The Largest Star in the Universe

​Stephenson 2-18 took the title of the largest star known from the previous record holders, the red super-giants 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 St2-18. 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 to only 755 solar radii, which is comparable to that of Antares, but smaller than the radii of Betelgeuse, Mu Cephei, VY Canis Majoris, and HR 5171A. 

​Stephenson 2-18 does not have a proper name. Its designation comes from the name of its discoverer, Charles Bruce Stephenson. The star is also known as RSGC2-18 and Stephenson 2 DFK 1. RSGC2 stands for Red Super-giant Cluster 2. DFK 1 comes from the initials of Ben Davies, Don F. Figer, and Rolf-Peter Kudritzki, who identified a core group of 26 physically associated red super-giants in Stephenson 2 in 2007. St2-18 was given the identifier 1 as the brightest star appearing in the region. 

Stephenson 2-18 appears as 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 known as RSGC2 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 2-18 is located in the constellation Scutum, in the Scutum-Centaurus Arm of the Milky Way galaxy. 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 of “ten faint, dust-reddened stars, several of them probable M super-giants, in a 1.5 x 3 arcmin area of sky” in a study published in The Astronomical Journal in June 1990. Stephenson discovered the cluster during a space-deep infrared objective-prism survey of the northern Milky Way with the Burrell Schmidt Telescope of the Warner and Swasey Observatory. He estimated a distance of 30 kiloparsecs (98,000 light-years) for the cluster based on the assumption that the stars were all red super-giants. 
​

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

​Stephenson 2-18 (St2-18) is a red supergiant or red hypergiant star of the spectral type M6, 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 appears in the same region as the open cluster Stephenson 2. With an effective temperature of 3,200 K, St2-18 shines with 436,516 solar luminosities. The stellar properties were derived in a 2012 study that used the Australia Telescope Compact Array (ATCA) and the DUSTY model. The star’s exact parameters are uncertain, but estimates of its energy output are in the range from 90,000 to 630,000 solar luminosities. If it replaced the Sun at the centre of our solar system, it would extend past the orbit of Saturn. The star’s size corresponds to a volume about 10 billion times greater than the Sun. The only two stars that may come even close to this size are MY Cephei in the constellation Cepheus with a radius 1,134 – 2,061 times that of the Sun and WY Velorum in Vela with 2,028 solar radii. With a current radius estimate of St2-18, it would take almost 9 hours to complete a circle around the star’s surface travelling at the speed of light. In comparison, making the same trip around the Sun would take only 14.5 seconds.

The Stephenson 2 cluster stretches across about 6’ of the sky and contains a core group of 26 red super-giants, the largest known population in the Milky Way, first identified in a study published in The Astrophysical Journal in 2007. Study authors estimated a spectral type of M5 or M6 for St2-18 and suggested that the star would soon expel its outer layers and evolve into a luminous blue variable (LBV) or Wolf-Rayet star. The estimated age of the cluster is 17 ± 3 million years. A 2012 study reported about 80 red super-giants appearing in the same line of sight as Stephenson 2 (RSGC2), 40 of which had radial velocities indicating that they were members of the cluster. Since these super-giants are spread across a wider area of the sky, they likely form an extended stellar association. The cluster has an estimated mass between 30,000 and 50,000 solar masses and a physical radius of about 4 parsecs (13 light-years). The cluster is believed to be located at the intersection of the northern end of Milky Way’s Long Bar and the inner region of the Scutum-Centaurus Arm, one of our galaxy’s two major spiral arms (the other being the Perseus Arm). Stephenson 2 is one of several massive open clusters containing multiple red super-giants located in the constellation of Scutum. The others include RSGC1, RSGC 3, Alicante 7 (RSGC5), Alicante 8 (RSGC4), and Alicante 10 (RSGC6). These clusters appear in the same area of the sky. RSGC1 hosts the red supergiant RSGC1-F01, a star whose radius would extend past the orbit of Jupiter in placed at the centre of the solar system.
​

The clusters RSGC2 (Stephenson 2) and RSGC1 contain about 20% of all the known red super-giants in the Milky Way and are frequent targets for observations studying pre-supernova evolution. 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. A 2013 study of the class M super-giants 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.

Stephenson 2-18
Spectral class:   M6

Apparent magnitude (G):   15.2631 ± 0.0092

Apparent magnitude (J):  7.150

Apparent magnitude (H):  4.698

Apparent magnitude (K):  2.9

Distance:  18,900 light years (5,800 parsecs)

Parallax:  −0.1524 ± 0.2330 mas

Proper motion:   RA: −1.697 ± 0.266 mas/yr
                                 Dec.: -4.637 ± 0.221 mas/yr

Luminosity: 436,516L☉(90,00–630,000L☉)

Radius:   2,150 R☉

Temperature:   3,200 K

Constellation:  Scutum

Right ascension:   18h 39m 02.3694806915s

Declination:   −06° 05′ 10.556738789″

Names and designations:   Stephenson 2-18, St2-18, Stephenson 2 DFK 1, Gaia        DR2 4253084565963481856, Gaia DR3 4253084565963481856, MSX6C G026.1044-00.0283, IRAS 18363-0607, 2MASS J18390238-0605106

Reference:
Stephenson 2 – 18. (2023, June 24). Star Facts. Retrieved from https://www.star-facts.com/stephenson-2-18/