Stephenson 2-18

red supergiant star in the open cluster Stephenson 2

Stephenson 2-18 (St2-18, also known as Stephenson 2-18 and RSGC2-18)[3] is a red supergiant (RSG) star. It is close to, and a possible member of, the open cluster Stephenson 2. It is about 6,000 parsecs (20,000 ly) away from Earth in the constellation of Scutum and is one of the largest known stars.[4]

Stephenson 2-18
Stephenson 2-18 zoomed in, 2MASS survey, 2003.png
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
Declination −06° 05′ 10.5357″
Evolutionary stage Red supergiant
Spectral type ~M6[1]
Apparent magnitude (G) 15.2631±0.0092
Apparent magnitude (J) 7.150
Apparent magnitude (H) 4.698
Apparent magnitude (K) 2.9
Proper motion (μ) RA: −3.045±0.511 mas/yr
Dec.: −5.950±0.480 mas/yr
Parallax (π)−0.0081 ± 0.3120 mas
Distance19,560 ly
(6,000 pc)
Radius2,150 R
Luminosity90,000–440,000; 630,000[2] L
Temperature3,200 K
Other designations
Stephenson 2-18, Stephenson 2 DFK 1, RSGC2-18, 2MASS J18390238-0605106, IRAS 18363-0607
Database references

It is also one of the most luminous of its type. It has a radius of 2,150 times that of the Sun,[5] and has a volume 10 billion times that of the Sun.[5] This star has a spectral type of M6,[6] which is pretty unusual for a supergiant star and corresponds to its cool temperature of 3,200 Kelvin.[5]


  3. Stephenson, C. B. (1990-06). "A possible new and very remote galactic cluster". The Astronomical Journal. 99: 1867. doi:10.1086/115464. ISSN 0004-6256. {{cite journal}}: Check date values in: |date= (help)
  4. Bicaj, Ardit (2020-06-11). "Stephenson 2-18: The Largest Star in the Universe". Cosmoknowledge. Archived from the original on 2020-08-10. Retrieved 2020-08-18.
  5. 5.0 5.1 5.2 Fok, Thomas K. T.; Nakashima, Jun-ichi; Yung, Bosco H. K.; Hsia, Chih-Hao; Deguchi, Shuji (2012-11-05). "MASER OBSERVATIONS OF WESTERLUND 1 AND COMPREHENSIVE CONSIDERATIONS ON MASER PROPERTIES OF RED SUPERGIANTS ASSOCIATED WITH MASSIVE CLUSTERS". The Astrophysical Journal. 760 (1): 65. doi:10.1088/0004-637x/760/1/65. ISSN 0004-637X.