WOH G64 (IRAS 04553–6825) is a symbiotic binary in the Large Magellanic Cloud, roughly 50 kiloparsecs (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, calculated to be around 1,540 times that of the Sun (R☉). At this radius, an object travelling at the speed of light would take over 6 hours to go around its surface, compared to just 14.5 seconds for the Sun. It is also one of the most luminous and massive red supergiants, with a luminosity around 282,000 times the solar luminosity (L☉) and a mass roughly 20 times that of the Sun (M☉). If placed at the center of the Solar System, the star's photosphere would engulf the orbit of Jupiter. The secondary component, being recently confirmed, is comparatively nowhere near as well-studied but it is at least understood to be a B-type star. 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, Nils 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 Large Magellanic Cloud. Westerlund also discovered four red supergiants in the massive super star cluster Westerlund 1 in the constellation Ara, notably including W26. 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.
Distance WOH G64 is assumed to be around 50 kiloparsecs (163,000 light-years) away from Earth, since it appears to be in the Large Magellanic Cloud. The Gaia Data Release 3 parallax for WOH G64 is −0.2477±0.0430 mas and the negative parallax does not provide a reliable distance.
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 (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. The variability has since been observed to transition from semi-regular to irregular circa 2014.
Physical properties WOH G64 A was discovered to be a prominent source of OH, H2O, and SiO masers emission, which is typical of an OH/IR supergiant star. It also 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 by line emission due to highly excited carbon monoxide. The spectral type of WOH G64 A is given as M5, but it is usually found to have a much cooler M7.5, which is highly unusual for a supergiant star. The combination of the star's temperature and luminosity, based on most estimates, placed it toward the upper right corner of the Hertzsprung–Russell diagram. It has an average mass loss rate of 3.1 to 7.4×10−4 M☉ per year, among the highest known and unusually high even for a red supergiant. Based on spectroscopic measurements assuming spherical shells, the red supergiant primary was originally calculated to have luminosity between 490,000 and 600,000 L☉, suggesting an initial mass of at least 40 M☉ and consequently larger values for the radius between 2,575 and 3,000 R☉. One measurement 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 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 escapes through the cavity (i.e., toward us). Many other disks have also been observed around other heavily mass-losing hypergiant stars, such as VY Canis Majoris, Mu Cephei, and the post-red supergiant IRC +10420. A much lower luminosity of 280,000 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.
Largest known star In 2009, Levesque et al. (2009) calculated an effective temperature of 3,400±25 K by spectral fitting of the optical and near-UV SED. Taking the flux contribution of the dusty torus into account gives a luminosity of 282,000+34,400−30,700 L☉, similar to the luminosity calculated by Ohnaka et al. (2008). Combining this luminosity with the newly-derived 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 A is possibly the largest known star and the most luminous and coolest red supergiant in the Large Magellanic Cloud. 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 7.4×10−4 M☉ per year, among the highest known and unusually high even for a red supergiant.
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