Zeta Aquarii
Zeta Aquarii is a triple star system, the central star of the "water jar" asterism in the equatorial constellation of Aquarius. Its identifier is a Bayer designation that is Latinized from ζ Aquarii, and abbreviated Zet Aqr or ζ Aqr, respectively. 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 from Earth.
Etymology
In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Achr al Achbiya, which was translated into Latin as Postrema Tabernaculorum, meaning the end of luck of the homes . This star, along with γ Aqr, π Aqr and η Aqr, were al-Akḣbiya, the Tents.In Chinese, 墳墓, meaning Tomb, refers to an asterism consisting of ζ Aquarii, γ Aquarii, η Aquarii and π Aquarii. Consequently, the Chinese name for ζ Aquarii itself is 墳墓一
Properties
Orbits
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 plane of the sky.Spectral class and stellar mass
The brighter component, ζ Aquarii A, is a yellow-white-hued F-type main sequence star with an apparent magnitude of +4.42 and a stellar classification of F3 V. Its companion, ζ Aquarii B, is a yellow-white-hued F-type subgiant with an apparent magnitude of +4.51 and a class of F6 IV. Their similar brightness 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.
Observational history
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 was in orbit around Zeta Aquarii B. The first claimed direct images of this third star were in 1979, by Ebersberger and Weigelt, 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. 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 "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.