Mu Herculis (μ Herculis) is a nearby quadruple star system about 27.1 light years from Earth in the constellation Hercules. Its main star, Mu Herculis A is fairly similar to the Sun although more highly evolved with a stellar classification of G5 IV. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. Its mass is about 1.1 times that of the Sun, and it is beginning to expand to become a giant.
In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Marfak Al Jathih Al Aisr, which was translated into Latin as Cubitum Sinistrum Ingeniculi, meaning the left elbow of kneeling man.
In Chinese, Chinese: 天市左垣 (Chinese: Tiān Shì Zuǒ Yuán), the Left Wall of Heavenly Market Enclosure, refers to an asterism which represents eleven old states in China, marking the left borderline of the enclosure, consisting of μ Herculis, δ Herculis, λ Herculis, ο Herculis, 112 Herculis, ζ Aquilae, θ1 Serpentis, η Serpentis, ν Ophiuchi, ξ Serpentis and η Ophiuchi.[1] Consequently, the Chinese name for μ Herculis itself is Chinese: 天市左垣三 (Chinese: Tiān Shì Zuǒ Yuán sān, English: the Third Star of Left Wall of Heavenly Market Enclosure), represent Jiuhe (九河, lit. meaning nine rivers), possibly for Jiujiang, the prefecture-level city in Jiangxi, China, which is the same literally meaning with Jiuhe.[2] [3] From this Chinese title, the name Kew Ho appeared.[4]
Mu Herculis is a quadruple star system. The brightest star is a well-studied G-type subgiant, whose parameters are precisely determined from asteroseismology. It was believed to be a close binary with a low-mass stellar or a large substellar companion. This was confirmed when low-mass companion was resolved using near-infrared spectroscopy. The companion star is a red dwarf with a spectral type of M4V and a mass of . This pair is also known as Mu1 Herculis.
The secondary component, also known as Mu2 Herculis,[5] consists of a pair of stars that orbit about each other with a period of about 43 years. Mu Herculis A and the binary pair B-C are separated by some 35 arcseconds.[6] The stars B and C, which orbit each other, are separated from each other by 1.385 arcseconds, and have a slightly eccentric orbit, at 0.1796.