Alpha Serpentis

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Alpha Serpentis
Diagram showing star positions and boundaries of the Serpens constellation and its surroundings
Cercle rouge 100%.svg

Location of α Serpentis (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Serpens
Right ascension 15h 44m 16.07431s[1]
Declination +06° 25′ 32.2633″[1]
Apparent magnitude (V) 2.623[2]
Spectral type K2 III
U−B color index +1.248[2]
B−V color index +1.167[2]
Radial velocity (Rv) +2.63[3] km/s
Proper motion (μ) RA: +133.84[1] mas/yr
Dec.: +44.81[1] mas/yr
Parallax (π) 44.10 ± 0.19[1] mas
Distance 74.0 ± 0.3 ly
(22.68 ± 0.10 pc)
Absolute magnitude (MV) +0.88 ± 0.03[4]
Radius 12[5] R
Luminosity 70[6] L
Surface gravity (log g) 2.5[7] cgs
Temperature 4,498[7] K
Metallicity [Fe/H] 0.03[7] dex
Rotational velocity (v sin i) 4.3[7] km/s
Other designations
Unukalhai, