Sigma Draconis

σ Draconis
Sigma Draconis is located in the constellation Draco
Sigma Draconis is located in the constellation Draco
σ
Location of σ Draconis (in red circle).

Observation data
Epoch J2000      Equinox ICRS
Constellation Draco
Right ascension 19h 32m 21.59021s
Declination +69° 39′ 40.2358″
Apparent magnitude (V) 4.674
Characteristics
Evolutionary stage Main sequence
Spectral type K0 V
U−B color index +0.386
B−V color index +0.791
Variable type None
Astrometry
Radial velocity (Rv)26.55±0.13 km/s
Proper motion (μ) RA: 597.384 mas/yr
Dec.: −1738.286 mas/yr
Parallax (π)173.4939±0.0748 mas
Distance18.799 ± 0.008 ly
(5.764 ± 0.002 pc)
Absolute magnitude (MV)5.89
Details
Mass0.84±0.02 M☉
Radius0.776±0.020 R☉
Luminosity0.44±0.03 L☉
Surface gravity (log g)4.33±0.04 cgs
Temperature5,336±40 K
Metallicity [Fe/H]−0.254 dex
Rotation27.7±0.77 days
Rotational velocity (v sin i)1.4 km/s
Age4.54±1.54 Gyr
Other designations
Alsafi, σ Draconis, Sig Dra, σ Dra, 61 Draconis, BD+69°1053, GJ 764, HD 185144, HIP 96100, HR 7462, LHS 477
Database references
SIMBADdata
ARICNSdata

Sigma Draconis is a single star in the northern constellation of Draco. It has the proper name Alsafi /ælˈseɪfi/, while Sigma Draconis, which is latinised from σ Draconis and abbreviated Sig Dra or σ Dra, is the Bayer designation. It has an apparent visual magnitude of 4.7, which is bright enough to be faintly visible to the naked eye. Based on parallax measurements, this star is located at a distance of 18.8 light years from the Sun. It is receding from the Sun with a radial velocity of 26.6 km/s.

Name

Photograph of Sigma Draconis (field of view about 0.5°)

σ Draconis (Latinised to Sigma Draconis) is the star's Bayer designation, established in 1603 as part of the Uranometria, a star catalogue produced by German celestial cartographer Johann Bayer.

It bore the traditional name Alsafi, derived from the Arabic Athāfi, itself erroneously transcribed from the Arabic plural Athāfiyy, by which the Bedouin nomads designated the tripods of their open-air kitchens. It was the name of an association of this star, Tau Draconis and Upsilon Draconis. According to a 1971 NASA memorandum, Athāfi or Alsafi was the title for three stars: Sigma Draconis as Alsafi, Tau Draconis as Athāfi I and Upsilon Draconis as Athāfi II. In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN approved the name Alsafi for Sigma Draconis on 30 June 2017 and it is now so included in the List of IAU-approved Star Names.

In Chinese, 天廚 (Tiān Chú), meaning Celestial Kitchen, refers to an asterism consisting of Sigma Draconis, Delta Draconis, Epsilon Draconis, Rho Draconis, 64 Draconis and Pi Draconis. Consequently, the Chinese name for Sigma Draconis itself is 天廚二 (Tiān Chú èr, English: the Second Star of Celestial Kitchen.)

Properties

Sigma Draconis is a main sequence dwarf which has long served as a K0 V spectral standard star. Its classification as K0 V defines one of the anchor points of the Morgan–Keenan system that have remained unchanged since the original 1943 MKK Atlas. However, some modern spectroscopy gives it as classification of G9 V.

The radius of Sigma Draconis has been directly measured using interferometry with the CHARA array, which yields a result of 77.6% of the Sun's radius at the star's distance. It has 84% of the Sun's mass, but the luminosity of this star is only 42% that of the Sun. The projected rotation rate (v sin i) is relatively low at 1.4 km/s. It is considered a slightly metal-poor star, meaning that it has a lower proportion of elements with masses greater than helium when compared to the Sun.

The temperature, luminosity and surface activity appear to vary slightly in a manner very similar to the sunspot cycle, with a changing duration of 5 to 7 years. The total variability is among the lowest of all stars that have been measured by the Hipparcos spacecraft.

Sigma Draconis has a high proper motion, advancing across the celestial sphere at a rate of 1.835 arc seconds per year. The star made its perihelion passage about 46,300 years ago, when it came within 16.6 ly (5.1 pc). 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).

As of 2013, 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).

