Location of WOH G64 (circled) in the Large Magellanic Cloud
Credit: NASA/JPL-Caltech/M. Meixner (STScI) & the SAGE Legacy Team
Epoch J2000,0 Equinox J2000,0
|Right ascension||04h 55m 10.482s|
|Declination||−68° 20′ 29.81″|
|Apparent magnitude (V)||18.46|
|Apparent magnitude (K)||6.85|
|Radial velocity (Rv)||294 km/s|
|Distance||163,000 ly |
|Radius||1,540 ± 77, 1,730 R☉|
|Surface gravity (log g)||-0.5 cgs|
|Temperature||3,400 ± 25, 3,200 K|
WOH G64, 2MASS J04551048-6820298
WOH G64 is a red hypergiant star in the Large Magellanic Cloud satellite galaxy in the southern constellation of Dorado. It is 163,000 light years away from Earth and is one of the largest known stars, with a radius of 1,540 to 1,730 solar radii (1.07×109 to 1.20×109 km; 7.2 to 8.0 au), corresponding to a volume some 3.65 to 5.2 billion times bigger than the Sun. If placed at the center of the Solar System, the star's surface would engulf Jupiter.
The star was discovered in the 1970s by Westerlund, Olander and Hedin. Like NML Cygni, the "WOH" in the star's name comes from the 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 cool hypergiant star, Westerlund 1-26, found in the massive super star cluster Westerlund 1 in the constellation Ara.
WOH G64 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 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 combination of the star's temperature and luminosity places 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. The intervening dust clouds makes the study of the star very difficult. It may even be possible that it has a bright hot companion, but there has been no confirmation of this observation. The strong stellar wind has created a torus-shaped cloud roughly a light year in diameter containing 3-9 M☉ of expelled material.
In 2004, Monnier et al. calculated that the star has a radius around 3,000 R☉, in 2007 Ohnaka et al. calculated that the star has a luminosity of 284,000 L☉ and a radius around 1,730 R☉ based on the assumption of an effective temperature of 3,200 K and radiative transfer modelling of the surrounding torus. In 2009, Levesque calculated an effective temperature of 3,400 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 R☉. Earlier studies, ignoring the asymmetric radiation caused by the dusty torus, found luminosities around twice as large, and consequently larger values for the radius. The physical parameters found by the most recent research are consistent with the largest red supergiants found elsewhere and with theoretical models of the coolest and largest possible stars (e.g. the Hayashi limit).
- Orders of magnitude (length)
- List of most luminous stars
- UY Scuti
- VY Canis Majoris
- VV Cephei A
- NML Cygni
- Westerlund 1-26
- Mu Cephei
- Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.; Evans, T.; Fowler, J.; Gizis, J.; Howard, E.; Huchra, J.; Jarrett, T.; Kopan, E. L.; Kirkpatrick, J. D.; Light, R. M.; Marsh, K. A.; McCallon, H.; Schneider, S.; Stiening, R.; Sykes, M.; Weinberg, M.; Wheaton, W. A.; Wheelock, S.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". VizieR On-line Data Catalog: II/246. Originally published in: 2003yCat.2246....0C. 2246. Bibcode:2003yCat.2246....0C.
- Levesque, E. M.; Massey, P.; Plez, B.; Olsen, K. A. G. (2009). "The Physical Properties of the Red Supergiant Woh G64: The Largest Star Known?". The Astronomical Journal. 137 (6): 4744. arXiv: . Bibcode:2009AJ....137.4744L. doi:10.1088/0004-6256/137/6/4744.
- Ohnaka, K.; Driebe, T.; Hofmann, K. H.; Weigelt, G.; Wittkowski, M. (2009). "Resolving the dusty torus and the mystery surrounding LMC red supergiant WOH G64". Proceedings of the International Astronomical Union. 4: 454. Bibcode:2009IAUS..256..454O. doi:10.1017/S1743921308028858.
- Westerlund, B. E.; Olander, N.; Hedin, B. (1981). "Supergiant and giant M type stars in the Large Magellanic Cloud". Astronomy & Astrophysics Supplement Series. 43: 267–295. Bibcode:1981A&AS...43..267W.
- Westerlund, B. E. (1987). "Photometry and spectroscopy of stars in the region of a highly reddened cluster in ARA". Astronomy and Astrophysics. Supplement. 70 (3): 311–324. Bibcode:1987A&AS...70..311W. ISSN 0365-0138.
- Fraser, Oliver J.; Hawley, Suzanne L.; Cook, Kem H. (2008). "The Properties of Long-Period Variables in the Large Magellanic Cloud from MACHO". The Astronomical Journal. 136 (3): 1242. arXiv: . Bibcode:2008AJ....136.1242F. doi:10.1088/0004-6256/136/3/1242.
- Soszyñski, I.; Udalski, A.; Szymañski, M. K.; Kubiak, M.; Pietrzyñski, G.; Wyrzykowski, Ł.; Szewczyk, O.; Ulaczyk, K.; Poleski, R. (2009). "The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. IV. Long-Period Variables in the Large Magellanic Cloud". Acta Astronomica. 59: 239. arXiv: . Bibcode:2009AcA....59..239S.
- Monnier, J. D; Millan-Gabet, R; Tuthill, P. G; Traub, W. A; Carleton, N. P; Coudé Du Foresto, V; Danchi, W. C; Lacasse, M. G; Morel, S; Perrin, G; Porro, I. L; Schloerb, F. P; Townes, C. H (2004). "High-Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer". The Astrophysical Journal. 605: 436. arXiv: . Bibcode:2004ApJ...605..436M. doi:10.1086/382218.
- Van Loon, J. Th.; Cioni, M.-R. L.; Zijlstra, A. A.; Loup, C. (2005). "An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars". Astronomy and Astrophysics. 438: 273. arXiv: . Bibcode:2005A&A...438..273V. doi:10.1051/0004-6361:20042555.