Sigma Draconis

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Sigma Draconis
Observation data
Epoch J2000      Equinox J2000
Constellation Draco
Right ascension 19h 32m 21.59026s[1]
Declination +69° 39′ 40.2354″[1]
Apparent magnitude (V) 4.674[2]
Spectral type G9 V[3][4]
U−B color index +0.386[2]
B−V color index +0.791[2]
Variable type None
Radial velocity (Rv) 26.7[5] km/s
Proper motion (μ) RA: 598.07[1] mas/yr
Dec.: −1738.40[1] mas/yr
Parallax (π) 173.77 ± 0.18[1] mas
Distance 18.77 ± 0.02 ly
(5.755 ± 0.006 pc)
Absolute magnitude (MV) 5.87
Mass 0.87[6] M
Radius 0.778 ± 0.008[7] R
Luminosity 0.428 ± 0.013[7] L
Surface gravity (log g) 4.60[6] cgs
Temperature 5,299 ± 32[7] K
Metallicity [Fe/H] −0.20[7] dex
Rotational velocity (v sin i) 1.5[8] km/s
Age 4.7[9] Gyr
Other designations
σ Draconis, 61 Draconis, HD 185144, HR 7462, BD+69°1053, GCTP 4607.00, GJ 764, LHS 477, and HIP 96100.[5]
Database references

Sigma Draconis (σ Dra, σ Draconis) is a 4.7-magnitude star located at a distance of 18.8 light-years in the constellation Draco. Its traditional name is Alsafi.[10]


Sigma Draconis, the Bayer designation for this star, was established in 1603 as part of the Uranometria, a star catalogue produced by German celestial cartographer Johann Bayer. The proper name Alsafi was derived from Arabic Athāfi, erroneously transcribed from the Arabic plural Athāfiyy, by which the nomads designated the tripods of their open-air kitchens. It was the name of an association of this star, τ Dra and υ Dra.[10] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Athāfi or Alsafi were the title for three stars: σ Dra as Alsafi, τ Dra as Athāfi I and υ Dra as Athāfi II.[11]

In Chinese, 天廚 (Tiān Chú), meaning Celestial Kitchen, refers to an asterism consisting of σ Draconis, δ Draconis, ε Draconis, ρ Draconis, 64 Draconis and π Draconis.[12] Consequently, σ Draconis itself is known as 天廚二 (Tiān Chú èr, English: the Second Star of Celestial Kitchen.)[13]


The star is a main sequence dwarf of spectral type K0 V, which has long served as a K0 V spectral standard star [14] [15] .[16] Its classification as K0 V defines one of the rare anchor points of the MK system[17] that have remained unchanged since the original 1943 MKK Atlas. The radius has been directly measured using interferometery with the CHARA Array, which yields a result of 77.8% of the Sun's radius.[7] It has 87% of the Sun's mass, but the luminosity of this star is only 43% that of the Sun.[6][7] The projected rotation rate (v sin i) is relatively low at 1.5 km/s. It is considered a slightly metal-poor star, meaning that it has a lower proportion of isotopes with masses more than helium when compared to the Sun.[18]

The temperature, luminosity and surface activity appear to vary slightly in a manner very similar to the sunspot cycle, although the full length of the cycle has not yet been determined.[8] The total variability is among the lowest of all stars that have been measured by the Hipparcos spacecraft.[18]

The components of Sigma Draconis's space velocity are U=+36, V=+40 and W=-10 km/s. This gives the star an unusually large orbital eccentricity about the Milky Way galaxy of 0.30 (compared to 0.06 for the Sun.) The mean galactocentric distance for the orbit is 10.3 kiloparsecs (about 34,000 light-years).[18]

As of 2007, no Jupiter-size or larger companion had been detected about the star and there was no indication of excess infrared radiation that would be evidence of circumstellar matter (such as a debris disk).[19]

Planetary system[edit]

Between 2004 and 2013, extensive radial velocity measurements were gathered on Sigma Draconis using the High Resolution Echelle Spectrometer on the Keck Observatory. Towards the end of that period, a significant radial velocity trend peak at around 310 days and another possible trend near 2800 days were emerging from the accumulated data. The trend around 310 days indicated a planet orbiting inside the habitable zone with a minimum of 11.2 Earth masses. However, the radial velocity peak near 2800 days was suspected to be caused by a bias in the data set collected by the telescope and not an indication of a second planet. Combining the Keck/HIRES data with further measurements taken in 2013 from the new Automated Planet Finder telescope indeed did show a significantly strengthened trend at 308 days consistent with a Uranus sized planet, of greater than around 12 Earth masses, but it also showed a much reduced radial velocity trend near 2800 days.[20]

