KY Cygni

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KY Cygni
Sadr Region rgb.jpg
Red circle.svg
Location of KY Cygni (circled, east is up)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Cygnus
Right ascension 20h 25m 58.05s[1]
Declination +38° 21′ 07.6″[1]
Apparent magnitude (V) 11.14[2] (10.60 - 11.74[3])
Spectral type M3-4I[4] (M3.5Ia[5])
U−B color index +2.91[2]
B−V color index +3.39[2]
Variable type LC[5]
Proper motion (μ) RA: –3.574[6] mas/yr
Dec.: –6.279[6] mas/yr
Parallax (π)0.9151 ± 0.0920[6] mas
Distance~5,000 ly
(1,580[2] pc)
Absolute magnitude (MV)−8.18[4]
Mass25[7] M
Radius672[8]–2,850?[4] R
Diameter672[8]–2,850?[4] D
Luminosity (bolometric)138,000[2]–273,000 (1,107,000?)[4] L
Surface gravity (log g)−0.5 (−0.9?)[4] cgs
Temperature3,500[4] K
Other designations
KY Cyg, GSC 03152-01140, IRAS 20241+3811, IRC+40415, TYC 3152-1140-1, RAFGL 2575, UCAC2 45230193, 2MASS J20255805+3821076
Database references

KY Cygni is a red supergiant of spectral class M3.5Ia located in the constellation Cygnus. It is one of the largest and most luminous stars, with a luminosity about 300,000 or more times that of the Sun and a radius of over 1,000 times that of the Sun. If it was placed at the center of the Solar System, it would extend past the orbit of Jupiter (or Saturn). It is approximately 5,000 light-years away.


KY Cyg lies near the bright open cluster NGC 6913, but is not thought to be a member. The location is close to the bright star γ Cygni.[9] It was identified as a variable star in 1930,[10] and later named as KY Cygni.[11] The spectrum was given the MK classification of M3 Ia, with only minor adjustments since.[12]

KY Cygni is heavily reddened due to interstellar extinction, losing an estimate 7.75 magnitudes at visual wavelengths. It would be a naked eye star If no light was lost.[4]


Comparison of KY Cygni to Betelgeuse

KY Cygni is a large luminous cool supergiant with a strong stellar wind. It is losing mass at one of the highest rates known for a red supergiant and has been described as a cool hypergiant.[2][13]

Its properties are uncertain, but the temperature is around 3,500 K and the luminosity over 100,000 L. A model fit based on K-band infrared brightness gives a luminosity of 273,000 L, corresponding to a radius of 1,420 D. Another model based on visual brightness gives an unexpectedly large luminosity of 1,107,000 L, with the difference due mainly to the assumptions about the level of extinction. The radius corresponding to the higher luminosity would be 2,850 D. These parameters are larger and more luminous than expected for any red supergiant, making them doubtful.[4] More recently, integration of the spectral energy distributions across a full range of wavelengths from U band to the 60 micron microwave flux gives an even lower luminosity of 138,000 L.[2] Data from the Gaia Data Release 2 gives the star a much smaller radius of 672 solar radii.[8]

Size comparison of Betelgeuse, Mu Cephei, KY Cygni, and V354 Cephei, according to Emily Levesque.[4]

KY Cygni is a variable star with a large amplitude but no clear periodicity. At times it varies rapidly, at others it is fairly constant for long periods.[9] The photographic magnitude range is given as 13.5 - 15.5,[5] while a visual range is 10.60 - 11.74.[3]


  1. 1.0 1.1 Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.; Evans, T. et al. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". VizieR On-line Data Catalog: II/246. Originally published in: University of Massachusetts and Infrared Processing and Analysis Center 2246: 0. Bibcode 2003yCat.2246....0C.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Mauron, N.; Josselin, E. (2011). "The mass-loss rates of red supergiants and the de Jager prescription". Astronomy and Astrophysics 526: A156. arXiv:1010.5369. Bibcode 2011A&A...526A.156M. doi:10.1051/0004-6361/201013993.
  3. 3.0 3.1 Alfonso-Garzón, J.; Domingo, A.; Mas-Hesse, J. M.; Giménez, A. (2012). "The first INTEGRAL-OMC catalogue of optically variable sources". Astronomy & Astrophysics 548: A79. arXiv:1210.0821. Bibcode 2012A&A...548A..79A. doi:10.1051/0004-6361/201220095.
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 Levesque, Emily M.; Massey, Philip; Olsen, K. A. G.; Plez, Bertrand; Josselin, Eric; Maeder, Andre; Meynet, Georges (2005). "The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought". The Astrophysical Journal 628 (2): 973–985. arXiv:astro-ph/0504337. Bibcode 2005ApJ...628..973L. doi:10.1086/430901.
  5. 5.0 5.1 5.2 KY Cyg, database entry, The combined table of GCVS Vols I-III and NL 67-78 with improved coordinates, General Catalogue of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line November 12, 2010.
  6. 6.0 6.1 6.2 Brown, A. G. A. (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.
  7. List of Largest Stars Gets 3 New Chart Toppers, Robert Roy Britt,, 10 January 2005. Accessed on line November 12, 2010.
  8. 8.0 8.1
  9. 9.0 9.1 Romano, G. (1969). "Researches with the Schmidt telescopes. III. Variable stars in the field of gamma Cygni". Memorie della Società Astronomia Italiana 40: 375. Bibcode 1969MmSAI..40..375R.
  10. Hoffmeister, Cuno (1930). "Relative Koordinaten, Oerter und Karten neuer Veraenderlicher". Mitteilungen der Sternwarte zu Sonneberg 17: 1. Bibcode 1930MiSon..17....1H.
  11. Ahnert, P.; Van Schewick, H.; Hoffmeister, C. (1941). "Die Veraenderlichen Sterne der noerdlichen Milchstrasse. Teil II". Kleine Veroeffentlichungen der Universitaetssternwarte zu Berlin Babelsberg 6: 4.1. Bibcode 1941KVeBB...6....4A.
  12. White, N. M.; Wing, R. F. (1978). "Photoelectric two-dimensional spectral classification of M supergiants". Astrophysical Journal 222: 209. Bibcode 1978ApJ...222..209W. doi:10.1086/156136.
  13. Stickland, D. J. (1985). "IRAS observations of the cool galactic hypergiants". The Observatory 105: 229. Bibcode 1985Obs...105..229S.

External links