from view, while simultaneously allowing the hot companion to ionise the surrounding gas.
“We are essentially witnessing a ‘phoenix’ rising from the ashes,” adds van Loon. “The atmosphere of the red supergiant is being stretched out by the approach of the companion star, but it has not been stripped altogether. It persists.”
This discovery highlights the critical role of long-term monitoring with large-aperture instruments like SALT in understanding the final evolutionary stages of massive stars.
The findings are detailed in the paper A phoenix rises from the ashes: WOH G64 is still a red supergiant, for now, accepted for publication in the Monthly Notices of the Royal Astronomical Society (MNRAS).
ENDS
Image Captions:
Artist’s impression:
WOH G64 is a system composed of a red supergiant (at left, in orange) and a smaller hot star (at right, in blue). New observations with SALT suggest that the hot star may be causing the atmosphere of the red supergiant to spread out. Some of it may fuel a disk of gas around the hot star, giving rise to emission lines in the SALT spectrum. This is an artist’s impression of what that may look like (not to scale), created by Jacco van Loon using Copilot.
Spectrum:
The red portion of the SALT spectrum of WOH G64, plotted here, shows emission lines (in blue) from atoms caused by the hot star but also absorption dips (in red) from TiO molecules in the extended atmosphere of the red supergiant. The system is enveloped in a shroud of dust that formed in the red supergiant wind and that makes it more difficult to see the stars themselves. The absorption dip (in green) from oxygen molecules is caused by the Earth’s atmosphere through which we look out into space.
Link to Full Text:
https://ui.adsabs.harvard.edu/abs/2026arXiv260102057V/abstract
Media Contact
Daniel Cunnama
South African Astronomical Observatory (NRF-SAAO)
Email: d.cunnama@saao.nrf.ac.za
About the Southern African Large Telescope (SALT)
The Southern African Large Telescope (SALT) is the largest single optical telescope in the southern hemisphere and among the largest in the world. It has a hexagonal primary mirror array 11 metres in diameter, consisting of 91 individual 1-m hexagonal mirrors. It is situated at the South African Astronomical Observatory (SAAO) field station near the small town of Sutherland, in the Northern Cape province. SALT is an international partnership between institutes in South Africa, the United States, Poland, India, and the United Kingdom
About the South African Astronomical Observatory (NRF-SAAO)
The South African Astronomical Observatory (NRF-SAAO), a facility of the National Research Foundation, is the national centre for optical and infrared astronomy in South Africa. Its primary function is to conduct fundamental research in astronomy and astrophysics by providing a world-class facility to scientists. The NRF-SAAO also promotes astronomy and astrophysics in Southern Africa by sharing research findings and discoveries, and by participating in outreach activities to enthuse citizens about physics and astronomy.
About the National Research Foundation (NRF)
The National Research Foundation (NRF) is an independent statutory body established in accordance with the National Research Foundation Act. Its mandate is to support and promote research through funding, human resource development and the provision of the necessary research facilities to facilitate the creation of knowledge, innovation and development in all fields of science and technology, including indigenous knowledge, and thereby contribute to the improvement of the quality of life of all South Africans.