KY Cygni

KY Cygni is a variable red supergiant of spectral class M3Ia located in the constellation Cygnus. It is approximately 4,700 light-years away.

KY Cygni

Location of KY Cygni in the Sadr region (circled, east is up)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Cygnus
Right ascension	20h 25m 58.04s[1]
Declination	+38° 21′ 07.7″[1]
Apparent magnitude (V)	11.14[2] (10.60 - 11.74[3])
Characteristics
Spectral type	M3 Ia[4] (M3.5 Ia[5])
U−B color index	+2.91[2]
B−V color index	+3.39[2]
Variable type	Lc[5]
Astrometry
Proper motion (μ)	RA: −3.556[1] mas/yr Dec.: −6.061[1] mas/yr
Parallax (π)	0.6687±0.0517 mas[1]
Distance	4,680+350 −390 ly (1,436+106 −119 pc)[6]
Absolute magnitude (MV)	−8.18[7]
Details[8]
Mass	19 M☉
Radius	1,032[a] R☉
Luminosity	150,000 L☉
Surface gravity (log g)	−0.5[7] cgs
Temperature	3,535 K
Age	>9[b] Myr
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
SIMBAD	data

Observations

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.^[4]

KY Cygni is heavily reddened due to interstellar extinction, losing an estimated 7.75 magnitudes at visual wavelengths. It would be a naked eye star if no light was lost.^[7]

Properties

Size comparison of Betelgeuse, Mu Cephei, KY Cygni, and V354 Cephei, according to estimates derived in 2005^[7]

KY Cygni is classified as a luminous red supergiant with a strong stellar wind. It is losing mass at around 4.9×10⁻⁶ M_☉ and has been described as a cool hypergiant.^[2][12]

Its properties are uncertain, but the temperature is around 3,500 K. A model fit based on K-band infrared brightness gives a luminosity of 273,000 L_☉, corresponding to a radius of 1,420 R_☉. 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 R_☉. These parameters are larger and more luminous than expected for any red supergiant, making them doubtful.^[7] 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] and a 2020 paper published a luminosity of 150,000 L_☉, which give a smaller radius of 1,032 R_☉ using an effective temperature of 3,535 K.^[8]

A visual band light curve for KY Cygni, plotted from data published by Kochanek et al. (2017)^[13]

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]

See also

• List of largest known stars

Notes

1. Applying the Stefan–Boltzmann law with a nominal solar effective temperature of 5,772 K:

   ${\displaystyle {\sqrt {{\biggl (}{\frac {5,772}{3,535}}{\biggr )}^{4}\cdot 10^{5.176}}}=1,032.46\ R_{\odot }}$.

2. This is only the age at which the star enters the red supergiant phase.