KY Cygni

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.

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

Properties

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.

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 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. 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_☉.

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.