Delta Aquarii
The third-brightest star in Aquarius, a binary system named Skat.
Delta Aquarii, also known by its Bayer designation δ Aquarii and the Flamsteed number 76 Aquarii, is a binary star system located roughly 113 light-years from Earth. It ranks as the third-brightest star in the constellation Aquarius, with an apparent visual magnitude of 3.3, making it easily visible to the unaided eye on a clear night. The system is moving away from the Sun at a radial velocity of about +17 km/s.
The system’s official proper name, Skat, comes from the Arabic al-sāq, meaning “shin.” This name (also historically spelled Scheat, Seat, or Sheat) was mistakenly applied to Beta Pegasi in late medieval times. In 2016, the International Astronomical Union’s Working Group on Star Names approved Skat as the star’s official name. In Chinese astronomy, Delta Aquarii is part of an asterism called Palace Guard, which includes dozens of other stars; the star itself is known by a specific Chinese designation derived from that grouping.
Delta Aquarii is a chemically peculiar A-type main-sequence star, with a spectral classification of A3 Vp. It generates energy through hydrogen fusion in its core. The star has about three times the Sun’s mass and a radius roughly 4.16 times larger, radiating about 100 times the Sun’s luminosity from its outer atmosphere at an effective temperature near 9,000 K, which gives it a characteristic white hue. It rotates relatively quickly, with a projected rotational velocity of 81 km/s. No strong excess infrared emission has been detected, suggesting little to no circumstellar dust or debris. The system is a probable member of the Ursa Major Moving Group, which is estimated to be about 500 million years old.
Analysis of Hipparcos data indicated the presence of a close companion, and an orbit was derived with a period of 483 days, an eccentricity of 0.12, and an inclination of 41°. Initial searches for this companion found nothing, and any possible object beyond a certain separation was constrained to be less than a given mass. In 2017, infrared interferometric observations confirmed the companion, which is likely a G5 main-sequence star located at a specific angular separation from the primary.
Quick Facts
- Bayer designation
- δ Aquarii
- Flamsteed designation
- 76 Aquarii
- Apparent visual magnitude
- 3.3
- Distance
- about 113 ly
- Radial velocity
- +17 km/s
- Spectral type
- A3 Vp
- Mass
- three times the Sun's mass
- Radius
- 4.16 times the Sun's radius
Facts from the source article.
Lore & Background
Delta Aquarii bears the traditional name Skat (also rendered Scheat, Seat, Sheat, etc., which was erroneously applied to Beta Pegasi in late medieval times), from the Arabic الساق al-sāq 'shin'. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Skat for this star on 21 August 2016, and it is now so included in the List of IAU-approved Star Names. In Chinese, the asterism Palace Guard consists of Delta Aquarii along with many other stars; consequently, the Chinese name for Delta Aquarii itself is derived from that asterism.
The spectrum of Delta Aquarii matches a stellar classification of A3 Vp, indicating this is a chemically peculiar A-type main-sequence star that is generating energy through the nuclear fusion of hydrogen at its core. It has a relatively high rate of rotation, with a projected rotational velocity of 81 km s−1. Delta Aquarii does not display a strong signal of excess infrared emission that might indicate the presence of circumstellar matter. It is a probable stream star member of the Ursa Major Moving Group, which has an estimated age of 500 million years.
An analysis of Hipparcos data strongly suggested a close companion object. An orbit was derived with a 483-day period, an eccentricity of 0.12, and an inclination of 41°. When Delta Aquarii was first examined for a companion, none was found. Infrared interferometric observations did subsequently find a companion in 2017: a likely G5 main sequence star around from the primary.
Reader's Guide
Delta Aquarii holds significance as the third-brightest star in Aquarius, making it a key navigational and observational marker in the constellation. Its official name Skat, approved by the IAU in 2016, connects it to a long history of Arabic astronomical nomenclature, though the name was historically confused with Beta Pegasi. The star's classification as a chemically peculiar A-type main-sequence star places it among stars of interest for studies of stellar chemistry and rotation. Its membership in the Ursa Major Moving Group, an association of stars with a common origin about 500 million years ago, provides context for understanding stellar kinematics and evolution. The detection of a binary companion in 2017, after initial searches found none, highlights the challenges and advances in observing close stellar companions. The system's distance of about 113 light-years and its radial velocity of +17 km/s indicate it is moving away from the Sun, contributing to the mapping of stellar motions in the solar neighborhood.
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