Zeta Aquarii (ζ Aquarii, ζ Aqr) is the Bayer designation for a triple star system, the central star of the "water jar" asterism in the equatorial constellation of Aquarius. The combined apparent visual magnitude of this system is 3.65, which is readily visible to the naked eye. Parallax measurements yield a distance estimate of around 92abbr=offNaNabbr=off from Earth.
In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Achr al Achbiya (آخر ألأخبية - ākhir al-akhbiya), which was translated into Latin as Postrema Tabernaculorum, meaning the end of luck of the homes (tents). This star, along with γ Aqr (Sadachbia), π Aqr (Seat) and η Aqr (Hydria), were al-Akḣbiya (الأخبية), the Tents.
In Chinese, Chinese: 墳墓 (Chinese: Fén Mù), meaning Tomb, refers to an asterism consisting of ζ Aquarii, γ Aquarii, η Aquarii and π Aquarii.[1] Consequently, the Chinese name for ζ Aquarii itself is Chinese: 墳墓一 (Chinese: Fén Mù yī, English: the First Star of Tomb.)[2]
The two stars have an orbital period of about 587 years. The semimajor axis is 3.8 arcseconds and they have an orbital eccentricity of 0.40. The orbital plane is inclined by 138.2° to the line of sight.
The brighter component, ζ Aquarii A (also called ζ2 Aquarii), is a yellow-white-hued F-type main sequence star with an apparent magnitude of +4.42. Its companion, ζ Aquarii B (also called ζ1 Aquarii), is a yellow-white-hued F-type subgiant with an apparent magnitude of +4.51. The fact that their brightness is so similar makes the pair easy to measure and resolve.
Zeta Aquarii A is known to be an astrometric binary system, as it undergoes regular perturbations from its orbit. It has a 26-year orbital period and a semimajor axis of 0.11″. The secondary's mass is .
The binary was measured by William Herschel in 1779; however, Christian Mayer listed an earlier observation in his first double-star catalog in 1784, so it is not known who first identified Zeta Aquarii as a binary star system.
Photographic measurements available since the early 20th century were more precise than earlier micrometer estimates, and they revealed regular orbital deviations in the Zeta Aquarii system. Kaj Aage Gunnar Strand in 1942 was the first to note this pattern and identify Zeta Aquarii as a triple star system. Several other astronomers later published revised models of the system's orbits.
At first, Strand and the other astronomers assumed that the third star (which had still not been observed directly) was in orbit around Zeta Aquarii B. The first claimed direct images of this third star were in 1979, by Ebersberger and Weigelt (who claimed that it revolved around Zeta Aquarii A instead), and then in 1982 by McCarthy et al., who again attributed it to Zeta Aquarii B. In a 1984 paper, Wulff-Dieter Heintz dismissed both claims as bogus and proposed a new orbital model, demonstrating that the third star revolved around Zeta Aquarii A. (According to Andrei Tokovinin, the imaging technology available at the time would not have been capable of producing an image of the third star.) This third star, now known as Zeta Aquarii Ab, was not directly imaged until 2009, by the Southern Astrophysical Research Telescope. Initially, the third star was attributed to Zeta Aquarii B, but later observations (such as Hartkopf et al. in 2012) "securely" identified it with Zeta Aquarii A instead.
Zeta Aquarii is currently a northern hemisphere object. In 2004 it was directly above the celestial equator, and before that it was located south of it.