Photo: ESO/L. Calçada
One of the largest stars in the universe is doing something unusual, and scientists are still debating what it means.
WOH G64, a star in the Large Magellanic Cloud, is one of the biggest known red supergiants, with a radius 1,500 times that of the Sun. In 2013–2014, telescopes observed changes suggesting it had shifted from a classic red supergiant to a hotter, yellow state.
A team led by Gonzalo Muñoz-Sánchez at the National Observatory of Athens proposed in 2024 that WOH G64 had entered a rare yellow hypergiant phase—a possible step toward its eventual death. They suggested the transformation could result from either a partial ejection of a pseudo-atmosphere or a return to a calmer state after a major eruption lasting over 30 years. Observations indicated its temperature had risen, its radius shrank to 800 solar radii, and its atmospheric chemistry had changed. They also identified a hot companion star interacting with WOH G64.
However, follow-up observations revealed the star might never have stopped being a red supergiant. Red supergiants are among the largest stars by volume, evolving from massive stars (8–30 times the Sun’s mass) at the end of their nuclear-burning life. As heavier elements form in their cores, these stars expand dramatically, often hundreds of times larger than the Sun, and are inherently unstable, showing rapid changes in brightness or color as they eject material into space.
Located about 160,000 light-years away, WOH G64 is under close observation. Interpreting the behavior of such unstable stars is complex; changes in brightness or color don’t necessarily indicate a change in the star’s evolutionary stage.
From November 2024 to December 2025, astronomers Jacco van Loon (University of Kiel, UK) and Keiichi Onaka (Andres Bello University, Chile) conducted further observations using the Southern African Large Telescope (SALT). Their findings, published in January 2026, detected titanium oxide in WOH G64’s atmosphere. Since a yellow hypergiant is too hot to maintain titanium oxide, this suggests WOH G64 remains a red supergiant and may never have left that stage.
In van Loon’s words: “While it was suggested WOH G64 transformed into a yellow hypergiant, our new spectra show a hot companion and clear molecular absorption bands of titanium oxide, indicating WOH G64 is still a red supergiant.”
