Astronomers Debate the Sudden Transformation of a Colossal Star in the Large Magellanic Cloud

WOH G64 is a massive star located in the Large Magellanic Cloud, a satellite galaxy to the Milky Way. It is positioned approximately 160,000 light-years away from Earth. Astronomers have long identified it as one of the largest red supergiants in the known Universe.

Recent measurements indicate that the star has a radius more than 1,500 times that of the Sun. Red supergiants like WOH G64 represent a final stage in the life of massive stars. These stars typically begin their lives with a mass between 8 and 30 times that of the Sun.

As these stars exhaust their primary hydrogen fuel, they begin to burn heavier elements in their cores. This nuclear transition causes the outer layers of the star to expand significantly. The resulting volume can be hundreds of times larger than the star’s original size.

An Abrupt Change in Appearance

In 2013 and 2014, telescopes recorded a distinct change in the light coming from WOH G64. The star appeared to be moving away from its status as a red supergiant. Its light signature suggested it was becoming hotter and shifting toward a yellow hue.

A research team led by Gonzalo Muñoz-Sanchez of the National Observatory of Athens analyzed this data. They presented their findings on the arXiv preprint server in November 2024. The team concluded that the star had entered a rare phase known as a yellow hypergiant.

Yellow hypergiants are considered a short-lived evolutionary stage. This phase often precedes a core-collapse supernova, which is the violent death of a massive star. The study suggested that WOH G64 had taken a decisive step toward its eventual destruction.

The Yellow Hypergiant Hypothesis

The transformation described by Muñoz-Sanchez and his colleagues was significant. Their analysis indicated that the star had reduced its size to approximately 800 solar radii. This represented a substantial contraction from its previous state.

The team also noted an increase in the star’s surface temperature. They observed shifts in the chemical composition of the stellar atmosphere during this period. These changes were presented as evidence of an abrupt evolutionary transition.

The researchers proposed two primary explanations for this behavior. One possibility involved the partial ejection of the star’s pseudo-atmosphere. This could occur during a common-envelope phase, where the star interacts closely with a companion.

Alternatively, the star may have been returning to a stable state following a massive eruption. Such an eruption could have lasted for more than 30 years. In either scenario, the observations pointed to a star that was fundamentally changing its identity.

The Role of a Binary Companion

The study also identified a second star in the system. This hot binary companion appears to be interacting with WOH G64. Such interactions can complicate how astronomers interpret the light coming from a star.

A binary companion can pull material away from its larger neighbor. It can also create a messy environment of gas and dust around the primary star. This environment can mimic the signals of an evolutionary shift even if the star remains stable.

Evidence of a Red Supergiant Identity

Following the publication of the Muñoz-Sanchez paper, other astronomers began their own investigations. Jacco van Loon of Keele University and Keiichi Ohnaka of Universidad Andrés Bello conducted new observations. They used the Southern African Large Telescope (SALT) between late 2024 and late 2025.

Their findings were published in the Monthly Notices of the Royal Astronomical Society in January 2026. The team looked specifically at the molecular composition of the star’s atmosphere. They discovered the presence of titanium oxide.

Titanium oxide is a molecule that is highly sensitive to temperature. It can only exist in the relatively cool atmospheres of red supergiants. A yellow hypergiant is too hot for titanium oxide to remain intact.

Interpreting the Spectral Data

The presence of titanium oxide suggests that WOH G64 has not reached the temperatures required for a yellow hypergiant phase. Jacco van Loon noted that the spectra obtained with SALT showed clear molecular absorption bands. These bands are a hallmark of red supergiants.

Van Loon stated that while the hot companion is present, WOH G64 appears to still be a red supergiant. The research suggests the star may never have stopped being a red supergiant during the 2013-2014 event. The perceived change in color and size may have a different origin.

The researchers believe the interactions between WOH G64 and its companion star are responsible. These interactions could produce a spectral shift that resembles an evolutionary leap. The environment surrounding the two stars is likely very complex.

Lessons from Other Unstable Stars

The behavior of WOH G64 is not entirely unique in the field of astronomy. Other red supergiants have demonstrated similar periods of instability. The star Betelgeuse is a well-known example of this phenomenon.

In recent years, Betelgeuse experienced a dramatic drop in brightness of nearly 25 percent. This event caused some to speculate that a supernova was imminent. However, the star eventually returned to its normal state, proving that red supergiants are inherently prone to “tantrums”.

Such stars are known to be unstable as they shed material into the surrounding space. They can undergo rapid shifts in brightness or color without fundamentally changing their evolutionary stage. This inherent messiness makes them difficult to study with certainty.

Why This Matters

Monitoring stars like WOH G64 provides a rare opportunity to see how massive stars behave just before they die. Because WOH G64 is so large and relatively close, it serves as a laboratory for stellar physics. Understanding its transitions helps scientists refine models of how heavy elements are distributed in galaxies.

If WOH G64 is indeed approaching a supernova, it would be the first chance for modern astronomers to witness such an event in a known red supergiant. If it is merely undergoing a period of instability, it teaches researchers about the complex dynamics of binary star systems.

The debate between the Muñoz-Sanchez team and the van Loon team highlights the rigorous nature of the scientific process. New data often challenges existing theories, forcing astronomers to look closer at the evidence.

Future Observations

Continued monitoring of WOH G64 is considered crucial by the scientific community. The star’s future evolution will eventually reveal which interpretation was correct. If it continues to heat up, the yellow hypergiant theory may gain more support.

If the star remains cool and continues to show titanium oxide, it will confirm its status as a stable, albeit messy, red supergiant. The system remains one of the most unpredictable and fascinating areas of study in the Large Magellanic Cloud.

Every new observation adds a piece to the puzzle of how the Universe’s largest stars reach their end. For now, WOH G64 remains a colossal enigma, 160,000 light-years away.

The research was published in Nature Astronomy on February 23, 2026.