Sigma Cygni is a star in the constellation of Cygnus. Its name is a Bayer designation that is Latinized from σ Cygni and abbreviated Sigma Cyg or σ Cyg. This star is visible to the naked eye with an apparent visual magnitude of 4.2. It is located approximately 2,800 light years away from Earth and it belongs to the Cygnus OB4 stellar association of co-moving stars.
The stellar classification of Sigma Cygni is B9 Iab, which presents as a blue supergiant star. Because of its location in the galactic disk, σ Cyg is obstructed by interstellar dust. It is reddened by around 0.2 magnitudes and loses about 0.6 magnitudes at visual wavelengths. Allowing for this, the star is over 38,000 times as luminous as the sun.
Spectral analysis of the star showed that photospheric SiII and HeI lines display a simultaneous, periodic variability. The periodicity was calculated at 1.59 hours in all three lines and it might be the result of stellar oscillations. No clear variation in the brightness has been detected.
The elemental composition of Sigma Cygni is anomalous. The star is enriched in helium, and has extremely high concentrations of nitrogen, calcium, cerium and europium, while carbon and aluminum are depleted relative to the Sun.
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