Stephenson 2-18

Stephenson 2-18, also known as RSGC2-18, is a red supergiant (RSG) star within the open cluster Stephenson 2 located around 6,000 parsecs away in the constellation of Scutum.[2] It is currently among the largest known stars, if not the largest, and one of most luminous cool supergiants, with an estimated radius around 2,200 times that of the Sun (R☉), which corresponds to a volume around 10 billion times bigger than the Sun. If placed at the center of the Solar System, its photosphere would engulf the orbit of Saturn.

Stephenson 2-18

Dense starfield around the red supergiant star St2-18 (brightest star in the image) as seen from the 2MASS astronomical survey. The picture was published in 2003.
Credit: Université de Strasbourg/CNRS
Observation data
Epoch J2000      Equinox J2000
Constellation Scutum
Right ascension  18h 39m 02.3709s[1]
Declination −06° 05′ 10.5357″[1]
Characteristics
Evolutionary stage Red supergiant
Astrometry
Proper motion (μ) RA: −3.045[1] mas/yr
Dec.: −5.950[1] mas/yr
Parallax (π)−0.0081 ± 0.3120[1] mas
Distance19,560[2] ly
(6,000[2] pc)
Details
Radius2,150[3] R☉
Luminosity90,000[4] - 440,000[3] L☉
Temperature3,200[3] K
Other designations
Stephenson 2-18, Stephenson 2 DFK 1, RSGC2-18, RSGC2 SW St2-18, 2MASS J18390238-0605106, IRAS 18363-0607
Database references
SIMBADdata

Observation history

The open cluster Stephenson 2 was discovered by American astronomer Stephenson in 1990 in the data obtained by a deep infrared survey.[5][6]

Physical properties
Comparison of Stephenson 2-18 to VY Canis Majoris

Stephenson 2-18 is classified as an extremely large and high luminosity cool supergiant star. In 2012, a calculation of the bolometric luminosity by fitting the Spectral Energy Distribution (SED) give the star a luminosity of 440,000 L☉, with an effective temperature of 3,200 K, which corresponds to a very large but unusually high radius of 2,150 R☉ (1.50×109 km; 10.0 au; 930,000,000 mi), which is considerably larger and more luminous than expected for any red supergiant and extreme.[3] An alternate calculation gives a much lower luminosity of 90,000 L☉ and correspondingly lower radius values.[4]

See also

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. Davies, B.; Figer, D. F.; Kudritzki, R. P.; MacKenty, J.; Najarro, F.; Herrero, A. (2007). "A Massive Cluster of Red Supergiants at the Base of the Scutum‐Crux Arm". The Astrophysical Journal. 671 (1): 781–801. arXiv:0708.0821. Bibcode:2007ApJ...671..781D. doi:10.1086/522224.
  3. Fok, Thomas K. T; Nakashima, Jun-ichi; Yung, Bosco H. K; Hsia, Chih-Hao; Deguchi, Shuji (2012). "Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters". The Astrophysical Journal. 760 (1): 65. arXiv:1209.6427. Bibcode:2012ApJ...760...65F. doi:10.1088/0004-637X/760/1/65.
  4. Deguchi, Shuji; Nakashima, Jun-Ichi; Zhang, Yong; Chong, Selina S. N.; Koike, Kazutaka; Kwok, Sun (2010). "SiO and H2O Maser Observations of Red Supergiants in Star Clusters Embedded in the Galactic Disk". Publications of the Astronomical Society of Japan. 62 (2): 391–407. arXiv:1002.2492. Bibcode:2010PASJ...62..391D. doi:10.1093/pasj/62.2.391.
  5. Davies, B.; Figer, D. F.; Kudritzki, R. P.; MacKenty, J.; Najarro, F.; Herrero, A. (2007). "A Massive Cluster of Red Supergiants at the Base of the Scutum‐Crux Arm". The Astrophysical Journal. 671 (1): 781–801. arXiv:0708.0821. Bibcode:2007ApJ...671..781D. doi:10.1086/522224.
  6. Stephenson, C. B. (1990). "A possible new and very remote galactic cluster". The Astronomical Journal. 99: 1867. Bibcode:1990AJ.....99.1867S. doi:10.1086/115464.
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