Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||20h 25m 58.05s|
|Declination||+38° 21′ 07.6″|
|Apparent magnitude (V)||11.14 (10.60 - 11.74)|
|Spectral type||M3-4I (M3.5Ia)|
|U−B color index||+2.91|
|B−V color index||+3.39|
|Absolute magnitude (MV)||−8.18|
|Radius||1,420 (2,850?) R☉|
|Luminosity (bolometric)||138,000, 270,000 (1,100,000?) L☉|
|Surface gravity (log g)||−0.5 (−0.9?) cgs|
KY Cygni is a red supergiant of spectral class M3.5Ia located in the constellation Cygnus. It is one of the largest stars known, Its value is very poorly known, Its estimated size is 1,420 R☉ (990,000,000 km; 6.6 au) or (up to 2,850 R☉ (1.98×109 km; 13.3 au) by some estimates). If It is placed at the center of the Solar System, it would extend past the orbit of Jupiter (or Saturn). And it's also one of the most luminous, with about 300,000 or more times the Sun's luminosity. 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. It was identified as a variable star in 1930, and later named as KY Cygni. The spectrum was given the MK classification of M3 Ia, with only minor adjustments since.
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.
The 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☉. One based on visual brightness gives an unexpectedly large luminosity of 1,100,000 L☉, with the difference due mainly to the assumptions about the level of extinction. The radius corresponding to the higher luminosity would be over 2,800 R☉, it is somewhat on par with the largest stars known. More recently, integration of the spectral energy distributions across a full range of wavelengths from U band to the 60 micron microwave flux gives a luminosity of 138,000 L☉. The radius corresponding to the lower luminosity would be less than 1,000 R☉.
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. The photographic magnitude range is given as 13.5 - 15.5, while a visual range is 10.60 - 11.74.
- Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.; Evans, T.; Fowler, J.; Gizis, J.; Howard, E.; Huchra, J.; Jarrett, T.; Kopan, E. L.; Kirkpatrick, J. D.; Light, R. M.; Marsh, K. A.; McCallon, H.; Schneider, S.; Stiening, R.; Sykes, M.; Weinberg, M.; Wheaton, W. A.; Wheelock, S.; Zacarias, N. (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.
- Mauron, N.; Josselin, E. (2011). "The mass-loss rates of red supergiants and the de Jager prescription". Astronomy and Astrophysics. 526: A156. arXiv: . Bibcode:2011A&A...526A.156M. doi:10.1051/0004-6361/201013993.
- Alfonso-Garzón, J.; Domingo, A.; Mas-Hesse, J. M.; Giménez, A. (2012). "The first INTEGRAL-OMC catalogue of optically variable sources". Astronomy & Astrophysics. arXiv: [astro-ph.IM]. Bibcode:2012A&A...548A..79A. doi:10.1051/0004-6361/201220095.
- 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: . Bibcode:2005ApJ...628..973L. doi:10.1086/430901.
- 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.
- 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.
- Hoffmeister, Cuno (1930). "Relative Koordinaten, Oerter und Karten neuer Veraenderlicher". Mitteilungen der Sternwarte zu Sonneberg. 17: 1. Bibcode:1930MiSon..17....1H.
- 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.
- 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.
- Stickland, D. J. (1985). "IRAS observations of the cool galactic hypergiants". The Observatory. 105: 229. Bibcode:1985Obs...105..229S.
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