For decades, astronomers have been watching WOH G64, an enormous heavyweight star in the Large Magellanic Cloud, a galaxy visible with the naked eye from the Southern Hemisphere. This star is more than 1,500 times larger than the Sun and emitting over 100,000 times more energy. For a long time, red supergiant WOH G64 looked like a star steadily reaching the end of its life, shedding material and swelling in size as it began to run out of fuel.
Astronomers didn’t think its final demise would happen anytime soon, because no-one has ever seen a known red supergiant die. But in recent years astronomers – including our team working with the Southern African Large Telescope (SALT) – discovered that this star has started to change, growing dimmer than before and seemingly warmer. This has surprised scientists and suggests the star’s final stages of life may be more complicated, and perhaps unfold faster, than once thought.
Massive stars, more than about eight times the mass of the Sun, produce so much energy, which we see as light, that they run out of fuel within millions of years, instead of the billions of years of the Sun’s lifespan.
Most massive stars become gigantic, cool stars in the final million years or so of their life – so-called red supergiants. All red supergiants blow gaseous winds, losing weight as they do so. Some do this so strongly that the star becomes enveloped in a shroud of the ejected material containing gas and solid particles like tiny sand grains – called dust in astronomy. This makes them look dim in visual light, but very bright in the infrared where the dust shines.
In the 1960s Swedish astronomers Westerlund, Olander and Hedin discovered number 64 in their catalogue of red stars. They thought nothing of it, as it looked like an unremarkable red giant star, something the Sun and most other stars will become later in life. But when in the 1980s Nasa, the UK and The Netherlands launched the InfraRed Astronomical Satellite into space, astronomers Elias, Frogel and Schwering discovered that WOH G64 is the most luminous, coolest and dustiest red supergiant in the entire Large Magellanic Cloud, which harbours over a thousand red supergiants. More observations over the following decades showed the strong, steady modulations of the brightness expected of a pulsating star of that kind.
Then, in 2024, our team (both authors of this article and our collaborators in Germany and the US) succeeded in taking a close-up image of WOH G64 using the European Southern Observatory’s telescopes and revealed a fresh cloud of dust close to the star. It was the sharpest picture of a star in another galaxy ever taken (comparable to being able to spot an astronaut walk on the Moon from Earth). We discovered that in the last decade, unexpectedly, the star had started to eject much more dust than before. At that time, we did not have an idea about why and how.
It turns out, WOH G64 had also become dimmer, possibly because of the dust cloud it had ejected, and started to pulsate less and a little more quickly, suggesting it had shrunk. At the same time, the star seemed to look a lot warmer, leading some to believe it might have entered a new stage of its life – a so-called yellow hypergiant on its final path to doom.
All these phenomena are happening on a human time scale, which is usually not the case when we observe stars. This makes WOH G64 even more special. Is this star offering us an opportunity not to be missed to witness the final death throes of massive stars?
Now, as we start 2026, we have announced that observations we have obtained using the Southern African Large Telescope give us some clues about what is going on with WOH G64. The SALT observations show the overwhelming presence of ions in the vicinity of the star, which means that the gas is heated up to high temperatures by what must be a much hotter star. This should not have surprised anyone as the hot gas had been spotted in the 1980s and ever since. But we also found the imprint of molecules, implying cool gas (because molecules break up at high temperatures) likely in the atmosphere of the red supergiant. It did not appear to have changed into a yellow hypergiant, at least not yet.
For a long time, astronomers have suspected that the red supergiant has a smaller, hotter twin living alongside it, but they have somehow been reluctant to claim this in publications. And now it looks to be the elephant in the room. One way of making sense of our observations is that this hotter star, looking blue in contrast to its bigger, cooler, red sibling, heats gas it might have captured from the red supergiant’s wind. Now that the red supergiant has faded, the presence of the heated gas has just become more conspicuous.
If the orbit of the blue star is not a circle but quite elongated (Earth’s orbit around the Sun only slightly deviates from a circle), the distance between the blue star and the red supergiant varies. It may have got closer in recent years, and its gravity might have caused the atmosphere of the red supergiant to stretch out. This would make it more transparent overall, allowing us to see the warmer interior, but with cool, dark molecular patches left in places. That would also have made it easier for dust to form further out in its wind.
If that is true, then once the blue star starts to recede again on its orbit, WOH G64 might regain its former red supergiant glory. On the other hand, if it did throw off its coat entirely, then the molecules would disappear, and with it, the dust, and we would gain a clean view of the star. Then again, WOH G64 might do something else unexpected. It certainly teaches astronomers to be humble.
This article originally appeared on The Conversation. You can read it here.
