Epsilon Cygni

Epsilon Cygni (ε Cygni, abbreviated Epsilon Cyg, ε Cyg) is binary star in the constellation of Cygnus. With an apparent visual magnitude of 2.48, it is readily visible to the naked eye at night, and is the third-brightest star in the constellation. Based upon parallax measurement, Epsilon Cygni is about 73 light-years distant.

Epsilon Cygni

Location of ε Cyg (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Cygnus
Right ascension	20h 46m 12.68236s[1]
Declination	+33° 58′ 12.9250″[1]
Apparent magnitude (V)	2.480[2]
Characteristics
Evolutionary stage	Horizontal branch[3]
Spectral type	K0 III[4]
U−B color index	+0.860[2]
B−V color index	+1.030[2]
Astrometry
Radial velocity (Rv)	−12.41[5] km/s
Proper motion (μ)	RA: 355.66[1] mas/yr Dec.: 330.60[1] mas/yr
Parallax (π)	44.86±0.12 mas[1]
Distance	72.7 ± 0.2 ly (22.29 ± 0.06 pc)
Absolute magnitude (MV)	+0.74 ± 0.02[4]
Orbit[3]
Primary	ε Cyg Aa
Companion	ε Cyg Ab
Period (P)	53.693+0.321 −0.328 yr
Semi-major axis (a)	15.8±0.2 AU
Eccentricity (e)	0.9295±0.0003
Argument of periastron (ω) (primary)	275.30±0.06°
Semi-amplitude (K1) (primary)	4.6 km/s
Details
A
Mass	1.103±0.042[3] M☉
Radius	11.13[6] R☉
Luminosity	56.4±0.6[7] L☉
Surface gravity (log g)	2.79±0.23[7] cgs
Temperature	4,699[6] K
Metallicity [Fe/H]	−0.126[6] dex
Rotation	0.67[4] years
Rotational velocity (v sin i)	2.3 ± 0.2[4] km/s
Age	9.62±0.12[3] Gyr
B
Mass	≥0.265±0.007[3] M☉
Other designations
Aljanah, Gienah,[8] ε Cyg, 53 Cygni, BD+33°4018, FK5 780, GCTP 4959.00, Gl 806.1, HD 197989, HIP 102488, HR 7949, LHS 5358, SAO 70474, WDS 20462+3358[9]
Database references
SIMBAD	data

The system has two confirmed constituents, Epsilon Cygni Aa (officially named Aljanah /ˈældʒənə/) and Ab. Additionally, a visual companion (Epsilon Cygni C) is likely bound to the system.^[3]

Nomenclature

ε Cygni (Latinised to Epsilon Cygni) is the system's Bayer designation. The designations of the three constituents as Epsilon Cygni A, B and C, and those of A's components - Epsilon Cygni Aa and Ab - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).^[10]

Epsilon Cygni bore the traditional name Gienah from the Arabic al janāħ (Arabic: جناح) meaning "the wing".^[11] However that name was more usually applied to Gamma Corvi.^[8] For reasons of disambiguation it was sometimes called Gienah Cygni. In 2016, the IAU organized a Working Group on Star Names (WGSN)^[12] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.^[13] It approved the name Aljanah for the component Epsilon Cygni Aa on 30 June 2017. It had previously approved the name Gienah for Gamma Corvi A on 6 November 2016. Both are now so included in the List of IAU-approved Star Names.^[14]

In Chinese astronomy, the "Celestial Ford" (Chinese: 天津; pinyin: Tiān Jīn) refers to an asterism consisting of Epsilon Cygni, Gamma Cygni, Delta Cygni, 30 Cygni, Alpha Cygni, Nu Cygni, Tau Cygni, Upsilon Cygni and Zeta Cygni.^[15] Consequently, the Chinese name for Epsilon Cygni Cygni itself is "the Ninth Star of Celestial Ford" (Chinese: 天津九; pinyin: Tiān Jīn jiǔ).^[16]

Companions

Epsilon Cygni A has an optical companion, Epsilon Cygni B, with which it is not physically associated, and a 13th magnitude candidate common proper motion companion, Epsilon Cygni C, at an angular separation of 78 arcseconds.^[17] If the latter star is gravitationally bound to Epsilon Cygni A, then they are currently separated by 1,700 AU or more, and have an orbital period of at least 50,000 years.^[18]

Properties

ε Cygni appears north of the Veil nebula towards the bottom left

Epsilon Cygni A is a single-lined spectroscopic binary.^[19] The components take 53 years to complete an orbit and are in a highly eccentric orbit, which put their distances from 1.1 astronomical units in apoapsis to 30.5 astronomical units in periapsis.^[3][a] Its main component is a giant star with a stellar classification of K0 III.^[20] This indicates that the star has left the main sequence and has begun the final stages in its stellar evolution. After passing through the red-giant branch stage, it underwent a helium flash event and is now a horizontal branch star generating energy through the thermonuclear fusion of helium at its core.^[3] The effective temperature of its photosphere is 4,700 K,^[6] giving an orange hue that is a characteristic of K-type stars.^[21] Being 10% more massive than the Sun and nine billion years old,^[3] It has 11 times the radius of the Sun^[6] and is about 56 times more luminous.^[7]

The secondary has never been observed directly, its existence was inferred solely on astronomical spectroscopy. Based on this data, it should have a mass of at least 0.265 M_☉.^[3]

Since 1943, the spectrum of Epsilon Cygni A has served as one of the stable anchor points by which other stars are classified.^[22]

Veil Nebula

Main article: Veil nebula

ε Cygni lies about three degrees north of the Veil nebula, a probable ancient supernova remnant. The nebula is far more distant than the star.

Notes

1. Calculated using semi-major axis of 15.8 AU and eccentricity of 0.93 via the equations SMA(1−e) for apoastron and SMA(1+e) for periastron.