KY Cygni

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KY Cygni
Observational Data
Designation KY Cyg
Right ascension 20h 25m 58.0435s[1][2]
Declination +38° 21′ 07.6752″[1][2]
Constellation Cygnus
Type of object Supergiant star
Magnitude Apparent Mag: +11.14[3]
Absolute Mag: +0.13[4]
Distance from Earth 5,200 ly[5]
Proper motion RA: -3.574 mas/yr[1][2]
Dec: -6.279 mas/yr[1][2]
Parallax 0.9151 ± 0.0920 mas[1][2]

KY Cygni (KY Cyg) is a supergiant star in the constellation of Cygnus and one of the largest stars known.[5] The star is thought to have a radius 1,420 times larger than the Sun's making it slightly smaller than the radius of the largest star known, UY Scuti, at 1,700 solar radii.[6][7] The star has a surface temperature of 3,500 kelvin, a little cooler than Sol, but the enormous radius means it outputs over 300,000 times more energy than the Sun.[8]

It is situated roughly 5,200 light years away from Earth. This large distance means that is shines at only the eleventh (some sources place it higher at roughly fourteenth) magnitude, far too dim to be observed with the unaided eye; a telescope is required for observation. The two letters at the start of the star's name indicate star has been identified as a variable star, meaning its apparent brightness changes over time.[3] Specifically, it has been classified as an irregular variable and its brightness varies by about two magnitudes.[9][10] As an irregular variable star, it does not vary in brightness with any discernible period.


  1. 1.0 1.1 1.2 1.3 1.4 KY Cygni. Simbad Astronomical Database. Retrieved on July 28, 2020.
  2. 2.0 2.1 2.2 2.3 2.4 Gaia Collaboration (2018). "VizieR Online Data Catalog: Gaia DR2". VizieR Online Data Catalog I/345: I/345. Bibcode2018yCat.1345....0G.  arXiv:0708.1752
  3. 3.0 3.1 KY Cygni (Supergiant Star) Star Facts. Retrieved on July 28, 2020.
  4. Computed using definition of absolute magnitude with an apparent magnitude of +11.14 and distance of 5,200 light years.
  5. 5.0 5.1 Janet Mason (January 11, 2005). Three Largest Stars Discovered. Retrieved on July 28, 2020.
  6. Nola Taylor Redd (July 26, 2018). What Is the Biggest Star?. Retrieved on July 28, 2020.
  7. A couple of estimations put KY Cygni's radius at 2,850 solar radii, but these are generally thought to be dubious to observational error (see reference).
  8. KY Cygni. Retrieved on July 28, 2020.
  9. Samus, N. N.; Kazarovets, E. V.; Durlevich, O. V. et al. (2017). "General Catalogue of Variable Stars: Version GCVS 5.1". Astronomy Reports 61 (1): 80-88. doi:10.1134/S1063772917010085. Bibcode2017ARep...61...80S. 
  10. KY Cygni. Retrieved on July 28, 2020.