Astronomers have observed one of the universe’s most colossal stars undergo a dramatic transformation, expanding our understanding of stellar evolution. The change may mark the prelude to a powerful supernova, potentially leading to the birth of a black hole from this colossal object.
The subject of this rare event is WOH G64 (also known as IRAS 04553–6825), a star situated in the Large Magellanic Cloud, a satellite galaxy of the Milky Way about 163,000 light-years away. At roughly 1,540 times the Sun’s radius, it possesses nearly 30 solar masses and shines with around 282,000 times the Sun’s brightness. Since its discovery in the 1970s, WOH G64 has generally appeared as a red supergiant encircled by a dense dust torus.
Yet in 2014, the star began to change its appearance. A team led by Gonzalo Muñoz-Sanchez of the National Observatory of Athens detected shifts in color that accompanied an uptick in surface temperature. Their analysis indicated a red supergiant is evolving into a rare yellow hypergiant, a transition that may signify a star nearing the end of its life in real time.
“The fate of stars with initial masses between roughly 23 and 30 solar masses after they become red supergiants is still uncertain,” Muñoz-Sanchez explained to Space.com. “WOH G64 stood out as the most extreme red supergiant known, with an estimated mass near 28 solar masses. It’s still unclear whether such stars explode as supernovae, collapse directly into black holes, or pass through a yellow hypergiant phase before ending their lives. WOH G64 might hold the answer.”
What’s remarkable is that this study provides the first evidence of an extreme stellar object changing its temperature and moving from red to yellow over the span of a single year—remarkably gradual and quiet compared to typical stellar upheavals.
“This rapid kind of change is surprising because stars usually undergo dramatic, abrupt events to shift their state,” Muñoz-Sanchez noted.
The researchers also uncovered that WOH G64 is not alone in this behavior. It appears to be part of a binary system, which could influence its evolution in significant ways.
In this “live fast, die young” scenario, stars like WOH G64 burn through their nuclear fuel far more quickly than smaller stars. At around 5 million years old, it is comparatively youthful next to our Sun’s ~4.6 billion-year lifespan, yet its fate remains uncertain because the terminal stages of massive stars are not fully understood.
Yellow hypergiants are exceedingly rare, representing a brief transitional phase between red supergiants and the ultimate supernova explosion. Only a few tens of confirmed yellow hypergiants are known, making this finding especially intriguing.
For a yellow hypergiant to form, a strong stellar wind must strip away an outer envelope, increasing the surface temperature. However, only the brightest red supergiants usually generate winds strong enough to drive this transition. The discovery that WOH G64 is in fact part of a binary system adds another layer of complexity: mass exchange with a companion could help peel away its envelope and push it toward higher temperatures.
If the evolution is driven by binary interactions, the two stars would have been enveloped by a shared gas cloud, or common envelope, initially making the primary appear as a red supergiant. Partial ejection of this envelope could reveal the two components and accelerate the transition. Alternatively, the change might be intrinsic to the star, perhaps the result of a prolonged eruptive episode lasting decades, after which it settles back into a yellow, quieter state. Both possibilities are rare, and witnessing either on human timescales is nearly unprecedented.
Thus, the team has not yet determined whether WOH G64’s change is primarily due to its binary partner or internal stellar processes alone. The researchers note that other extreme red supergiants may also be in binary systems, underscoring the broader importance of discerning how intrinsic properties versus binary interactions shape the life cycles of massive stars and the supernovae they produce.
Understanding the binary nature of WOH G64 is crucial not only for mapping its life story but also for predicting how its end might unfold. Continued mass transfer or a forthcoming stellar merger could lead to a dramatic collision, while milder interactions might allow the primary to proceed toward core collapse, resulting in a supernova or a direct black hole formation. In astronomical terms, WOH G64 is highly evolved, and researchers estimate that core collapse could occur within a timespan ranging from a hundred to a few thousand years—though such an event is unlikely to happen within our lifetimes.
A Nature paper published on February 23 summarizes these findings. Rob Lelis, a science journalist based in the U.K., has contributed to several major outlets and is known for science communication work with Elsevier and the European Journal of Physics. Follow his reporting on related topics for ongoing updates.
Would you say this kind of real-time stellar evolution challenges your intuition about how stars live and die, or does it reinforce what you already believed about these cosmic giants? Share your thoughts in the comments.
