Xi Aquarii
A binary star system in Aquarius, visible to the naked eye.
Xi Aquarii is a binary star system located in the equatorial constellation Aquarius. It appears to the unaided eye as a single point of light with an apparent visual magnitude of 4.7. The system is classified as a single-lined spectroscopic binary: the presence of an unseen companion is detected through periodic Doppler shifts in the star's spectral absorption lines. The two components orbit each other once every 8,016 days (about 22 years), following an eccentric path with an eccentricity of 0.54.
Based on parallax measurements, the system lies roughly 179 light-years from the Sun and is moving closer at a heliocentric radial velocity of –26 km/s. Its position near the ecliptic makes it subject to lunar occultations. The primary star, designated Xi Aquarii A (also named Bunda), is an A-type main-sequence star with a stellar classification of A7 V. It has about 1.7 times the Sun's mass and spins rapidly, with a projected rotational velocity of 170 km/s. Its photosphere radiates 36 times the Sun's luminosity at an effective temperature of 8,140 K. The orbital data suggests the secondary component, Xi Aquarii B, is likely either a red dwarf or a white dwarf.
The Bayer designation ξ Aquarii (Latinized to Xi Aquarii) applies to the binary system. The component designations A and B follow the convention of the Washington Multiplicity Catalog, adopted by the International Astronomical Union. Along with Beta Aquarii (Sadalsuud), Xi Aquarii once formed the Persian lunar mansion Bunda. In 2016, the IAU's Working Group on Star Names approved the name Bunda specifically for Xi Aquarii A on 1 June 2018, and it now appears in the List of IAU-approved Star Names. In the Calendarium of Al Achsasi al Mouakket, the star was called Thanih Saad al Saaoud, meaning "the second of luck of lucks," and together with Beta Aquarii and 46 Capricorni, it formed the asterism known as "the Luck of Lucks." In Chinese astronomy, Xi Aquarii belongs to an asterism called Celestial Ramparts, which also includes 46, 47, 50, and 29 Capricorni, Lambda Capricorni, and several Aquarii stars (18, 9, 8, Nu, 14, 17, and 19 Aquarii). The Chinese name for Xi Aquarii itself is derived from this asterism.
Quick Facts
- Bayer designation
- ξ Aquarii
- Apparent visual magnitude
- 4.7
- Distance
- approximately 179 ly from the Sun
- Radial velocity
- –26 km/s (drifting closer)
- Orbital period
- 8,016 days (22 years)
- Orbital eccentricity
- 0.54
- Primary component spectral type
- A7 V (A-type main sequence star)
Facts from the source article.
Lore & Background
Xi Aquarii carries the Bayer designation ξ Aquarii, Latinized from the Greek. Its two components are designated Xi Aquarii A (also named Bunda) and B. The system lies near the ecliptic, making it subject to lunar occultation. Based on parallax measurements, it is approximately 179 light-years from the Sun and drifting closer with a heliocentric radial velocity of –26 km/s.
The primary component, Xi Aquarii A, is an A-type main sequence star with a stellar classification of A7 V. It has about 1.7 times the mass of the Sun and is spinning rapidly with a projected rotational velocity of 170 km/s. It radiates 36 times the luminosity of the Sun from its photosphere at an effective temperature of 8,140 K. The two bodies orbit each other with a period of 8,016 days (22 years) and an eccentricity of 0.54. The orbital data is consistent with the secondary component, Xi Aquarii B, being either a red dwarf or a white dwarf star.
In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN approved the name Bunda for the component Xi Aquarii A on 1 June 2018, and it is now included in the List of IAU-approved Star Names. Along with Beta Aquarii (Sadalsuud), it constituted the Persian lunar mansion Bunda. In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Thanih Saad al Saaoud, translated into Latin as 'the second of luck of lucks'. This star, along with Beta Aquarii and 46 Capricorni, were 'the Luck of Lucks'.
Reader's Guide
Xi Aquarii holds significance as a named star in multiple cultural and astronomical contexts. Its proper name Bunda, approved by the IAU in 2018, derives from the Persian lunar mansion that it shared with Beta Aquarii (Sadalsuud). This connection to ancient Persian astronomy reflects the star's role in timekeeping and celestial organization. In the Calendarium of Al Achsasi al Mouakket, it was called Thanih Saad al Saaoud, meaning 'the second of luck of lucks', and together with Beta Aquarii and 46 Capricorni formed the asterism 'the Luck of Lucks', indicating its importance in Arabic astrological traditions. In Chinese astronomy, Xi Aquarii is part of the asterism Celestial Ramparts, which includes numerous stars from Aquarius and Capricornus. Scientifically, Xi Aquarii is notable as a single-lined spectroscopic binary system, where the primary star's spectral lines reveal the presence of an unseen companion through Doppler shifts. The system's orbital parameters—a period of 22 years and an eccentricity of 0.54—allow astronomers to infer the nature of the secondary component, which may be either a red dwarf or a white dwarf. Its location near the ecliptic makes it a target for lunar occultation studies, providing opportunities to refine its position and orbital characteristics. The primary star's rapid rotation (170 km/s) and A-type classification contribute to understanding stellar evolution and binary system dynamics.
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