Alpha Piscium
Triple star system with variable brightness in Pisces.
Alpha Piscium is a triple star system located in the equatorial constellation Pisces. Its three components are designated Alpha Piscium A, Ba, and Bb. The system is about 151 light-years from the Solar System, based on dynamical parallax measurements.
The traditional name for the system is Alrescha, derived from the Arabic for "the cord." Less common names include Kaitain and Okda, the latter from the Arabic word for "knot." In 2016, the International Astronomical Union approved the name Alrescha specifically for the component Alpha Piscium A. In Chinese astronomy, Alpha Piscium is part of an asterism called Outer Fence, which also includes Delta, Epsilon, Zeta, Mu, Nu, and Xi Piscium.
The system has a hierarchical structure. The outer pair consists of Alpha Piscium A and the subsystem Alpha Piscium B, separated by 1.85 arcseconds. Alpha Piscium A is the primary, with a visual magnitude of +4.33 and spectral type kA0hA7 Sr. The two components of Alpha Piscium B have a combined magnitude of 5.23 and a combined spectral class of kA2hF2mF2 (IV). These two bodies take roughly 2,800 years to orbit each other, and they will make their closest approach around the year 2188. The inner pair, Ba and Bb, orbit each other every 25 days with a semi-major axis of about 0.04 astronomical units.
Alpha Piscium is classified as an α² Canum Venaticorum variable star, where brightness changes are driven by stellar rotation. The variations are about 0.01 magnitude, detected by Hipparcos photometry. The primary component is thought to be the source of these variations, with an initial rotation period of 0.845 days later revised to 1.49 days. A variation with a period of 6.65 days has also been identified.
The system also forms a four-star system with the G1/2V star HIP 9519, which lies 404.7 arcseconds away. For observers away from polar regions, the star culminates at midnight on 21 October, making it visible from dusk to dawn. Six months later, on 22 April, it reaches conjunction with the Sun.
- Bayer designation
- Alpha Piscium
- Traditional name
- Alrescha
- Apparent magnitude (primary)
- +4.33
- Spectral type (primary)
- kA0hA7 Sr
- Combined magnitude (b components)
- 5.23
- Combined spectral class (b components)
- kA2hF2mF2 (IV)
- Variable star type
- α² Canum Venaticorum
Lore & Background
The system bears the traditional name Alrescha, derived from the Arabic ar-Rishā’ meaning 'the cord', and less commonly Kaitain and Okda, the latter from the Arabic ʽuqdah 'knot'. In 2016, the International Astronomical Union approved the name Alrescha for the component Alpha Piscium A. In Chinese astronomy, Alpha Piscium is part of an asterism called Outer Fence, which also includes Delta, Epsilon, Zeta, Mu, Nu, and Xi Piscium.
Alpha Piscium comprises a hierarchical architecture. The outer system consists of Alpha Piscium A and the subsystem Alpha Piscium B, separated by 1.85 arcseconds. The two bodies take roughly 2,800 years to orbit one another and will make their closest approach around 2188. The components of Alpha Piscium B, called Ba and Bb, have an orbital period of roughly 25 days.
The primary component is thought to be the source of the brightness variations, with a period of 0.845 days believed to be the rotation period, later revised to 1.49 days. Variations with a period of 6.65 days have also been identified. The brightness varies by about 1/100 of a magnitude, identified from Hipparcos photometry. Alpha Piscium also composes a four-star system together with the G1/2V star HIP 9519, separated by 404.7 arcseconds.
Reader's Guide
Alpha Piscium serves as a notable example of a hierarchical multiple star system and an α² Canum Venaticorum variable. Its variability, driven by the rotation of the primary component, has been studied through Hipparcos photometry, revealing subtle brightness changes of about 1/100 of a magnitude. The system's orbital dynamics are complex: the outer pair takes roughly 2,800 years to complete an orbit, with a predicted closest approach around 2188, while the inner binary components orbit each other every 25 days. The star's traditional name Alrescha, meaning 'the cord', reflects its historical significance in Arabic astronomy, and it is also part of the Chinese Outer Fence asterism. The system's inclusion in the Washington Multiplicity Catalog and the IAU's standardization of star names underscores its importance in modern stellar nomenclature. Additionally, Alpha Piscium forms a four-star system with the star HIP 9519, separated by over 400 arcseconds, highlighting the extended gravitational architecture of the region.
Did You Know?
- Alpha Piscium is an α² Canum Venaticorum variable star, with brightness changes caused by the rotation of the star.
- The system's traditional name Alrescha derives from the Arabic ar-Rishā’ meaning 'the cord'.
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