WOH G64

WOH G64 is a red supergiant or hypergiant star located at a distance of 160,000 light years from Earth in the constellation Dorado. It is one of the largest stars discovered to date, as well as one of the most massive and most luminous stars of its type. WOH G64 does not belong to the Milky Way galaxy, but to one of its satellites, the Large Magellanic Cloud (LMC). With a visual magnitude of 18.46, the star is invisible to the unaided eye and cannot be seen in amateur telescopes.

Star type

WOH G64 has the stellar classification M5 I, indicating a luminous red supergiant. Its estimated initial mass is 25 times that of the Sun, sealing its fate as a supernova down the line. The star is between 1,540 and 2,575 times larger than the Sun. With an effective temperature between 3,008 and 3,400 K, it shines with a luminosity between 280,000 and 490,000 times that of the Sun.

WOH G64 is surrounded by a thick envelope of dust about a light year across. The envelope consists of material expelled from the star. WOH G64 has so far expelled between 3 and 9 solar masses of material – between a tenth and a third of its initial mass – through a strong stellar wind.

The star WOH G64. An artist’s impression is provided of the thick, massive torus of matter surrounding the star as inferred from observations made with ESO’s Very Large Telescope Interferometer. Image: European Southern Observatory

The estimated distance of 160,000 light years or 50,000 parsecs is based on the star’s membership in the Large Magellanic Cloud. WOH G64 is likely the largest, coolest and the most luminous red supergiant in the galaxy. A study published in 2009 determined a radial velocity of 294 ± 2 km s^–1 for the star, the same as that of the rotating gas in the disk of the LMC, confirming the star’s membership in the galaxy, even though it shows a number of characteristics that set it apart from other red supergiants in the LMC. These include the late spectral type, the thick torus-shaped dust cloud, nebular emission lines, and maser activity. The nebular emission lines have an unusual spectrum, with the hot gas that is rich in nitrogen and has a radial velocity that is significantly more positive that the star’s.

WOH G64 is known to be a variable star, but the exact type is uncertain. The star’s visual brightness fluctuates by more than a magnitude with a primary period of about 800 days. However, at visual wavelengths, the star is hidden by more than 6 magnitudes of interstellar extinction, and the variation in brightness is considerably smaller in the infrared part of the spectrum. The star has been characterized as a carbon-rich Mira variable or long-period variable. Mira variables are pulsating variable red giant stars on the asymptotic giant branch (AGB) – not supergiants – with periods of more than 100 days. However, even though the variability of WOH G64 has been confirmed by multiple studies, the type is still uncertain.

The star has a possible companion, a dwarf star of the spectral type O, with a luminosity about 100,000 times that of the Sun. However, the companion has not been confirmed by subsequent observations and the dust obscuring the star makes it a difficult object to study.

WOH G64, image: European Southern Observatory

Size

The estimated radius of WOH G64 is between 1,540 and 2,575 solar radii, or between 7.16 and 11.97 astronomical units. If it were placed in the centre of our solar system, it would extend well beyond the orbit of Jupiter, possibly even Saturn. Taking into account the margin of error of 192 solar radii for the current record holder, UY Scuti (1,708 solar radii), there is a possibility that WOH G64 is in fact the largest star known.

The star’s radius is uncertain and different studies have given different values. Observations with the Very Large Telescope (VLT) in 2007 yielded a bolometric luminosity of 282,000 solar luminosities, an initial mass about 25 times that of the Sun, and a radius 1,730 times solar with an effective temperature of 3,200 K.

A 2009 study gave an effective temperature of 3,400 ± 25 K and a radius 1,540 times that of the Sun.  Another study gave an estimated radius of 1,970 to 1,990 solar radii based on a temperature of 3,372 K to 3,400 K and a luminosity of about 450,000 solar luminosities.

The estimated radius and other properties of WOH G64 are consistent with those of the largest red supergiants in the Milky Way, including VY Canis Majoris, Mu Cephei and NML Cygni, as well as with theoretical modelling of the most luminous, coolest and largest possible supergiant stars.

Facts

WOH G64 was discovered by Bengt Westerlund, Nils Olander and B. Hedin at the Uppsala Astronomical Observatory (UAO) in Sweden in the 1970s. The “WOH” in WOH G64 is taken from the initials of the three astronomers and is used for an entire catalogue of supergiant and giant stars in the Large Magellanic Cloud. Westerlund is also credited for the discovery of Westerlund 1-26, another exceptionally large red supergiant or hypergiant, located in the Westerlund 1 super star cluster, also known as the Ara Cluster, in the constellation Ara. With an estimated radius between 1,530 and 1,580 solar radii, with some estimates going as high as 2,550 solar radii, Westerlund 1-26 is currently the third largest star known, after UY Scuti and WOH G64.

The results of interferometric observations with ESO’s Very Large Telescope Interferometer (VLTI), published in 2008, revealed that WOH G64 is surrounded by an optically thick torus which appears almost pole-on. The results were consistent with an initial mass of 25 solar masses.

Location

WOH G64 is located in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way and the fourth largest galaxy in the Local Group, after Andromeda (M31), the Milky Way, and the Triangulum Galaxy (M33). Lying at a distance of about 163,000 light years, the LMC is the second or third nearest galaxy to our own. It is classified as a Magellanic spiral galaxy, a dwarf galaxy with a single spiral arm. With an apparent magnitude of 0.9, the galaxy appears as a hazy cloud on a clear night, but due to its southern location, it is invisible to most northern observers. It can only be seen from locations south of latitude 20° N.

Constellation

WOH G64 is located in the constellation Dorado, the Dolphinfish. Even though it is one of the fainter constellations – its brightest star has an apparent magnitude of 3.27 – Dorado contains a number of interesting stars and deep sky objects. The stars include the luminous blue variable S Doradus, the red giant Mira variable R Doradus, and the pulsating variable Gamma Doradus, the prototype for its own class of variable stars. Dorado also hosts the Wolf-Rayet stars BAT99-98, possibly the most massive star known, and R136a1, the most luminous star discovered to date.

Dorado constellation map by IAU and Sky&Telescope magazine

The best known deep sky object in Dorado is the Large Magellanic Cloud (LMC), which contains a number of notable features, among them the star-forming regions known as the Tarantula Nebula (NGC 2070) and the Ghost Head Nebula (NGC 2080). The star cluster R136 at the centre of the Tarantula Nebula is home to some of the most massive and luminous stars discovered to date, including R136a1, R136a7, and R136c.

The best time of year to observe the stars and deep sky objects in Dorado is during the month of January.

The 10 brightest stars in Dorado are Alpha Doradus (mag. 3.27), Beta Doradus (mag. 3.46 – 4-08), Gamma Doradus (mag. 4.25), Delta Doradus (mag. 4.34), HD 40409 (mag. 4.65), Zeta Doradus (mag. 4.82), Theta Doradus (mag. 4.82), Eta² Doradus (mag. 5.01), Nu Doradus (mag. 5.06), and Epsilon Doradus (mag. 5.11).

WOH G64