Search for planets

Between 2004 and 2013, extensive radial velocity measurements were gathered on Sigma Draconis using the High Resolution Echelle Spectrometer on the Keck Observatory. The Keck/HIRES data indicated a possible period of about 300 days and a likely alias period of 2,800 days. Adding data taken with the Automated Planet Finder at the Lick Observatory strengthened and narrowed the 300-day period while reducing the significance of the 2,800-day period. The combined analysis suggests there may be a Uranus-mass planet on a 308-day orbit, though the authors do not yet consider the discovery to be publishable as they have not yet attempted to rule out other non-planetary explanations for the velocity variations.

A 2017 study also using Keck/HIRES data did not find evidence of a planet; while a signal with a 2,600-day period was found, it was attributed to the star's magnetic activity cycle. A 2025 study detected variations with a period of 409.7 days, which is tentatively attributed by its activity cycle.

See also

References

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  4. ↑ Radick, Richard R.; et al. (September 1998), "Patterns of Variation among Sun-like Stars", The Astrophysical Journal Supplement Series, 118 (1): 239–258, Bibcode:1998ApJS..118..239R, doi:10.1086/313135.
  5. ↑ Kim, Bokyoung; et al. (February 2016), "Spectroscopic Survey of G and K Dwarfs in the Hipparcos Catalog. I. Comparison between the Hipparcos and Photometric Parallaxes", The Astrophysical Journal Supplement Series, 222 (2): 29, arXiv:1601.01459, Bibcode:2016ApJS..222...19K, doi:10.3847/0067-0049/222/2/19, S2CID 15793261, 19.
  6. 1 2 3 Hon, Marc; Huber, Daniel; Li, Yaguang; Metcalfe, Travis S.; Bedding, Timothy R.; Ong, Joel; Chontos, Ashley; Rubenzahl, Ryan; Halverson, Samuel (2024-07-30), "Asteroseismology of the Nearby K-Dwarf Sigma Draconis using the Keck Planet Finder and TESS", The Astrophysical Journal, 975 (1): 147, arXiv:2407.21234, Bibcode:2024ApJ...975..147H, doi:10.3847/1538-4357/ad76a9
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  9. ↑ Olspert, N.; Lehtinen, J. J.; Käpylä, M. J.; Pelt, J.; Grigorievskiy, A. (2018-10-30). "Estimating activity cycles with probabilistic methods". Astronomy & Astrophysics. 619. EDP Sciences: A6. arXiv:1712.08240. Bibcode:2018A&A...619A...6O. doi:10.1051/0004-6361/201732525. ISSN 0004-6361.
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  14. ↑ Allen, R. H. (1963) [1899], 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{{citation}}: CS1 maint: ignored ISBN errors (link)
  15. ↑ Rhoads, Jack W. (November 15, 1971), Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars (PDF), Jet Propulsion Laboratory, California Institute of Technology
  16. ↑ "IAU Working Group on Star Names (WGSN)". Archived from the original on 23 April 2020. Retrieved 22 May 2016.
  17. ↑ 陳久金 [Chen Jiujin] (2005). 中國星座神話 [Chinese Constellation Mythology] (in Chinese). 台灣書房出版有限公司 [Taiwan Shufang Publishing Co., Ltd.] ISBN 978-986-7332-25-7.
  18. ↑ "香港太空館 - 研究資源 - 亮星中英對照表" [Hong Kong Space Museum - Research Resources - Bright Star Chinese-English Table] (in Chinese). Hong Kong Space Museum. Archived from the original on 2008-10-25. Retrieved November 23, 2010.
  19. ↑ Johnson, H. L.; Morgan, W. W. (1953). "Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas". Astrophysical Journal. 117: 313. Bibcode:1953ApJ...117..313J. doi:10.1086/145697.
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  26. ↑ Bruevich, E. A.; et al. (September 2016). "Evolution of the cycles of magnetic activity of the Sun and Sun-like stars in time". arXiv:1609.05335 [astro-ph.SR].
  27. ↑ Lépine, Sébastien; Shara, Michael M. (March 2005), "A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)", The Astronomical Journal, 129 (3): 1483–1522, arXiv:astro-ph/0412070, Bibcode:2005AJ....129.1483L, doi:10.1086/427854, S2CID 2603568.
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  32. Boyajian, Tabetha S.; et al. (February 2012), "Stellar Diameters and Temperatures. I. Main-sequence A, F, and G Stars", The Astrophysical Journal, 746 (1): 101, arXiv:1112.3316, Bibcode:2012ApJ...746..101B, doi:10.1088/0004-637X/746/1/101, S2CID 18993744. See Table 10.