The Southern African Large Telescope (SALT) has been used, by international astronomers, to solve a “cosmic mystery”. Located in the South African Astronomical Observatory complex at Sutherland in the Karoo, in the Northern Cape province, SALT was used to observe one of the most extreme stars ever found, designated WOH G64, which lies in the Large Magellanic Cloud (LMC – a satellite galaxy of the Milky Way).

WOH G64 is one of the coolest but also most luminous, as well as dustiest, red supergiant stars in the LMC. This kind of star ends its days in a core-collapse supernova explosion. And, over the past ten years WOH G64 has faded to a significant degree, while its pulsations have been suppressed, and the spectrum of the light that it emits has become dominated by emission lines from ionised gas, rather than those typical of red supergiants. Moreover, in 2024, it was discovered that the star was being veiled by a fresh dust cloud.

These developments led astronomers to theorise that WOH G64 had ended its red supergiant phase and developed into a yellow hypergiant, which is a brief unstable phase before the star went supernova.

(In parallel, these developments also revealed the existence of a smaller hot companion star to WOH G64.) 

To ascertain what was actually happening to WOH G64, an international team of astronomers, led by Dr Jacco van Loon of Keele University in the UK and Dr Keiichi Ohnaka of the Universidad Andrés Bello in Chile, turned to SALT. They made observations from the South African instrument over the period from November 2024 to December last year, employing its Robie Stobie Spectrograph.

“WOH G64 has been claimed to have turned into a yellow hypergiant, which could signal a pre-supernova post-red supergiant evolution,” points out 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 significance of the TiO is that it shows that the star is still cool enough to be a red supergiant.

Van Loon, Ohnaka and their team theorise that the “bizarre” behaviour of WOH G64 is the result of its interaction with its smaller companion star. As that companion approaches WOH G64, its gravity could be stretching the massive star’s extended atmosphere, triggering its previously observed and significant loss of mass, and leading to the creation of a new dust cloud that obscured WOH G64. At the same time, the smaller and hotter companion star ionised the gas in this new cloud, resulting in the emission lines detected by astronomers.

“We are essentially witnessing a ‘phoenix’ rising from the ashes,” explains 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.”