The Enigmatic WOH G64: A Red Supergiant’s Shifting Identity

WOH G64, a colossal star located approximately 163,000 light-years away in the Large Magellanic Cloud, has captivated astronomers with its dramatic fluctuations. Once considered one of the largest known red supergiants, recent observations have challenged its classification, revealing a complex system that continues to defy effortless categorization.

A Giant Among Stars

WOH G64 is a symbiotic binary system, comprised of a red supergiant and a smaller, hot companion star. The primary component, the red supergiant, boasts a radius around 1,540 times that of the Sun [1]. At this immense size, light would take over six hours to travel around its surface [1]. Its luminosity is approximately 282,000 times that of the Sun [1], and it has a mass roughly 20 times greater than our Sun [1].

A Dramatic Transformation?

In 2013 and 2014, telescopes detected significant changes in WOH G64. The star appeared to brighten and its spectrum shifted towards higher temperatures [2]. Initial analysis, led by Gonzalo Muñoz-Sanchez of the National Observatory of Athens, suggested a transition from a red supergiant to a rare yellow hypergiant phase [2]. This phase is a short-lived evolutionary stage that can precede a core-collapse supernova.

Researchers proposed two possible explanations for this change: a partial shedding of the star’s outer layers during a common envelope phase within the binary system, or a return to a quiescent state following a substantial eruption lasting over 30 years [2]. The analysis also indicated a decrease in the star’s estimated radius to around 800 solar radii and changes in its atmospheric chemistry [2].

Back to Red? Recent Observations Challenge Earlier Findings

However, more recent observations conducted between November 2024 and December 2025 using the Southern African Large Telescope (SALT) have cast doubt on the yellow hypergiant hypothesis [3]. Astronomers Jacco van Loon of Keele University and Keiichi Ohnaka of Andrés Bell University detected prominent molecular absorption bands of titanium dioxide (TiO) in the star’s atmosphere [3].

The presence of TiO is significant because these molecules are characteristic of cool stars, specifically red giants. They cannot stably exist in the atmospheres of hotter yellow hypergiants [3]. Van Loon stated that the new spectra indicate WOH G64 is currently a red giant and may never have transitioned to a yellow hypergiant [3].

The Complexity of Binary Systems

Astronomers now believe that WOH G64’s observed changes are likely due to the interaction between the red supergiant and its hot companion star [3]. This interaction can cause fluctuations in brightness, spectrum, and mass loss, creating the illusion of a significant evolutionary shift without a fundamental change in the star’s internal structure.

A Continuing Mystery

Distinguishing between genuine evolutionary transitions and the effects of binary interactions and stellar instability remains a challenge for such extreme objects. Long-term monitoring will be crucial to understanding the true nature of WOH G64 and its ultimate fate. Regardless of its future, WOH G64 remains a valuable “laboratory” for studying the final stages of massive star evolution [1].