# WOH G64

WOH G64 (IRAS 04553-6825) is a symbiotic binary in the Large Magellanic Cloud (LMC), roughly 160,000 light-years from Earth. The main component of this system was once recognized as the best candidate for the largest known star when it was a red supergiant, until it gradually became a yellow hypergiant with half of its original size. The secondary is a B-type star. This system also exhibits features of B(e) stars.
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.

## Observational history

WOH G64 was discovered in the 1970s by Bengt Westerlund, N. Olander and B. 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 1-26, 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 (VLT) 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.

## Variability

As a red supergiant, WOH G64 A varies 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 (AGB 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. It is now classified as an irregular variable.

## Physical properties
### Red supergiant stage

The spectral type of WOH G64 A in its red supergiant stage was given as M5, but it is usually found to have a much cooler spectral type of M7.5, highly unusual for a supergiant star. Later observations showed that while the star used to be a M5–7.5 red supergiant, with a temperature of between 3200 K and 3400 K, it rapidly evolved, reaching a temperature of 4700 K and becoming a yellow hypergiant.
WOH G64 was classified as an extremely luminous M class supergiant and was likely to be the largest 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. The star's evolved state means that it can no longer hold on to its atmosphere due to low density, high radiation pressure, and the relatively opaque products of thermonuclear fusion. It had an average mass loss rate of 3.1 to 5.8×10$^{−4}$ M$_☉$ per year, among the highest known and unusually high even for a red supergiant.
Based on spectroscopic measurements assuming spherical shells, the star was originally calculated to have luminosity around between 490,000 and 600,000 L$_☉$, suggesting initial masses at least 40 M$_☉$ and consequently larger values for the radius between 2,575 and 3,000 R$_☉$. One such of these measurements from 2018 gives a luminosity of 432,000 L$_☉$ and a higher effective temperature of 3,500 K, based on optical and infrared photometry and assuming spherically-symmetric radiation from the surrounding dust. This would suggest a radius of 1,788 R$_☉$.
The dust surrounding WOH G64 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 (i.e. toward us). A much lower luminosity of 282,000+40,000 −30,000 L$_☉$ was derived based on radiative transfer modelling of the surrounding torus, suggesting an initial mass of 25±5 M$_☉$ and a radius around 1,730 R$_☉$ for an effective temperature of 3,200 K. In 2009, Emily Levesque calculated an effective temperature of 3,400±25 K by spectral fitting of the optical and near-UV SED. Adopting the Ohnaka luminosity with this new temperature gives a radius of 1,540±77 R$_☉$. 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 (e.g. the Hayashi limit or the Humphreys–Davidson limit).
WOH G64 was discovered to be a prominent source of OH, H $_2$O, and SiO 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.

### Yellow hypergiant stage

WOH G64 has shrunk in size since 2014, and has become a smaller yellow hypergiant. It is now about half of its size in the red supergiant phase, at 800 R$_☉$. The variability of WOH G64 also changed, from semiregular to irregular. Its change would only be recognized in a 2024 preprint, which also discovered WOH G64 is a symbiotic star with a smaller B-type companion.

## Companion

Since 2016, the spectrum of WOH G64 exhibits features of both B[e] stars and yellow stars, which is interpreted as the spectral signature of a massive symbiotic binary consisting of a yellow hypergiant losing material to an accreting B-type star companion. The persistent presence of surrounding hot dust, elongated appearance of the hypergiant in interferometric imaging, and the lack of a violent outburst during WOH G64's transition out of the red supergiant stage further supports the binary nature of WOH G64. The interacting binary system HR 5171 is considered an analog to WOH G64, as it also contains a yellow hypergiant with a B-type star companion. The presence of a hot stellar companion of WOH G64 was first suspected by Levesque et al. in 2009, who proposed that a late O-type main-sequence star companion could be ionizing the nebula surrounding WOH G64 in order to explain the 50 km/s shift between the nebular emission lines and WOH G64's spectral features.