Delta Cygni

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δ Cygni
Cygnus constellation map.svg
Red circle.svg
Location of δ Cygni (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Cygnus
Right ascension 19h 44m 58.47854s[1]
Declination +45° 07′ 50.9161″[1]
Apparent magnitude (V) 2.87[2]
Spectral type A0 IV[3] (B9 III + F1 V[4])
U−B color index –0.10[5]
B−V color index –0.02[5]
Variable type suspected[6]
Radial velocity (Rv)–20.1[7] km/s
Proper motion (μ) RA: +44.07[1] mas/yr
Dec.: +48.66[1] mas/yr
Parallax (π)19.77 ± 0.48[1] mas
Distance165 ± 4 ly
(51 ± 1 pc)
Absolute magnitude (MV)−0.74[8]
Period (P)780.27 yr
Semi-major axis (a)3.0″
Eccentricity (e)0.4670
Mass2.93 M
Radius5.13 R
Diameter5.13 D
Luminosity155 L
Surface gravity (log g)3.49 cgs
Temperature10,150 K
Rotational velocity (v sin i)135[11] km/s
Other designations
Fawaris, δ Cyg, 18 Cygni, 18 Cyg, BD+44 3234, HD 186882, HIP 97165, HR 7528, SAO 48796, WDS J19450+4508AB.
Database references

Delta Cygni (δ Cygni, abbreviated Delta Cyg, δ Cyg) is a binary star of a combined third-magnitude in the constellation of Cygnus. It is also part of the Northern Cross asterism whose brightest star is Deneb. Based upon parallax measurements obtained during the Hipparcos mission, Delta Cygni is located roughly 165 light-years (51 parsecs) distant from the Sun.[1]

Delta Cygni's two components are designated Delta Cygni A (also named Fawaris[12]) and B. More widely separated is a faint third component, a 12th magnitude star that is moving along with the others. Together they form a triple star system.[13]


δ Cygni (Latinised to Delta Cygni) is the binary's Bayer designation. The designations of the two components as Delta Cygni A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[14]

Traditionally, Delta Cygni had no proper name.[13] It belonged to the Arabic asterism al-Fawāris (الفوارس), meaning "the Riders" in indigenous Arabic,[15] together with Zeta, Epsilon, and Gamma Cygni, the transverse of the Northern Cross. In 2016, the IAU organized a Working Group on Star Names (WGSN)[16] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[17] It approved the name Fawaris for the component Delta Cygni A on 1 June 2018 and it is now so included in the List of IAU-approved Star Names.[12]

In Chinese, 天津 (Tiān Jīn), meaning Celestial Ford, refers to an asterism consisting of Delta Cygni, Gamma Cygni, 30 Cygni, Alpha Cygni (Deneb) and Nu, Tau, Upsilon, Zeta and Epsilon Cygni.[18] Consequently, Delta Cygni itself is known as 天津二 (Tiān Jīn èr, English: the Second Star of Celestial Ford).[19]


The primary, Delta Cygni A, is a blue-white giant star of spectral class B9,[4] with a temperature of 10,500 K. It is nearing the end of its main-sequence life stage with a luminosity 155 times that of the Sun, a radius of 5.13 solar radii, and a mass approximately 2.93 solar masses. Like many hot stars, it spins rapidly, at least 135 kilometers per second at the equator, about 60 times that of the Sun.[10]

The close companion Delta Cygni B is a yellow-white class F of the sixth magnitude (6.33) with a luminosity about 6 times that of the Sun, and a mass about 1.5 times the Sun's. The two stars orbit each other at an average distance of 157 AU and a period of 780 years.[13]

The much more distant third companion is an orange (class K) twelfth magnitude star, and only two thirds as massive.[13]

The two main stars together appear with a spectral type of A0 IV.[3] As seen from Earth, the entire triple star system of Delta Cygni shines at a combined apparent magnitude of 2.87.[2]

Pole Star[edit]

Delta Cygni is a visible star located within 3° of the precessional path traced across the celestial sphere by the Earth's North pole. For at least four centuries around 11,250 AD it will probably be considered a pole star, a title currently held by Polaris which is just 0.5° off of the precessional path.

Preceded by Pole Star Succeeded by
Deneb ~11,000 AD
~11,500 AD

Suspected variability[edit]

Both δ Cygni A and B have been suspected to vary in brightness. δ Cygni A was reported in 1951 as varying between magnitudes 2.85 and 2.89, and δ Cygni B was reported in 1837 to vary between magnitudes 6.3 and 8.5. The variability of the stars has not been confirmed.[6]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode 2007A&A...474..653V, doi:10.1051/0004-6361:20078357
  2. 2.0 2.1 Malagnini, M. L.; Morossi, C. (November 1990), "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars", Astronomy and Astrophysics Supplement Series 85 (3): 1015–1019, Bibcode 1990A&AS...85.1015M
  3. 3.0 3.1 . Bibcode 2009A&A...501..297Z.
  4. 4.0 4.1 Edwards, T. W. (April 1976), "MK classification for visual binary components", Astronomical Journal 81: 245–249, Bibcode 1976AJ.....81..245E, doi:10.1086/111879
  5. 5.0 5.1 Johnson, H. L. et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory 4 (99): 99, Bibcode 1966CoLPL...4...99J
  6. 6.0 6.1 Samus, N. N.Expression error: Unrecognized word "etal". (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally Published In: 2009yCat....102025S 1: B/gcvs. Bibcode 2009yCat....102025S.
  7. Evans, D. S. (June 20–24, 1966), Batten, Alan Henry; Heard, John Frederick, eds., "The Revision of the General Catalogue of Radial Velocities", Determination of Radial Velocities and Their Applications 30: 57, Bibcode 1967IAUS...30...57E
  8. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters 38 (5): 331, arXiv:1108.4971, Bibcode 2012AstL...38..331A, doi:10.1134/S1063773712050015.
  9. . Bibcode 2012A&A...546A..69M.
  10. 10.0 10.1 Challouf, M. et al. (2014), "Improving the surface brightness-color relation for early-type stars using optical interferometry⋆", Astronomy & Astrophysics 570: A104, arXiv:1409.1351, Bibcode 2014A&A...570A.104C, doi:10.1051/0004-6361/201423772.
  11. Abt, Helmut A.; Levato, Hugo; Grosso, Monica (July 2002), "Rotational Velocities of B Stars", The Astrophysical Journal 573 (1): 359–365, Bibcode 2002ApJ...573..359A, doi:10.1086/340590
  12. 12.0 12.1 Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  13. 13.0 13.1 13.2 13.3 Kaler, James B., "Delta Cygni", Stars (University of Illinois),, retrieved 2018-10-03
  14. Lua error in Module:Citation/CS1 at line 876: attempt to call method 'sub' (a nil value).
  15. Lua error in Module:Citation/CS1 at line 925: attempt to concatenate local 'str' (a table value).
  16. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  17. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  18. (in 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.
  19. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 4 日

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