Eta Ophiuchi (η Ophiuchi, abbreviated Eta Oph, η Oph) is a binary star in the constellation of Ophiuchus. With a combined apparent magnitude of +2.43, it is the second-brightest of the constellation and one of the list of brightest stars, brightest stars in the night sky. Based on stellar parallax, parallax measurements taken during the Hipparcos mission, it is approximately 88 light-years away. Eta Ophiuchi is part of a multiple star system designated WDS J17104-1544. It itself is designated WDS J17104-1544AB and its two components WDS J17104-1544A (also called Sabik , the traditional name for the system) and WDS J17104-1544B. The 'C' component is UCAC4 372-080717 and 'D' is UCAC2 26022336.

Nomenclature

''η Ophiuchi'' (Latinisation of names, Latinised to ''Eta Ophiuchi'') is the system's Bayer designation. WDS J17104-1544AB is its designation in the Washington Double Star Catalog. The designations of the two components as WDS J17104-1544 A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for Star system, multiple star systems, and adopted by the International Astronomical Union (IAU). It bore the traditional name ''Sabik'', from the Arabic السابق ''al-sābiq'' "the preceding one", of uncertain reference. In 2016, the International Astronomical Union organized a IAU Working Group on Star Names, Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name ''Sabik'' for the component WDS J17104-1544 A on 21 August 2016 and it is now so included in the List of IAU-approved Star Names. In Chinese astronomy, Chinese, this star is considered part of (), meaning ''Heavenly Market enclosure, Left Wall of Heavenly Market Enclosure'', which refers to an Asterism (astronomy), asterism representing eleven old states in China that mark the left borderline of the enclosure, consisting of Eta Ophiuchi, Delta Herculis, Lambda Herculis, Mu Herculis, Omicron Herculis, 112 Herculis, 112 Herculis, Zeta Aquilae, Theta Serpentis, Eta Serpentis, Nu Ophiuchi and Xi Serpentis. Consequently, the Chinese star names, Chinese name for Eta Ophiuchi itself is (, ), representing the state Song (state), Song (宋).English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name
, Hong Kong Space Museum. Accessed on line November 23, 2010.

Namesake

USS Sabik (AK-121) was a United States Navy Crater class cargo ship named after the star.

Properties

Eta Ophiuchi consists on two A-type main-sequence stars, currently fusing hydrogen into helium at their stellar core, core. The primary, of apparent magnitude +3.05, has about 2.5 times the Sun's radius and 2.25 times its solar mass, mass, with an effective temperature around 9,000 K. The secondary, of apparent magnitude +3.27, is 2 times as large and massive than the Sun, with an effective temperature of 8,700 K. The estimated age of the system is 500 million years, albeit with significant uncertainty. The stars take 87.8 years to complete an orbit, which is highly elliptical: While the semi-major axis is 31 astronomical units, the high orbital eccentricity of 0.931 take them to a distance of 2.15 au during their closest approach (the periastron); the last of such occurring in 2024. Such large eccentricity hindered accurate measurements of the stellar mass with the Kepler's third law, until observations with the Very Large Telescope during the most recent periastron accurately determined orbital elements such as the eccentricity, hence a considerable mass sum for A2V-type stars.

References

{{DEFAULTSORT:Eta Ophiuchi Bayer objects, Ophiuchi, Eta Binary stars Flamsteed objects, Ophiuchi, 35 Henry Draper Catalogue objects, 155125 Hipparcos objects, 084012 Ophiuchus A-type main-sequence stars Bright Star Catalogue objects, 6378 Durchmusterung objects, BD-15 4467 Gliese and GJ objects, 0656.1 Stars with proper names, Sabik