# Zeta Andromedae

Zeta Andromedae, also named Shimu, is a star system in the constellation Andromeda. It is approximately 189 light-years from Earth.

## Location

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.

## Naming

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 ψ$^1$ Piscium. Consequently, the Chinese name for ζ Andromedae itself is 奎宿二 (Kuí Sù èr, English: the Second Star of Legs).
An older Chinese name is Tian Shi Mu, The Eye of the Celestial Pig. The IAU Working Group on Star Names approved the name Shimu for this star on 16 March 2025 and it is now so entered in the IAU Catalog of Star Names.

## 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. Its brightness varies from magnitude +3.92 to +4.14 with 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.

## Resolved images

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.
Additional star parameters :
| Parameter | Value |
| Oblateness | 1.060 ± 0.011 |
| Polar radius | 15.0 ± 0.8 R$_☉$ |
| Axis Inclination | 70.0 ± 2.8 ° |
| Pole angle | 126 ± 1.9 ° |
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. A Sun-like differential rotation of the star was observed instead.

## Visual companions

The WDS notes three visual companions to the eclipsing binary (Aa and Ab, forming binary A). 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. The closest companion, B, is likewise a background object. The companion C at 97″ shares a common proper motion and a similar parallax.
Multiple/double star designation: WDS 00473+2416 :
| Component | Primary | Right ascension (α) Equinox J2000.0 | Declination (δ) Equinox J2000.0 | Epoch of observed separation | Angular distance from primary | Position angle (relative to primary) | Apparent magnitude (V) | Database reference |
| B | A | 00$^h$ 47$^m$ 20.2$^s$ | +24° 16′ 33″ | 1959 | 32.6″ | 0° | 15.3 | |
| C | A | 00$^h$ 47$^m$ 15.2$^s$ | +24° 15′ 03″ | 1997 | 97.0″ | 231° | 13.6 | |
| D | A | 00$^h$ 47$^m$ 09.0$^s$ | +24° 15′ 33″ | 2006 | 155.5″ | 260° | 10.80 | SIMBAD |