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The dramatic transition of the extreme red supergiant WOH G64 to a yellow hypergiant

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Abstract

Red supergiants (RSGs) are cool, evolved massive stars in their final evolutionary stage before exploding as a supernova. However, the evolution and fate of the most luminous RSGs remain uncertain. Observational evidence for luminous warm, post-RSG objects and the apparent lack of luminous RSGs as supernova progenitors suggest a bluewards evolution. Since the 1980s, WOH G64 has been considered the most extreme RSG in the Large Magellanic Cloud, given its large obscuration, outstanding size, luminosity and mass-loss rate. Here we report a sudden, yet smooth change in its apparent nature. Time-series photometry and subsequent spectroscopy reveal an extreme transition in the optical spectral features. We conclude that WOH G64 is a rare, massive symbiotic binary system where the RSG component has transitioned to a yellow hypergiant. This drastic transformation can be explained either by the partial ejection of the pseudo-atmosphere during a common-envelope phase or the return to a quiescent state after an outstanding eruption exceeding 30 years in duration. WOH G64 offers an opportunity to witness stellar evolution in real time and assess the role of binarity on the final phases of massive stars and their resulting supernovae.

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Fig. 1: Light curves of WOH G64.
Fig. 2: Evolution of the optical colours of WOH G64.
Fig. 3: Optical spectra of WOH G64 illustrating its dramatic transition from a late-M star to a B[e] star.
Fig. 4: Constraining the Teff of WOH G64 in the near-infrared.
Fig. 5: Evolution of the SED of WOH G64 before and after the transition.
Fig. 6: Hertzsprung–Russell diagram showing the transition of WOH G64.

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Data availability

The photometric datasets are publicly available in the respective survey repositories, except for the OGLE data, which can be accessed via figshare134. The spectra obtained with ESO facilities are available through the ESO archive, while the MagE spectrum is provided at the same figshare link.

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Acknowledgements

G.M.-S., M.K., S.d.W., K.A., A.Z.B., E.C. and G.M. acknowledge funding support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (‘ASSESS’, grant agreement number 772086). E.Z. acknowledges support from the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the ‘3rd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers’ (project number 7933). This work has been co-funded by the National Science Centre, Poland, grant number 2022/45/B/ST9/00243. We acknowledge helpful discussions with N. Blagorodnova, W.-J. de Wit, A. Ercolino, Y. Götberg, D. Hatzidimitriou, M. Kraus, E. Koumpia, M. Kourniotis, R. Oudmaijer and L. Patrick. The work of K.B. is supported by NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the US National Science Foundation. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

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Authors and Affiliations

  1. IAASARS, National Observatory of Athens, Penteli, Greece

    Gonzalo Muñoz-Sanchez, Maria Kalitsounaki, Stephan de Wit, Konstantinos Antoniadis, Alceste Zoe Bonanos, Evangelia Christodoulou & Grigoris Maravelias

  2. Department of Physics, National and Kapodistrian University of Athens, Zografos, Greece

    Gonzalo Muñoz-Sanchez, Maria Kalitsounaki, Stephan de Wit, Konstantinos Antoniadis, Alceste Zoe Bonanos, Emmanouil Zapartas & Evangelia Christodoulou

  3. Institute of Astrophysics, FORTH, Heraklion, Greece

    Emmanouil Zapartas & Grigoris Maravelias

  4. Cerro Tololo Inter-American Observatory/NSF, NOIRLab, La Serena, Chile

    Konstantina Boutsia

  5. Las Campanas Observatory, Carnegie Observatories, La Serena, Chile

    Konstantina Boutsia

  6. Astronomical Observatory, University of Warsaw, Warsaw, Poland

    Igor Soszyński & Andrzej Udalski

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  1. Gonzalo Muñoz-Sanchez
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Contributions

All authors helped with the interpretation of the data and provided comments on the paper. G.M.-S. led the analysis of the data and is the main author of the text. M.K. and S.d.W. contributed notably to the analysis of the data. K.A. built the spectral energy distribution after the transition. A.Z.B. supervised the project and contributed considerably to the text. E.Z. led the discussion on stellar evolution and the binary scenario. E.C. performed the telluric cleaning of the near-infrared spectroscopy. G.M. provided insight on the B[e] analysis. K.B. obtained the MagE spectrum. A.U. and I.S. provided the time-series photometry from OGLE.

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Correspondence to Gonzalo Muñoz-Sanchez.

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Extended data

Extended Data Fig. 1 Comparison of observed color-magnitude variability with synthetic photometry.

Left: Photometric observations are shown with dots (OGLE, between January 2010 and October 2017) and squares (MACHO, between June 1993 and December 1999). The dotted line represents the MACHO synthetic colors for a model with Teff = 3300 K and variable AV, while the dot-dashed line corresponds to OGLE synthetic colors for a model with variable Teff and a fixed AV = 4 mag. Black arrows indicate the direction of increasing AV and Teff in the diagram. Shaded areas represent the full range of observed color variation. Top right: Values of AV required to reproduce the observed MACHO colors using synthetic photometry, as a function of Teff. Bottom right: Values of Teff required to reproduce the observed OGLE colors using synthetic photometry, as a function of AV.

Extended Data Fig. 2 Comparison of the optical spectra of WOH G64.

Top: Spectral region between 6200-7200 Å. Bottom: Spectral region of the Ca II triplet region. The X-Shooter (2016) and MagE (2021) spectra are normalized, and the UVES spectrum is shown with an offset. The main spectral features are indicated for each epoch.

Extended Data Fig. 3 Comparison of the main forbidden lines found in WOH G64 spectra.

The UVES spectrum is shown in brown, X-Shooter in blue, and MagE in magenta. The vertical gray dashed-dotted line indicates the center of the line assuming a RV of 270 km s−1.

Extended Data Fig. 4 Near-infrared spectrum of WOH G64 from X-Shooter (2016).

The normalized spectrum of WOH G64 is shown in Y J-band (top), H-band (middle), and K-band (bottom), indicating identified lines. Unidentified lines are marked with a ’?’.

Extended Data Fig. 5 Radial velocity evolution of atomic lines with time.

Top: Median radial velocity of each ion, with the standard deviation derived from individual lines. Filled squares indicate measurements from emission lines, and empty squares from absorption lines. Bottom: Median radial velocity of the forbidden and double-peaked emission lines, with error bars representing the spectral resolution. The squares indicate single emission, the diamonds and circles are the red-shifted and blue- shifted peaks, respectively, in the double-peaked emission. Only the red-peaked emission from Hα is shown. Ca II triplet measures are included as reference. All the error bars are the largest uncertainty between the spectral resolution and the.

Extended Data Table 1 Optical spectral classifications of WOH G64
Full size table
Extended Data Table 2 Spectroscopic observations
Full size table

Supplementary information

Supplementary Information

Supplementary Discussion (sections ‘The binary system’ and ‘The cause of the transition’) and Supplementary Fig. 1.

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Muñoz-Sanchez, G., Kalitsounaki, M., de Wit, S. et al. The dramatic transition of the extreme red supergiant WOH G64 to a yellow hypergiant. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02789-7

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