Xi Scorpii
Quintuple star system in Scorpius with two close groups.
Xi Scorpii (ξ Sco) is a star system in the constellation Scorpius containing five stars. It received its Bayer designation from Johann Bayer, though Ptolemy had previously listed it in the constellation Libra. John Flamsteed assigned it the number 51 Librae, but this designation is no longer used because modern constellation boundaries place the star in Scorpius.
The system has no proper name, though it was once mistakenly called Graffias before that name was reassigned to Beta Scorpii. The five stars are divided into two groups separated by about 4.7 arcminutes (0.08°) on the sky. The brighter group consists of three stars: Xi Scorpii A, B, and C. A and B are both yellow-white F-type stars, with A being slightly brighter and hotter. They orbit each other every 45.9 years at an average separation of 0.744 arcseconds. The fainter, seventh-magnitude Xi Scorpii C orbits this pair at about ten times that distance, with a separation of roughly 7.6 arcseconds.
The second group contains Xi Scorpii D and E, both K-type main-sequence stars separated by about 11.9 arcseconds. All five stars share similar proper motions, confirming they are physically associated. In multiple-star catalogs, the pair AB and C are listed together as Σ1998, while D and E are listed as Σ1999.
- Bayer designation
- ξ Scorpii (Xi Scorpii)
- Flamsteed designation
- 51 Librae (fallen out of use)
- Constellation
- Scorpius
- Number of stars
- 5
- Separation between groups
- 4.7 arcminutes (0.08°)
- Orbital period (a and b)
- 45.9 years
- Separation (a and b)
- 0.744 arcseconds
- Separation (c from a/b)
- 7.6 arcseconds
- Separation (d and e)
- 11.9 arcseconds
Lore & Background
Xi Scorpii was assigned its Bayer designation by Johann Bayer, although Ptolemy had catalogued the star in Libra. John Flamsteed later gave it the designation 51 Librae, but this fell out of use when modern constellation boundaries placed the star in Scorpius. The system has no proper name, though it was erroneously known as Graffias before that name was applied to Beta Scorpii.
The five stars are divided into two groups. The brighter group contains Xi Scorpii A, B, and C. A and B are both yellow-white F-type stars, with A slightly brighter and warmer. They orbit each other every 45.9 years at an average separation of 0.744 arcseconds. The dimmer, seventh-magnitude Xi Scorpii C orbits this pair at about ten times the distance, with a separation of around 7.6 arcseconds. The second group contains components D and E, both K-type main-sequence stars, separated by about 11.9 arcseconds. All five stars share similar proper motions, confirming their association.
In multiple-star catalogues, the pair AB and C are grouped as Σ1998, while D and E are grouped as Σ1999.
Reader's Guide
Xi Scorpii is significant as a quintuple star system, demonstrating hierarchical orbital architecture with two distinct groups separated by a wide angular distance. The system's history reflects shifting constellation boundaries: originally catalogued in Libra by Ptolemy and later given a Flamsteed number there, it was reassigned to Scorpius when modern boundaries were drawn, causing the Flamsteed designation to fall out of use. The erroneous use of the name Graffias for Xi Scorpii before it was applied to Beta Scorpii illustrates historical naming confusions. The system's five components are all physically linked through common proper motion, making it a valuable example of a multiple-star system with both close (A and B) and wide (D and E) binaries. The grouping of stars under separate multiple-star designations (Σ1998 and Σ1999) highlights how visual catalogues treat close and wide pairs differently. The system's properties—such as the 45.9-year orbit of A and B and the spectral types of its components—provide concrete data for studies of stellar evolution and dynamics in multiple systems.
Did You Know?
- Xi Scorpii was originally catalogued by Ptolemy in Libra, but modern boundaries place it in Scorpius.
- The system's Flamsteed designation 51 Librae fell out of use after the constellation boundaries were redrawn.
More in Binary and Multiple Stars, Part 3 1-24
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