Eta Ophiuchi
Template:Short description Template:Starbox begin Template:Starbox image Template:Starbox observe Template:Starbox character Template:Starbox astrometry Template:Starbox orbit Template:Starbox detail Template:Starbox catalog Template:Starbox reference Template:Starbox end 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 brightest stars in the night sky. Based on 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 Template:IPAc-en, the traditional name for the system)<ref name=Kunitzsch>Template:Cite book</ref><ref name="IAU-LSN">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and WDS J17104-1544B. The 'C' component is UCAC4 372-080717<ref name=SIMBAD2>Template:Citation</ref> and 'D' is UCAC2 26022336.<ref name=SIMBAD3>Template:Citation</ref>
NomenclatureEdit
η Ophiuchi (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 multiple star systems, and adopted by the International Astronomical Union (IAU).<ref name="planetnaming">Template:Cite arXiv</ref>
It bore the traditional name Sabik, from the Arabic السابق al-sābiq "the preceding one", of uncertain reference.<ref name=Kunitzsch/> In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)<ref name="WGSN">Template:Citation</ref> 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.<ref name="IAU-LSN"/>
In Chinese, this star is considered part of {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), meaning Left Wall of Heavenly Market Enclosure, which refers to an 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, Zeta Aquilae, Theta Serpentis, Eta Serpentis, Nu Ophiuchi and Xi Serpentis.<ref>Template:In lang 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, Template:ISBN.</ref> Consequently, the Chinese name for Eta Ophiuchi itself is {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}, Template:Langx), representing the state Song (宋).<ref>Template:In lang 香港太空館 - 研究資源 - 亮星中英對照表 Template:Webarchive, Hong Kong Space Museum. Accessed on line November 23, 2010.</ref><ref>Template:In lang English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name Template:Webarchive, Hong Kong Space Museum. Accessed on line November 23, 2010.</ref>
NamesakeEdit
USS Sabik (AK-121) was a United States Navy Crater class cargo ship named after the star.
PropertiesEdit
Eta Ophiuchi consists on two A-type main-sequence stars,<ref name=Waisberg2025/> currently fusing hydrogen into helium at their core. The primary, of apparent magnitude +3.05,<ref name=aj133_4_1209/> has about 2.5 times the Sun's radius and 2.25 times its mass, with an effective temperature around 9,000 K.<ref name=Waisberg2025/> The secondary, of apparent magnitude +3.27,<ref name=aj133_4_1209/> 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.<ref name=Waisberg2025/>
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.<ref name=Waisberg2025/>