Zeta Andromedae
Star system in the constellation Andromeda
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Zeta Andromedae, officially named Shimu,[9] is a binary star system in the northern constellation Andromeda. It is visible to the naked eye with an apparent visual magnitude that varies from 3.92 to 4.14 over the course of its 17.7 day orbit.[2] Based on parallax measurements, it is located at a distance of approximately 181 light-years from the Sun. The system is drifting closer with a heliocentric radial velocity of −24.4 km/s.[5]
Naming
The star's location is in the northern constellation Andromeda, in which it is the second-most southerly of the stars in this often drawn characteristic shape representing the mythical princess asterism, after η Andromedae.
Zeta Andromedae (Zeta And, ζ Andromedae, ζ And) is the star's Bayer designation. It also has the Flamsteed designation 34 Andromedae and multiple other designations in stellar catalogues.
In Chinese, 奎宿 (Kuí Sù), meaning Legs (asterism), refers to an asterism consisting of ζ Andromedae, η Andromedae, 65 Piscium, ε Andromedae, δ Andromedae, π Andromedae, ν Andromedae, μ Andromedae, β Andromedae, σ Piscium, τ Piscium, 91 Piscium, υ Piscium, φ Piscium, χ Piscium and ψ1 Piscium. Consequently, the Chinese name for ζ Andromedae itself is 奎宿二 (Kuí Sù èr, English: the Second Star of Legs).[10]
An older Chinese name is Tian Shi Mu, The Eye of the Celestial Pig. Based on this, the IAU Working Group on Star Names approved the name Shimu for Zeta Andromedae A on 16 March 2025 and it is now so entered in the IAU Catalog of Star Names.[9]
System
The system is a spectroscopic binary whose primary is classified as an orange K-type giant with a mean apparent magnitude of +4.08. Due to brightness changes caused by the ellipsoidal shape of that object, the system is also an RS Canum Venaticorum-type variable star. It is a magnetically active star with a brightness that varies from magnitude +3.92 to +4.14 over a period of 17.77 days, and its spectrum shows strong and variable Ca II H and K lines. The orbital period of the binary is 17.77 days.[3]
The primary component of this binary system, Zeta Andromedae Aa, is one of the few stars who has been resolved using Doppler imaging and long-baseline infrared interferometry. With resolved images astronomers can recover additional information about this star.
Resolved images also allowed observation of starspots (the analogue to sunspots), on this star, and their asymmetric distribution showed that the magnetic field of the star is generated by a mechanism different from the solar dynamo.[12] A Sun-like differential rotation of the star was observed instead.[3]
Visual companions
The WDS notes three visual companions to the eclipsing binary (Aa and Ab, forming binary A).[4] The parallax of the D star has been measured by Gaia proving its distance to be much greater than Zeta Andromedae, probably a distant red giant.[13] The closest companion, B, is likewise a background object.[14] The companion C at 97″ shares a common proper motion and a similar parallax.[15]
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