Magnitude 2.6 Apparent magnitude (V) 2.6 | Constellation Sagittarius | |
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Similar Lambda Sagittarii, Delta Sagittarii, Epsilon Sagittarii, Sigma Sagittarii, Gamma2 Sagittarii | ||
Zeta Sagittarii (ζ Sagittarii, abbreviated Zeta Sgr, ζ Sgr), also named Ascella, is a binary star and the third-brightest star in the constellation of Sagittarius. Based upon parallax measurements, it is about 88 ly (27 pc) from the Sun.
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Nomenclature
ζ Sagittarii (Latinised to Zeta Sagittarii) is the system's Bayer designation.
It bore the traditional name Ascella, from a Late Latin word meaning armpit. In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Thalath al Sadirah, which was translated into Latin as Tertia τού al Sadirah, meaning third returning ostrich. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Ascella for this star on 12 September 2016 and it is now so entered in the IAU Catalog of Star Names.
This star, together with Gamma Sagittarii, Delta Sagittarii, Epsilon Sagittarii, Lambda Sagittarii, Sigma Sagittarii, Tau Sagittarii and Phi Sagittarii comprise the asterism Teapot.
In Chinese, 斗 (Dǒu), meaning Dipper, refers to an asterism consisting of Zeta Sagittarii, Phi Sagittarii, Lambda Sagittarii, Mu Sagittarii, Sigma Sagittarii and Tau Sagittarii. Consequently, Zeta Sagittarii itself is known as 斗宿一 (Dǒu Sù yī, English: the First Star of Dipper.)
Namesake
Ascella was a United States Navy Crater class cargo ship named after the star.
Properties
Zeta Sagittarii has an apparent visual magnitude of +2.59. It is moving away from the Solar System with a radial velocity of 22 km s−1, and some 1.0–1.4 million years ago, came within 7.5 ± 1.8 ly (2.30 ± 0.55 pc) of the Sun.
The two components orbit each other over a period of 21 years at an eccentricity of 0.211. The combined mass of the system is 5.26 ± 0.37 times the mass of the Sun and their blended stellar classification is A2.5 Va.
The system consists of a spectral class A2 giant with an apparent magnitude of +3.27, and an A4 subgiant with apparent magnitude of +3.48. The pair have a mean separation of 13.4 astronomical units (AU). The primary has a faint, 10th magnitude companion, separated from it by a distance of 75 arcseconds.