The dramatic transition of the extreme Red Supergiant WOH G64 to a Yellow Hypergiant

Red Supergiants (RSGs) are cool, evolved massive stars in their final evolutionary stage before exploding as a supernova [1, 2]. However, the evolution and fate of the most luminous RSGs is uncertain. Observational evidence for luminous warm, post-RSG objects [3] and the lack of luminous RSGs as supernova progenitors [4] suggest a blueward evolution. Since the 1980s, WOH G64 has been considered the most extreme RSG in the Large Magellanic Cloud given its outstanding size, luminosity, and mass-loss rate [5–7]. Here we report a sudden, yet smooth change in its nature. Time-series photometry and subsequent spectroscopy revealed the most extreme transition ever seen in the optical spectral features of a RSG. We discovered that WOH G64 is a rare, massive symbiotic system formed by a RSG, which transitioned to a Yellow Hypergiant, and a B-star companion. The dramatic transition can be explained by: a) binary interactions partially stripping the envelope, b) the return of WOH G64 to a quiescent state after an outstanding eruption exceeding 30 years, and c) the expulsion of its outer layers due to a pre-supernova superwind phase, indicating its imminent explosion. WOH G64 offers a unique opportunity to witness stellar evolution in real-time, providing crucial clues for the final phases of massive stars and their resulting supernovae.