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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.
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.
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.
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.
WOH G64 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. 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☉.
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.
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.
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.
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.
WOH G64 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. 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☉.
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