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|
|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 "single" star within the constellation.
Stellar system 
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.
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)
See also 
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- Gray, R. O. et al. (October 2003), "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.", The Astronomical Journal 126 (4): 2048–2059, arXiv:astro-ph/0308182, Bibcode:2003AJ....126.2048G, doi:10.1086/378365
- Feinstein, A. (November 1974), "Photoelectric UBVRI observations of AM stars", Astronomical Journal 79: 1290, Bibcode:1974AJ.....79.1290F, doi:10.1086/111675
- Wilson, Ralph Elmer (1953). General Catalogue of Stellar Radial Velocities. Washington: Carnegie Institution of Washington. Bibcode:1953QB901.W495.....
- Batten, A. H.; Fletcher, J. M. (April 1992), "A new spectroscopic orbit for Delta Capricorni", Journal of the Royal Astronomical Society of Canada 86 (2): 99–109, Bibcode:1992JRASC..86...99B
- Trilling, D. E. et al. (April 2007), "Debris disks in main-sequence binary systems", The Astrophysical Journal 658 (2): 1264–1288, arXiv:astro-ph/0612029, Bibcode:2007ApJ...658.1289T, doi:10.1086/511668. See p. 33.
- Royer, F. et al. (October 2002), "Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i", Astronomy and Astrophysics 393: 897–911, arXiv:astro-ph/0205255, Bibcode:2002A&A...393..897R, doi:10.1051/0004-6361:20020943
- "del Cap -- Eclipsing binary of Algol type", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2012-02-06
- Eggleton, P. P.; Tokovinin, A. A. (September 2008). "A catalogue of multiplicity among bright stellar systems". Monthly Notices of the Royal Astronomical Society 389 (2): 869–879. arXiv:0806.2878. Bibcode:2008MNRAS.389..869E. doi:10.1111/j.1365-2966.2008.13596.x.
- Lloyd, C.; Wonnacott, D. (January 1994), "Is the ROSAT Wide Field Camera EUV source and AM eclipsing binary, Delta Capricorni, also a Delta Scuti variable?", Monthly Notices of the Royal Astronomical Society 266: L13–L16, Bibcode:1994MNRAS.266L..13L
- "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
- Wright, Anne (2003), The Fixed Stars: Deneb Algedi, archived from the original on June 16, 2005, retrieved July 28, 2005
- 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
- (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
- Malasan, H. L.; et al. (1989). "Delta Capricorni – an evolved binary or a main-sequence binary?". Astronomical Journal 97: 499–504. Bibcode:1989AJ.....97..499M. doi:10.1086/114999.
- "Delta Capricorni". Heavens Above. Hipparcos Star Catalog Entry. Retrieved 2008-06-21.
- Kaler, Jim. "Deneb Algedi". University of Illinois. STARS. Retrieved 2008-06-21.