Epoch J2000 Equinox J2000
|Right ascension||20h 46m 12.68236s|
|Declination||+33° 58′ 12.9250″|
|Apparent magnitude (V)||2.480|
|U−B color index||+0.860|
|B−V color index||+1.030|
|Radial velocity (Rv)||–12.41 km/s|
|Proper motion (μ)||RA: 355.66 mas/yr
Dec.: 330.60 mas/yr
|Parallax (π)||44.86 ± 0.12 mas|
|Distance||72.7 ± 0.2 ly
(22.29 ± 0.06 pc)
|Absolute magnitude (MV)||+0.78 ± 0.03|
|Radius||10.82 ± 0.14 R☉|
|Luminosity||62 ± 3 L☉|
|Surface gravity (log g)||2.8 cgs|
|Metallicity [Fe/H]||–0.27 dex|
|Rotational velocity (v sin i)||3.0 ± 1.0 km/s|
Epsilon Cygni (ε Cyg, ε Cygni), also known Gienah, is a star in the constellation Cygnus. This star's Bayer designation is from Johann Bayer's 1603 Uranometria atlas of bright stars. With an apparent visual magnitude of 2.48, it is readily visible to the naked eye at night as one of the brighter members of Cygnus. Based upon parallax measurement, Epsilon Cygni is about 73 light-years from Earth.
Epsilon Cygni is a giant star with a stellar classification of K0 III. This indicates that the star has left the main sequence and has begun the final stages in its stellar evolution. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. The effective temperature of the star's photosphere is 4,710 K, giving Epsilon Cygni an orange hue that is a characteristic of K-type stars. This star has nearly 11 times the solar radius and is about 62 times the luminosity of the Sun.
In 1920 it was suggested that the star is a spectroscopic binary system, but this was later brought into question. It has an optical companion Epsilon Cygni B that is not physically associated with Epsilon Cygni A, and a 13th magnitude candidate common proper motion companion Epsilon Cygni C at an angular separation of 78 arcseconds. If the latter star is gravitationally bound to Epsilon Cygni A, then they are currently separated by 1700 AU or more, and have an orbital period of at least 50,000 years. Radial velocity observations of Epsilon Cygni suggest a possible companion with an orbital period of at least 15 years.
In Chinese astronomy, the "Celestial Ford" (Chinese: 天津; pinyin: Tiān Jīn) refers to an asterism consisting of ε Cygni, γ Cygni, δ Cygni, 30 Cygni, α Cygni, ν Cygni, τ Cygni, υ Cygni and ζ Cygni. Consequently, ε Cygni itself is known as the "Ninth Star of Celestial Ford" (Chinese: 天津九; pinyin: Tiān Jīn jiǔ.
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