Keele-led team reveals massive star WOH G64 is still a red supergiant

An international team of astronomers led by a researcher at Keele University has solved a long-standing cosmic mystery surrounding one of the most extreme stars ever observed.

The star, known as WOH G64, is located in the Large Magellanic Cloud – a satellite galaxy of the Milky Way – and has long been regarded as the most luminous, coolest and dustiest red supergiant in that galaxy. Such massive stars are expected to end their lives in dramatic core-collapse supernova explosions.

In recent years, however, WOH G64 began behaving in ways that puzzled astronomers. The star faded dramatically, its characteristic pulsations weakened, and its spectrum became dominated by emission from ionised gas rather than the cool absorption features typical of red supergiants. The discovery of a newly formed dust cloud obscuring the star in 2024 led to speculation that it had left the red supergiant phase altogether and evolved into a rare and unstable yellow hypergiant – a short-lived stage that can precede a supernova.

To uncover the star's true nature, a team led by Dr Jacco van Loon from Keele University, together with Dr Keiichi Ohnaka from Universidad Andrés Bello in Chile, carried out detailed observations using the Southern African Large Telescope (SALT). Between November 2024 and December 2025, the researchers obtained deep optical spectra of the fading system.

Their results reveal that, despite its unusual behaviour, WOH G64 remains a red supergiant.

Dr van Loon said: "WOH G64 has been claimed to have turned into a yellow hypergiant, which could signal a pre-supernova post-red supergiant evolution.

"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.

"We are essentially witnessing a 'phoenix' rising from the ashes," said Dr 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."

The findings are detailed in the paper A phoenix rises from the ashes: WOH G64 is still a red supergiant, for now, published in the Monthly Notices of the Royal Astronomical Society (MNRAS).