# 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.
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.

## 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).
Epsilon Cygni bore the traditional name Gienah from the Arabic al janāħ (Arabic: جناح) meaning "the wing". However that name was more usually applied to Gamma Corvi. For reasons of disambiguation it was sometimes called Gienah Cygni. In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. 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.
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. Consequently, the Chinese name for Epsilon Cygni Cygni itself is "the Ninth Star of Celestial Ford" (Chinese: 天津九; pinyin: Tiān Jīn jiǔ).

## 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. 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.

## Properties

Epsilon Cygni A is a single-lined spectroscopic binary. 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. Its main component 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. 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. The effective temperature of its photosphere is 4,700 K, giving an orange hue that is a characteristic of K-type stars. Being 10% more massive than the Sun and nine billion years old, It has 11 times the radius of the Sun and is about 56 times more luminous.
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$_☉$.
Since 1943, the spectrum of Epsilon Cygni A has served as one of the stable anchor points by which other stars are classified.

## 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.