Pi1 Cygni

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Pi¹ Cygni
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Cygnus
Right ascension  21h 42m 05.66458s[1]
Declination +51° 11′ 22.6415″[1]
Apparent magnitude (V) 4.66[2]
Spectral type B3 IV[3]
B−V color index −0.11[2]
Proper motion (μ) RA: +5.29[1] mas/yr
Dec.: −1.78[1] mas/yr
Parallax (π)1.89 ± 0.15[1] mas
Distance1,700 ± 100 ly
(530 ± 40 pc)
Absolute magnitude (MV)−3.91[4]
Period (P)26.33 d
Eccentricity (e)0.00
Periastron epoch (T)2431306.5 ± 10.0 JD
Argument of periastron (ω)
Semi-amplitude (K1)
16.5 km/s
π¹ Cyg A
Mass10.1±0.1[3] M
Radius5.6[6] R
Luminosity16,538[7] L
Temperature18,360±1,100[2] K
Rotational velocity (v sin i)55[8] km/s
Age25.1±0.8[3] Myr
Other designations
π¹ Cyg, 80 Cygni, BD+50° 3410, FK5 3733, HD 206672, HIP 107136, HR 8301, SAO 33665.[9]
Database references

Pi¹ Cygni (π¹ Cygni, abbreviated Pi¹ Cyg, π¹ Cyg) is a binary star[5] in the northern constellation of Cygnus. It is visible to the naked eye, having a combined apparent visual magnitude of 4.66.[2] The distance to this system can be roughly gauged by its annual parallax shift of 1.89 mas,[1] which yields a separation of around 1,700 light years from the Sun, give or take a hundred light years.

The two components are designated Pi¹ Cygni A (officially named Azelfafage /əˈzɛlfəf/, the traditional name for the system)[10] and B.


π¹ Cygni (Latinised to Pi¹ Cygni) is the star's Bayer designation. The designations of the two components as Pi¹ 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).[11]

It bore the traditional name Azelfafage, derived from the Arabic ظلف الفرس Dhilf al-faras meaning "the horse track" or (probably) ذيل الدجاجة Dhail al-dajājah meaning "the tail of hen".[12] In 2016, the IAU organized a Working Group on Star Names (WGSN)[13] to catalogue and standardize proper names for stars. The WGSN approved the name Azelfafage for Pi¹ Cygni on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.[10] For such names relating to members of multiple star systems, and where a component letter is not explicitly listed, the WGSN says that the name should be understood to be attributed to the brightest component by visual brightness[14] - in this case Pi¹ Cygni A.

In Chinese, 螣蛇 (Téng Shé), meaning Flying Serpent, refers to an asterism consisting of Pi¹ Cygni, Alpha Lacertae, 4 Lacertae, Pi² Cygni, HD 206267, Epsilon Cephei, Beta Lacertae, Sigma Cassiopeiae, Rho Cassiopeiae, Tau Cassiopeiae, AR Cassiopeiae, 9 Lacertae, 3 Andromedae, 7 Andromedae, 8 Andromedae, Lambda Andromedae, Kappa Andromedae, Psi Andromedae and Iota Andromedae. Consequently, the Chinese name for Pi¹ Cygni itself is 螣蛇四 (Téng Shé sì, English: the Fourth Star of Flying Serpent)[15]


This is a single-lined spectroscopic binary with a close, circular orbit, having a period of just 26.33 days. The primary, component A, is a slightly evolved B-type subgiant star with a stellar classification of B3 IV.[3] It has an estimated 10 times the mass of the Sun and around 5.6[6] times the Sun's radius. The star radiates 16,538[7] times the solar luminosity from its outer atmosphere at an effective temperature of roughly 18,360 K.[2] It is about 25[3] million years old and is spinning with a projected rotational velocity of 55 km/s.[8]


  1. ^ a b c d e f van Leeuwen, F. (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. ^ a b c d e Soubiran, C.; et al. (June 2010), "The PASTEL catalogue of stellar parameters", Astronomy and Astrophysics, 515: A111, arXiv:1004.1069, Bibcode:2010A&A...515A.111S, doi:10.1051/0004-6361/201014247.
  3. ^ a b c d e Tetzlaff, N.; et al. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x.
  4. ^ 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.
  5. ^ a b Pourbaix, D.; et al. (2004), "SB9: The ninth catalogue of spectroscopic binary orbits", Astronomy & Astrophysics, 424 (2): 727, arXiv:astro-ph/0406573, Bibcode:2004A&A...424..727P, doi:10.1051/0004-6361:20041213.
  6. ^ a b Pasinetti Fracassini, L. E.; et al. (February 2001), "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS)", Astronomy and Astrophysics (3rd ed.), 367 (2): 521–524, arXiv:astro-ph/0012289, Bibcode:2001A&A...367..521P, doi:10.1051/0004-6361:20000451.
  7. ^ a b Hohle, M. M.; et al. (April 2010), "Masses and luminosities of O- and B-type stars and red supergiants", Astronomische Nachrichten, 331 (4): 349, arXiv:1003.2335, Bibcode:2010AN....331..349H, doi:10.1002/asna.200911355.
  8. ^ a b Strom, Stephen E.; et al. (2005), "B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?", The Astronomical Journal, 129 (2): 809–828, arXiv:astro-ph/0410337, Bibcode:2005AJ....129..809S, doi:10.1086/426748.
  9. ^ "pi.01 Cyg -- Spectroscopic binary", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2017-02-19.
  10. ^ a b "Naming Stars". IAU.org. Retrieved 16 December 2017.
  11. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  12. ^ p. 197, Star-names and Their Meanings, Richard Hinckley Allen, New York, G. E. Stechert, 1899.
  13. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  14. ^ "Bulletin of the IAU Working Group on Star Names, No. 2" (PDF). Retrieved 16 December 2017.
  15. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 7 日