WR 104

WR 104 is a young star system consisting of a Wolf-Rayet star and two hot blue main sequence stars. The system lies about 8,400 light years from Earth in the constellation Sagittarius. The Wolf-Rayet star is expected to end its life as a supernova in the next few hundred thousand years. When it does, there is a slight chance that it may produce a long duration gamma-ray burst approximately in the direction of Earth.

Star system

WR 104 is a triple star system composed of a close binary pair – a Wolf-Rayet star and a B-type main sequence star – and a more distant O-type star. All three stars are exceptionally hot and luminous.

The Wolf-Rayet star has a mass 10 times that of the Sun and a radius 3.29 times solar. It is 40,000 times more luminous than the Sun with an effective temperature of about 45,000 K. The star has the stellar classification WC9d. The “C” indicates dominant lines of ionised carbon, while the suffix “d” indicates dust in the star’s spectrum.

False-color infrared image of Wolf-Rayet star WR 104 taken at the W. M. Keck Observatory. Image: NASA

The Wolf-Rayet star forms a spectroscopic binary pair with a main sequence star of the spectral type B0.5V. The individual components cannot be resolved even in the largest of telescopes because the physical distance between them is only about 2.34 astronomical units (Earth – Sun distances). At 8,400 light years, this corresponds to an angular separation of about 1 milliarcsecond. The stars take 241.5 days to complete an orbit. They have a combined apparent magnitude of 13.28.

The companion has a mass of about 20 solar masses and a radius 10 times that of the Sun. With a surface temperature of 30,000 K, it is 80,000 times more luminous than the Sun.

The Wolf-Rayet star is considered to be the primary component in the system. Even though it is 0.3 magnitudes fainter than its close companion in the visible part of the spectrum, it dominates the spectrum of the system.

The third component is fainter, with a visual magnitude of 15.36. It is a hot blue star of the spectral type O8V-O5V. It lies almost an arcsecond away from the binary pair and is believed to be physically associated with it.

WR 104 exhibits irregular variations in brightness caused by eclipses and other events. The eclipses are not believed to be caused by the companion but by dust formed from the material ejected by the Wolf-Rayet star. They do not affect the companion. The Wolf-Rayet star exhibits an almost continuous string of eclipses, which are sometimes concurrent and have a range of depths.

Supernova

Both components in the spectroscopic binary system are expected to meet their ends as core-collapse supernovae. The Wolf-Rayet star is expected to go out sooner, in the next few hundred thousand years. There has been a lot of debate on whether the supernova will affect life on Earth, but there are too many uncertainties to make an accurate prediction.

The rotational axis of the binary pair – and probably of the two nearest stars – in the WR 104 system points roughly in the direction of our planet, at an inclination of 0 to 16 degrees. However, the inclination may be greater. Spectroscopic observations with the Keck Telescope indicate that the system may be inclined 30° – 40° away from us.

The stars lie at too great a distance for the supernovae themselves to have a real impact on our planet’s biosphere. However, Wolf-Rayet stars with fast rotating cores are believed to produce long-duration gamma-ray bursts (LGRBs), which are emitted along their spin axes. If WR 104 does produce a LGRB, this could significantly affect Earth if it our planet lies within an opening angle of 12 degrees or less of the burst. Both the Wolf-Rayet star and its close companion may produce a LGRB based on current models, but there are still too many unknown parameters to determine the likelihood of this outcome. To produce a LGRB, a star has to be spinning very fast and it is uncertain if this is the case with WR 104. There has been some evidence that suggests that Wolf-Rayet stars in the Milky Way may be slowed down too much to produce a gamma-ray burst. These events have mostly been observed in low-metallicity galaxies, never in the Milky Way.

Pinwheel Nebula

The Wolf-Rayet star in the WR 104 system is surrounded by a Wolf-Rayet nebula, an extended dust shell formed by the interaction of the stellar winds of the two stars in close orbit. Spiral in shape, the nebula is often called the Pinwheel Nebula. It has a diameter of more than 200 astronomical units. The nebula was imaged by the Keck Telescope in April 1998.

The nebula’s dust would not have formed near the WR star if the star were solitary due to its intense radiation. However, in the binary system it is formed when material in the region where the stars’ stellar winds collide is compressed. The spiral shape of the nebula is the result of the system’s rotation. It indicates that the system appears almost pole-on. The nebula’s outflow pattern has allowed astronomers to derive an orbital period of 241.5 ± 0.5 days for the two stars.

