Scientists have observed something strange about one of the largest stars known in the Universe, and they aren’t entirely sure about what it means.
WOH G64, a star with a mass 28 times that of the sun, resides in a satellite galaxy of the Milky Way known as the Large Magellanic Cloud. Compared to the sun, its luminosity is about 300,000 times greater, and its diameter is about 1,500 times greater. If it were in place of the sun, its surface would span a distance between the orbits of Jupiter and Saturn.
“WOH G64 is a massive star and very different from the sun,” said astronomer Gonzalo Muñoz-Sanchez, from the National Observatory of Athens in Greece.
Scientists started noticing changes in the star in 2011, when the star started to dim. Then, in 2013 and 2014, not only was the star able to recover its initial brightness, but it significantly exceeded it. Its surface temperature rose up by more than 1000°C, causing the star to shift from a classic red supergiant to a hotter, yellower state. The star shrank while heating up, deflating to around 800 solar radii. Then, in 2025, the star began to fade again to less than half its brightness, dropping by roughly two magnitudes in less than a year.
WOH G64 appears to be near the end of its lifespan at around 10 million years old. In contrast, the sun, around 4.5 billion years old, has an estimated 5 billion years left. Stars between eight and 23 times the sun’s mass are expected to evolve into red supergiants that eventually explode as a supernova. As for stars in the range of 23 to 30 times the sun’s mass, it is not clear what would happen to them. They could explode as a supernova as well, collapse to form a black hole, or evolve from a red supergiant to a yellow hypergiant stage.
Between November 2024 and December 2025, astronomers Jacco van Loon of Keele University in the UK and Keiichi Ohnaka of Universidad Andrés Bello in Chile conducted observations using the Southern African Large Telescope. They found titanium oxide in the atmosphere of WOH G64, but such a yellow hypergiant is too hot to sustain titanium oxide.
“WOH G64 has been claimed to have turned into a yellow hypergiant, which could signal a pre-supernova post-red supergiant evolution,” said van Loon. “However, our new spectra obtained with SALT show the hot companion’s presence but also clear molecular absorption bands from titanium oxide. This implies that WOH G64 is currently a red supergiant and may never have ceased to be.”
This doesn’t mean that nothing striking is happening with the star. Van Loon and Ohnaka believe that the star has a binary companion. They believe interactions between the two stars may have complicated the environment around them, producing such remarkable changes.
It is difficult to say for certain what the reason is behind what is happening with WOH G64, and continued monitoring is crucial for scientists to uncover more details.
“We all agree that we are witnessing an unprecedented spectacle,” van Loon says, “and therefore are unsure of what exactly is going on. We’re all looking forward to new observations that might offer us new clues.”
