The Supergiant WOH G64

​The supergiant WOH G64 is a very cool, extreme red supergiant (RSG) with an effective temperature near 3,400 K and a radius of roughly 1,500–1,700 times the Sun’s radius, making it among the largest known stars. It is in the Large Magellanic Cloud (LMC), about 163,000 light‑years away, and is one of the largest known stars by radius. Around 280,000–300,000 times the solar luminosity, with an estimated initial mass of about 25–30,M_\odot, so it is a probable supernova‑progenitor.
 
WOH G64 sheds mass at an exceptionally high rate, embedded in a thick, asymmetric dust torus and surrounded by nitrogen‑rich nebular gas moving faster than the star itself. Long‑term monitoring shows that after appearing as a canonical red supergiant for decades, WOH G64 has dramatically changed since the 2010s: its spectrum shifted from cool absorption to more emission‑dominated features, and it has been argued to have transitioned into (or toward) a yellow hypergiant phase, possibly as part of a massive symbiotic binary system with a hot B‑type companion.
 
It sits at the extreme upper‑right of the Hertzsprung–Russell diagram and challenges standard RSG models because of its low temperature, huge radius, and intense mass loss. The system is now viewed as a rare laboratory for observing late‑stage, massive‑star evolution and binary interaction, possibly on the verge of a core‑collapse supernova or a major eruptive episode.
 
However, its exact evolutionary route is complicated by extreme mass loss and possible binary interaction. The default pathway as a massive star, WOH G64 has already burned hydrogen, helium, and heavier elements in its core, and now sits as a red supergiant with a cool, inflated envelope and a hot, dense core. Standard massive‑star models expect that once the core builds up an iron‑group core, fusion stops, gravity wins, and the core collapses in seconds to form a neutron star or black hole, ejecting the outer layers in a Type II‑P or similar core‑collapse supernova.
 
Over the last 30 years, WOH G64 has shown sudden photometric and spectroscopic changes: its variability shifted from semi‑regular to irregular, its spectrum became more emission‑line dominated, and it appears to have become warmer and less extended, consistent with a transition toward a yellow hypergiant (YHG)‑like or Be‑type state. This behavior is interpreted as either a pre‑supernova superwind phase expelling the outer envelope in a final, violent mass‑loss episode, or a binary‑driven evolution in which a hidden B‑type companion has stripped part of the RSG envelope, leading to a massive symbiotic system (RSG/YHG + hot companion).
 
In either scenario, the core mass and composition are what ultimately decide the explosion, but the surface evolution is messier. Instead of a canonical cool RSG, WOH G64 might explode while in a warm, inflated, or binary‑altered state (YHG‑like with strong mass loss plus a companion). The intense mass loss implies the hydrogen envelope could be significantly reduced or even partially removed before core collapse, potentially producing a stripped‑envelope or hybrid supernova (e.g., Type IIb or IIL/IIn) rather than a classic hydrogen‑rich Type II‑P.
 
In short, the nominal path is core‑collapse of a formerly red supergiant, but WOH G64’s recent transition and likely binary nature may turn it into a peculiar, stripped‑envelope or symbiotic‑system supernova progenitor, offering a rare real‑time probe of how some of the most massive stars die.
 
References
Levesque, E.M., Massey, P., Plez, B. & Knut, A.G. (2009, June). The Physical Properties of the Red Supergiant WHO G64: The Largest Known Star? The Astronomical Journal.
 
Munoz-Sanchez, G., Kalitsounaki, M., de Wit, S., Antoniadis, K., Bonanos, A.Z., et al. (2026, February 23). The Dramatic Transition of the Extreme Red Supergiant WHO G64 to a Yellow Hypergiant. Nature Astronomy.
 
Ohnaka, K. & van Loon, J. (2026, February 7). A Giant Star is Changing Before Our Eyes and Astronomers are Watching in Real Time. Phys.Org.
 
(2026, April 1). WHO G64. In Wikipedia: https://en.wikipedia.org/wiki/WOH_G64