Zeta Aquarii

Zeta Aquarii (ζ Aqr, ζ Aquarii) is a binary star, the central star in the "water jar" asterism [1] in the constellation Aquarius. It is approximately 103 light years from Earth.

Zeta Aquarii has the traditional name Sadaltager (or Altager), from the Arabic سعد التاجر sa‘d al-tājir "luck of the merchant".

In Chinese, 墳墓 (Fén Mù), meaning Tomb, refers to an asterism consisting of ζ Aquarii, γ Aquarii, η Aquarii and π Aquarii.^[1] Consequently, ζ Aquarii itself is known as 墳墓一 (Fén Mù yī, English: the First Star of Tomb.)^[2]

In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Achr al Achbiya (أجر ألأجبية - akhir al ahbiyah), which was translated into Latin as Postrema Tabernaculorum, meaning the end of luck of the homes (tents).^[3]. This star, along with γ Aqr (Sadachbia), π Aqr (Seat) and η Aqr (Hydria), were al Aḣbiyah (الأخبية), the Tent.^[4][5][6]

Structure

The brighter component, ζ² Aquarii, is a yellow-white F-type main sequence dwarf with an apparent magnitude of +4.42. Its companion, ζ¹ Aquarii, is a yellow-white F-type subgiant with an apparent magnitude of +4.59. The fact that their brightness is so close makes the pair easy to measure and resolve. The binary system has a combined brightness of +3.65 magnitudes. The two stars are 1.67 arcseconds apart and have an orbital period of 760 years.

Christian Mayer, director of the Mannheim Observatory, is considered the first to have observed Zeta Aquarii to be double, in 1777. A couple of years later, William Herschel also discovered this duality.

Zeta Aquarii has only been observed through half its orbit; hence, the size and shape of the orbit, and therefore, the orbital period, are undetermined. According to Martin Gaskell, the latest orbital calculation was made in 1968, and gave a period of 856 years. (This information was catalogued in Norton's, 2000.)

The greatest distance in the elliptical orbit of the two components, which appears to be a clockwise movement, is approximately four times greater than their closest distance, which is about the distance of our Sun and Pluto.

The stars currently have an ephemeris of about 2.5". It is projected that at the end of the twenty-third century, the stars should be about 6.4" apart. The change per year a very slight, however.

The two components, simply labeled star A and star B, each have a mass of 1.1 solar masses and 0.9 solar masses, respectively. Despite this similitude in mass, they are about seven times more luminous than our Sun, which means they are more evolved. The spectroscopic interpretation of both stars classifies them as subgiants. This supports the prediction that the hydrogen in their cores is exhausted and that they are becoming red giants.

There have been claims that there is a third component orbiting star B with a period of 25.5 years; but it is possible that this is simply an over-interpretation of residuals, as has been the case with other perceived third components of binary systems. Such tertiaries have a luminosity too low to be seen directly. The speculated tertiary in the Zeta Aquarii system allegedly has a mass of 0.28 solar masses and is probably a red or white dwarf. It is located at 9 astronomical units from star B, the same distance as Saturn from our Sun.

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.

Components

References

• Double Stars to Follow, Part IV: Zeta Aquarii and Mu Cygni. by Martin Gaskell, Prairie Astronomy Club Home Page.
• "This Month's Double Stars" by Richard Jaworski.
• Image ζ Aquarii