KY Cygni

KY Cygni is a red supergiant of spectral class M3.5Ia located in the constellation Cygnus. It is approximately 4,700 light-years away.

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. 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 estimated 7.75 magnitudes at visual wavelengths. It would be a naked eye star if no light was lost.

Properties
KY Cygni is classified as a luminous red supergiant with a strong stellar wind. It is losing mass at around $1,436$ and has been described as a cool hypergiant.

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, corresponding to a radius of. Another model based on visual brightness gives an unexpectedly large luminosity of, with the difference due mainly to the assumptions about the level of extinction. The radius corresponding to the higher luminosity would be. These parameters are larger and more luminous than expected for any red supergiant, making them doubtful. 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, and a 2020 paper published a luminosity of , which give a smaller radius of using an effective temperature of 3,535K.

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.