Facts

WR 104 was first identified as a binary system in 1977 by M. Cohen and L. V. Kuhi, who suggested that the companion was an early-type star. In 1987, Williams, van der Hucht and Thé estimated that the companion was 0.3 magnitudes brighter than the Wolf-Rayet star. In 1999, Tuthill led a study that found further evidence of a companion with an orbital period of 220 ± 30 days.

Name

The designation WR 104 comes from the names of the French astronomers Charles Wolf and Georges Rayet, who were the first to discover three stars in this class, now known as WR 134, WR 135 and WR 137, in 1867. Wolf and Rayet used the 40 cm Foucault telescope at the Paris Observatory. Located in the northern constellation Cygnus, the stars showed very broad emission lines that were originally believed to be hydrocarbon molecules, but are now associated with helium, nitrogen, carbon, silicon and oxygen, with weak or absent hydrogen lines. The stars were called Wolf-Rayet stars from their discovery, but the naming convention for this class was not created until 1962, when the fourth catalogue of Wolf-Rayet stars in our galaxy was published and the stars were numbered in order of right ascension. Even though the numbering scheme used today is different and catalogues include new discoveries, the Wolf-Rayet stars in the Milky Way are still catalogued in this order.

Location

WR 104 lies just northwest of the Teapot asterism, in an area of the sky that is very familiar to stargazers because it contains several bright, large nebulae.

WR 104 location, image: Wikisky

The star is located between the Trifid Nebula (Messier 20) and the Lagoon Nebula (Messier 8), just north of the fainter emission nebula NGC 6526. With an apparent magnitude of 13.28, WR 104 is invisible to the unaided eye and cannot be seen in binoculars. It can only be seen in 6-inch and larger telescopes.

M8, M20 and the Teapot in Sagittarius, image: Wikisky

Constellation

WR 104 is located in the constellation Sagittarius. First catalogued by Claudius Ptolemy in his Almagest in the 2nd century CE, Sagittarius is considered to be one of the 48 Greek constellations. The celestial Archer stretches across 867 square degrees of the southern sky and is the 15th largest of all 88 constellations.

Easily recognizable for the Teapot pattern, the constellation is a popular target for backyard telescopes because it contains more Messier objects (bright deep sky objects catalogued by the French astronomer Charles Messier) than any other constellation. In addition to the Trifid and Lagoon nebulae, these include the Omega (Swan) Nebula (Messier 17), the Small Sagittarius Star Cloud (Messier 24), the globular clusters Messier 22, Messier 28, Messier 54, Messier 55, Messier 69, and Messier 75, and the open clusters Messier 23 and Messier 25.

Sagittarius constellation map by IAU and Sky&Telescope magazine

Other notable deep sky objects in Sagittarius include the planetary nebulae NGC 6537 (the Red Spider Nebula), NGC 6445 (the Box Nebula), NGC 6818 (the Little Gem Nebula), and M 1-42 (the Eye of Sauron Nebula), the star-forming nebula NGC 6559, and the Sagittarius Dwarf Elliptical Galaxy, one of Milky Way’s satellite galaxies.

Sagittarius is also home to many interesting stars. These include the blue-white giant Kaus Australis, the constellation’s brightest star, the luminous blue variable known as the Pistol Star, the Wolf-Rayet star WR 102ka (the Peony Star), the latter two among the most luminous stars known in our galaxy, the red supergiants KW Sagittarii and VX Sagittarii, notable for their exceptional size, Sakurai’s Object (V4334 Sagittarii), a star believed to have been born again as a red giant after a brief period of being a white dwarf, and the bright orange giants Kaus Media, Kaus Borealis, Alnasl and Tau Sagittarii.

The best time of year to observe the stars and deep sky objects of Sagittarius is during the month of August, when the constellation rises high above the horizon in the evening sky. The entire constellation is visible from locations between the latitudes 55° N and 90° S.

The 10 brightest stars in Sagittarius are Kaus Australis (Epsilon Sgr, mag. 1.85), Nunki (Sigma Sgr, mag. 2.05), Ascella (Zeta Sgr, mag. 2.59), Kaus Media (Delta Sgr, mag. 2.70), Kaus Borealis (Lambda Sgr, mag. 2.82), Albaldah (Pi Sgr, mag. 2.89), Alnasl (Gamma² Sgr, mag. 2.98), Eta Sagittarii (mag. 3.11), Phi Sagittarii (mag. 3.17), and Tau Sagittarii (mag. 3.326).

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