Stephenson 2-18

Stephenson 2-18 (St2-18) is a red supergiant located in the constellation Scutum. Also catalogued as Stephenson 2-DFK 1 and RSGC2-18, it is the current record holder for the largest star known, with a size 2,150 times that of the Sun. The star is located at a distance of 18,900 light years from Earth. It is a member of the open cluster Stephenson 2.

Star type

Stephenson 2-18 is a red supergiant of the spectral type M6. It is one of the largest stars ever discovered, with a radius of 2,150 solar radii.

It is also one of the most luminous red supergiants known. With a surface temperature of 3,200 K, it shines with about 440,000 solar luminosities.

Size

Stephenson 2-18 has an estimated radius of 2,150 solar radii. If it replaced the Sun in our solar system, it would extend past the orbit of Saturn (1,940 – 2,169 R_☉). The star’s size corresponds to a volume about 10 billion times greater than the Sun. The only two stars that come even close to this size are MY Cephei in the constellation Cepheus with a radius 2,061 times that of the Sun and WY Velorum in Vela with 2,028 solar radii.

Stephenson 2-18 size compared to other large stars, image: Wikimedia Commons/THE COLOSSAL GALAXY NAMED IC1101 (CC BY-SA 4.0)

Stephenson 2-18 compared with UY Scuti

Stephenson 2-18 took the title of the largest star known from the previous record holders, the red supergiants WOH G64 in the constellation Dorado and UY Scuti in Scutum. WOH G64 has an estimated radius between 1,540 and 1,730 solar radii, which is considerably smaller than both St2-18 and the current runner-up, MY Cephei (2,061 R_☉).

A size comparison of the Sun, UY Scuti and Stephenson 2-18, image: Wikimedia Commons/THE COLOSSAL GALAXY NAMED IC1101 (CC BY-SA 4.0)

UY Scuti had an estimated radius of 1,708 solar radii until more accurate measurements of the star’s parallax in the Gaia Data Release 2 yielded a lower distance and therefore lower values for the star’s luminosity and radius. The estimated radius of UY Scuti is now only 755 solar radii, which is comparable to that of Antares (680 – 800 R_☉), but smaller than the radii of Betelgeuse (887 R_☉), Mu Cephei (972 – 1,260 R_☉), VY Canis Majoris (1,420 R_☉), and HR 5171A (1,060 – 1,160 R_☉).

Facts

The open cluster Stephenson 2 is one of the most massive open clusters in the Milky Way. It was first noticed by American astronomer Charles Bruce Stephenson, who reported the discovery in June 1990. Stephenson discovered the cluster after a space-deep infrared objective-prism survey.

The cluster stretches across about 6’ of the sky and contains a core group of 26 red supergiants, the largest known population in the Milky Way, first identified in a study published in 2007. The estimated age of the cluster is 17 ± 3 million years. In the study, the brightest star in the K-band that lay in the same line of sight as the cluster (St2-18) was given the identifier 1. The study concluded that the star likely had considerable infrared excess and may be a red hypergiant like VY Canis Majoris.

Stephenson 2-18, image: Université de Strasbourg (CC BY-SA 4.0)

Stephenson 2-18 has a radial velocity about 20 km/s below that of the other cluster stars, which indicates that it is not a foreground giant. It may be a cluster member and its observed radial velocity is offset by a thick expanding envelope. The same study stated that the star appeared to be on the verge of shedding its outer layers and evolving blueward into a luminous blue variable (LBV) or a Wolf-Rayet star.

In a study published in 2010, the supergiant was given the identifier 18 and assigned to the Stephenson 2 SW cluster, an aggregation of stars 5’ southwest of the core cluster, believed to lie at the same distance as Stephenson 2.

The clusters RSGC2 (Stephenson 2) and RSGC1 contain about 20% of all the known red supergiants in the Milky Way and are frequent targets for observations studying pre-supernova evolution.

A 2013 study of the class M supergiants in the cluster Stephenson 2 detected maser emission from the two brightest members of the cluster, indicating that the stars have the highest mass loss rates of all the members.

Location

Stephenson 2-18 is a member of the open cluster Stephenson 2, which occupies an area of 1.8’ of the sky but is not visible in amateur telescopes. The cluster cannot be detected in visible light at all because it is heavily obscured by dust, but it can be seen in infrared light. It lies in the region of the sky between Alpha and Beta Scuti.

Stephenson 2 location, image: Wikisky

Constellation

Stephenson 2-18 is located in the constellation Scutum. Originally known as Scutum Sobiescianum (the Shield of Sobieski), the constellation was named by Polish astronomer Johannes Hevelius in honour of the Polish King John III Sobieski’s victory in the Battle of Vienna in 1684.

Scutum is one of the smallest and faintest constellations in the sky. It is the 84th constellation in size, stretching across only 109 square degrees of the southern sky. It does not contain any stars brighter than magnitude 3.00. None of its stars make the list of the 300 brightest stars in the sky.

Scutum constellation map by IAU and Sky&Telescope magazine

Scutum is best known for being home to the variable white giant star Delta Scuti, which serves as a prototype for its own class of variables, and to the pulsating variable red supergiant UY Scuti, one of the largest known stars. Notable deep sky objects in Scutum include the open clusters Messier 11, better known as the Wild Duck Cluster, and Messier 26, the massive young clusters RSGC1, RSGC2 (Stephenson 2), RSGC3 and RSGC4 (Alicante 8), the globular cluster NGC 6712, and the planetary nebula IC 1295.

The best time of year to observe the stars and deep sky objects in Scutum is during the month of August, when the constellation rises high in the evening sky. The entire constellation is visible from locations between the latitudes 80° N and -90° S, i.e. from all inhabited places on Earth.

The 10 brightest stars in Scutum are Alpha Scuti (mag. 3.83), Beta Scuti (mag. 4.22), Zeta Scuti (mag. 4.66), Gamma Scuti (mag. 4.67), Delta Scuti (mag. 4.60 – 4.79), Eta Scuti (mag. 4.83), Epsilon Scuti (mag. 4.88), HD 175156 (mag. 5.08), HD 171391 (mag. 5.12), and R Scuti (mag. 4.2 – 8.6).

Stephenson 2-18