WOH G64
WOH G64 is a symbiotic binary in the Large Magellanic Cloud, roughly 163,000 light-years from Earth. The primary component is an extreme red supergiant or red hypergiant that may be the largest known star with a well-defined radius. It is also one of the most luminous stars|most luminous] and massive stars|massive] red supergiants, with a radius calculated to be around 1,540 times that of the Sun and a luminosity around 282,000 times the solar luminosity. If placed at the center of the Solar System, the star's photosphere would engulf the orbit of Jupiter.
WOH G64 is surrounded by an optically thick dust envelope of roughly a light year in diameter, containing 3 to 9 times the Sun's mass of expelled material that was created by the strong stellar wind of the red supergiant primary.
Observational history
WOH G64 was discovered in the 1970s by Bengt Westerlund, Noah Olander, and Björn Hedin. Like NML Cygni, the "WOH" in the star's name comes from the last names of its three discoverers, but in this case refers to a whole catalogue of giant and supergiant stars in the LMC. Westerlund also discovered another notable red supergiant star, Westerlund 1 W26, found in the massive super star cluster Westerlund 1 in the constellation Ara. In 1986, infrared observations showed that it was a highly luminous supergiant surrounded by gas and dust which absorbed around three quarters of its radiation.In 2007, observers using the Very Large Telescope showed that WOH G64 is surrounded by a torus-shaped cloud. In 2024, the dusty torus around WOH G64 was directly imaged by VLTI, showing the elongated and compact emission around the hypergiant. This is also the first interferometric imaging of a star outside the Milky Way.
Physical properties
The spectral type of WOH G64 A is given as M5, but it is usually found to have a much cooler spectral type of M7.5, highly unusual for a supergiant star.Based on spectroscopic measurements assuming spherical shells, the star was originally calculated to have luminosity around between, suggesting an initial mass of at least and consequently larger values for the radius between. One measurement from 2018 gives a luminosity of and a higher effective temperature of, based on optical and infrared photometry and assuming spherically-symmetric radiation from the surrounding dust. This would suggest a radius of.
The dust surrounding WOH G64 A was revealed in 2007 to have a torus-like shape which was being viewed pole-on, meaning that the previous radius and luminosity estimates which assumed spherical dust shells were overestimated, as the radiation escape through the cavity. A much lower luminosity of was derived based on radiative transfer modelling of the surrounding torus, suggesting an initial mass of and a radius around for an effective temperature of. In 2009, Emily Levesque calculated an effective temperature of by spectral fitting of the optical and near-UV SED. Adopting the Ohnaka luminosity with this new temperature gives a radius of. Those physical parameters are consistent with the largest galactic red supergiants and hypergiants found elsewhere such as VY Canis Majoris and with theoretical models of the coolest, most luminous and largest possible cool supergiants.
WOH G64 A is possibly the largest known star and the most luminous and coolest red supergiant in the LMC. The combination of the star's temperature and luminosity placed it toward the upper right corner of the Hertzsprung–Russell diagram. It has an average mass loss rate of 3.1 to per year, among the highest known and unusually high even for a red supergiant.
WOH G64 A was discovered to be a prominent source of OH,, and masers emission, which is typical of an OH/IR supergiant star. It shows an unusual spectrum of nebular emission; the hot gas is rich in nitrogen and has a radial velocity considerably more positive than that of the star. The stellar atmosphere is producing a strong silicate absorption band in mid-infrared wavelengths, accompanied a line emission due to highly excited carbon monoxide.
Variability
WOH G64 A used to vary regularly in brightness by over a magnitude at visual wavelengths with a primary period of around 800 days. The star suffers from over six magnitudes of extinction at visual wavelengths, and the variation at infra-red wavelengths is much smaller. It has been described as a carbon-rich Mira or long-period variable, which would necessarily be an asymptotic-giant-branch star rather than a supergiant. Brightness variability has been confirmed by other researchers in some spectral bands, but it is unclear what the actual variable type is. No significant spectral variation has been found. The variability has since been observed to transition from semi-regular to irregular circa 2014.Supposed yellow hypergiant transition
Photometric measurements spanning from 30 years showed a transition from semi-regular to irregular variability during around 2014, which together with the absence or strength of certain spectral lines in spectroscopic observations that would be inconsistent with a red supergiant, led to Munoz-Sanchez et al. to conclude that WOH G64 A transitioned from a red supergiant to a yellow hypergiant. The lack of a violent outburst and smooth transition would be explained by the presence of a B-type companion forming a symbiotic binary. As a yellow hypergiant, WOH G64 A would be half of its original size, at, and have a hotter effective temperature of 4,700 K with a likely spectral class of either early K or late G.This interpretation was subsequently challenged by van Loon & Ohnaka, which detected molecular absorption bands of titanium oxide in spectroscopic data taken between 2024 and 2025, implying that the central component is still a red supergiant. The anomalies observed by Munoz-Sanchez et al. were explained instead by the periastron passage of the companion: Its tidal forces stretched the outer layers of the primary's atmosphere, causing the atmospheric layer of optical depth 1 to be an inner layer that has a hotter temperature, instead of a cooler layer as it used to be. This, in turn, changes the nature of the variability. Since then, the star has returned to its original state.