Epsilon Herculis, Latinized from ε Herculis, is a fourth-magnitude multiple star system in the northern constellation of Hercules. The combined apparent visual magnitude of 3.9111[1] is bright enough to make this system visible to the naked eye. Based upon an annual parallax shift of 21.04 mas as seen from Earth,[1] it is located 155 light years from the Sun. The system is moving closer to the Sun with a radial velocity of −25 km/s.[11]
There is disagreement over the properties of this system. Petrie (1939) classified two components as class A0 and A2 with a visual magnitude difference of 1.5.[3] Batten et al. (1989) catalogued it as a double-lined spectroscopic binary system with an orbital period of four days and an eccentricity of 0.02.[6] However, Hipparcos was not able to detect the duplicity. Tokovinin (1997) and Faraggiana et al. (2001) catalogued it as a triple star system.[3] Cowley et al. (1969) gave it a combined stellar classification of A0 V,[2] whereas Gray & Garrison (1987) classified it as an A0 IV+. Wolff & Preston (1978) listed a magnesium overabundance.[3] Since 1995 it has been classified as a Lambda Boötis star,[12] although this has been brought into question.[13][3]
In Chinese, 天紀 (Tiān Jì), meaning Celestial Discipline, refers to an asterism consisting of ε Herculis, ξ Coronae Borealis, ζ Herculis, 59 Herculis, 61 Herculis, 68 Herculis, HD 160054 and θ Herculis.[14] Consequently, the Chinese name for ε Herculis itself is 天紀三 (Tiān Jì sān, English: the Third Star of Celestial Discipline.)[15]
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