Epoch J2000.0 Equinox J2000.0
|Right ascension||21h 47m 02.44424s|
|Declination||−16° 07′ 38.2335″|
|Apparent magnitude (V)||2.81|
|Spectral type||A7m III (kA5hF0mF2III)|
|U−B color index||+0.07|
|B−V color index||+0.31|
|Variable type||Delta Scuti
|Radial velocity (Rv)||-6.3 km/s|
|Proper motion (μ)||RA: +261.70 mas/yr
Dec.: -296.70 mas/yr
|Parallax (π)||84.27 ± 0.19 mas|
|Distance||38.70 ± 0.09 ly
(11.87 ± 0.03 pc)
|Period (P)||1.0227683 days|
|Eccentricity (e)||0 (assumed)|
|Periastron epoch (T)||2,448,105.793 ± 0.003 HJD|
|75.3 ± 1.0 km/s|
|δ Cap A|
|Surface gravity (log g)||3.66 cgs|
|Metallicity [Fe/H]||–0.13 dex|
|Rotational velocity (v sin i)||105 km/s|
|δ Cap B|
Delta Capricorni (δ Cap, δ Capricorni), also traditionally named Deneb Algedi and Scheddi, is a binary star system approximately 39 light-years away in the constellation of Capricornus (the Sea Goat). The primary star in the system is a white giant star and the combined light of its members makes it the brightest star within the constellation.
Delta Capricorni is an eclipsing binary star system similar to Algol, with an orbital period of 1.022768 days and an inclination close to the line of sight from the Earth. The peak apparent visual magnitude of the pair is 2.81. During an eclipse of the primary, this magnitude drops by 0.24. When the primary is eclipsing the secondary, the magnitude decreases by 0.09.
The primary component, Delta Capricorni A, has an overall stellar classification of A7m III, indicating that it is a giant star that has exhausted the supply of hydrogen at its core. More specifically, this is a chemically-peculiar Am star with a spectral type of kA5hF0mF2 III under the revised MK system. This notation indicates that the K-line matches the temperature of an A5 star, the hydrogen spectral type matches an F0 star, and the metallic absorption lines match an F2 star.
In the past this star was suspected of being a Delta Scuti variable, which is rare for an Am star. This categorization was brought into question during observations in 1994 and it is most likely not a variable star. Compared to the Sun, the primary has double the mass and nearly twice the radius. It is rotating rapidly with a projected rotational velocity of 105 km s−1. (This rotation rate is synchronous with the orbital period.) Note that it is unusual for an Am star to have such a high rotational velocity. The outer envelope of the star is radiating energy at an effective temperature of 7,301 K, giving it the white-hued glow of an A-type star. The secondary component is a type G or K star with around 90% of the mass of the Sun.
In 1906 astronomer Vesto Slipher of Lowell Observatory discovered that Delta Capricorni was a spectroscopic binary. The orbit was determined in 1921 by Clifford Crump using 69 radial velocity measurements obtained at Yerkes Observatory. However the eclipsing binary nature of the system was not discovered until 1956 by Olin J. Eggen at Lick Observatory.
Names and cultural associations
The traditional name 'Deneb Algedi' derives from the Arabic ذنب الجدي (ðanab al-jady), meaning "the tail of the goat", referring to the fishlike tail of the celestial sea-goat Capricorn. According to astrology, Deneb Algedi's representation of a flexible tail is reflected in its association with both good and bad fortune alike. It was one of the fifteen Behenian stars of medieval astrology, associated with chalcedony, marjoram and the kabbalistic symbol .
In Chinese astronomy, δ Capricorni is known as 壘壁陣四 (Lěi Bì Zhèn sì), meaning 'The Fourth Star of the Line of Ramparts'. This refers to its presence among an asterism known as 'The Line of Ramparts', which also includes κ Capricorni, ε Capricorni, γ Capricorni, ι Aquarii, λ Aquarii, σ Aquarii, φ Aquarii, 27 Piscium, 29 Piscium, 33 Piscium and 30 Piscium.(Chinese)
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