The unconfirmed Sigma Draconis system[20]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b (unconfirmed) ≳12 M 0.814[note 1] 308 ~0

See also[edit]


  1. ^ Kepler's Third Law, assuming a circular orbit - \begin{smallmatrix}\frac{4 \pi^2}{T^2} = \frac{G M}{R^3}\end{smallmatrix} Mass and the Period are known from paper[20] so equation can be written with semimajor axis as the subject - \begin{smallmatrix}R = \sqrt[3]{\frac{GMT^2}{4\pi^2}}\end{smallmatrix}.


  1. ^ a b c d e 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. ^ a b c Oja, T. (August 1986), "UBV photometry of stars whose positions are accurately known. III", Astronomy and Astrophysics Supplement Series 65 (2): 405–409, Bibcode:1986A&AS...65..405O 
  3. ^ Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Robinson, P. E. (2003). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 parsecs: The Northern Sample I". arXiv:0308182v1 [astro-ph].
  4. ^ Henry, Todd J. (October 1, 2006), The One Hundred Nearest Star Systems, Research Consortium on Nearby Stars, retrieved 2011-10-14 
  5. ^ a b "Query Result: NSV 12176 -- Variable Star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2007-06-15 
  6. ^ a b c Hearnshaw, J. B. (1974), "Carbon and iron abundances for twenty F and G type stars", Astronomy and Astrophysics 36: 191–199, Bibcode:1974A&A....36..191H 
  7. ^ a b c d e f Boyajian, Tabetha S. et al. (August 2008), "Angular Diameters of the G Subdwarf μ Cassiopeiae A and the K Dwarfs σ Draconis and HR 511 from Interferometric Measurements with the CHARA Array", The Astrophysical Journal 683 (1): 424–432, arXiv:0804.2719, Bibcode:2008ApJ...683..424B, doi:10.1086/589554 
  8. ^ a b Gray, David F. et al. (1992), "The activity cycle of Sigma Draconis", Astrophysical Journal 400 (2): 681–691, Bibcode:1992ApJ...400..681G, doi:10.1086/172030 
  9. ^ Mamajek, Eric E.; Hillenbrand, Lynne A. (November 2008), "Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics", The Astrophysical Journal 687 (2): 1264–1293, arXiv:0807.1686, Bibcode:2008ApJ...687.1264M, doi:10.1086/591785 
  10. ^ a b Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York, NY: Dover Publications Inc, p. 210, ISBN 0-486-21079-0, retrieved 2010-12-12 
  11. ^ Rhoads, Jack W. (November 15, 1971), Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars, Jet Propulsion Laboratory, California Institute of Technology 
  12. ^ (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.
  13. ^ (Chinese) 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.
  14. ^ Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas H.L. Johnson & W.W. Morgan, 1953, Astrophysical Journal, 117, 313
  15. ^ Spectral Classification W.W. Morgan & P.C. Keenan, 1973, Annual Review of Astronomy and Astrophysics, 11, 29
  16. ^ The Perkins Catalog of Revised MK Types for the Cooler Stars, P.C. Keenan & R.C McNeil, "Astrophysical Journal Supplement Series" 71 (October 1989), pp. 245–266.
  17. ^ MK ANCHOR POINTS, Robert F. Garrison
  18. ^ a b c Porto de Mello, Gustavo; del Peloso, Eduardo F.; Ghezzi, Luan (2006), "Astrobiologically Interesting Stars Within 10 Parsecs of the Sun", Astrobiology 6 (2): 308–331, arXiv:astro-ph/0511180, Bibcode:2006AsBio...6..308P, doi:10.1089/ast.2006.6.308, PMID 16689649 
  19. ^ Holmes, E. K. et al. (2003), "A Survey of Nearby Main-Sequence Stars for Submillimeter Emission", The Astronomical Journal 125 (6): 3334–3343, Bibcode:2003AJ....125.3334H, doi:10.1086/375202 
  20. ^ a b c Vogt, Steven S. et al. (February 2014), "APF - The Lick Observatory Automated Planet Finder" (PDF), Publications of the Astronomical Society of the Pacific, arXiv:1402.6684, Bibcode:2014arXiv1402.6684V 

External links[edit]