A Cosmic Mystery Solved: SALT Confirms Giant Star WOH G64 Is Still Alive

CAPE TOWN/SUTHERLAND – Astronomers using South Africa’s Southern African Large Telescope (SALT) have resolved a long-running cosmic puzzle, confirming that one of the most extreme stars known, WOH G64, has not entered a new evolutionary phase nor is it on the brink of collapse.

Despite dramatic changes in its brightness and behaviour over the past decade, the massive star remains a red supergiant.

The finding, announced this week by an international research team working with SALT in Sutherland, Northern Cape, overturns recent speculation that WOH G64 may have evolved into a rarer and more unstable stage known as a yellow hypergiant, a phase often associated with stars approaching a supernova explosion.

WOH G64 lies in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, and has long fascinated astronomers. It is regarded as the most luminous and one of the coolest and dustiest red supergiants in that galaxy. Stars of this size represent a brief but critical stage in stellar evolution and are destined to end their lives in spectacular core-collapse supernovae.

In recent years, however, WOH G64 began behaving in unexpected ways. It faded significantly, its characteristic pulsations weakened, and its spectrum shifted from showing the cool absorption features typical of red supergiants to emission lines associated with ionised gas. In 2024, astronomers also detected a fresh cloud of dust enveloping the star, further obscuring it from view.

Together, these changes prompted speculation that the red supergiant phase had ended, possibly giving way to a yellow hypergiant, a short-lived and highly unstable evolutionary stage. Some researchers also suggested that a previously hidden hot companion star might have become visible as the primary star dimmed.

To test these ideas, a team led by Dr Jacco van Loon of Keele University in the United Kingdom and Dr Keiichi Ohnaka of Universidad Andrés Bello in Chile turned to SALT, one of the largest optical telescopes in the southern hemisphere. Using the Robert Stobie Spectrograph, the team conducted deep optical observations of the system between November 2024 and December 2025.

Their results tell a different story.

“Our new spectra clearly show molecular absorption bands from titanium oxide,” van Loon explained. “These molecules can only exist in very cool stellar atmospheres. Their presence is the smoking gun that confirms WOH G64 is still a red supergiant, and may never have stopped being one.”

While the observations did confirm the presence of a hot companion star, the primary star itself remains cool and intact. Rather than signalling a dramatic evolutionary shift, the researchers propose that WOH G64’s strange behaviour is best explained by interaction within a binary system.

According to the study, the companion star’s gravity is likely stretching and disturbing the vast outer atmosphere of the red supergiant. This interaction may have triggered intense mass loss, producing the newly formed dust cloud that caused the star’s apparent fading. At the same time, radiation from the hot companion can ionise the surrounding gas, explaining the unusual emission lines seen in recent observations.

“We are essentially witnessing a phoenix rising from the ashes,” van Loon said. “The red supergiant’s atmosphere is being pulled and reshaped by its companion, but it has not been stripped away. The star persists.”

The discovery underscores the importance of long-term monitoring and large-aperture telescopes such as SALT in understanding the final stages of massive stars. These late evolutionary phases are complex, dynamic and often influenced by interactions with companion stars, factors that can easily be misinterpreted without detailed, sustained observation.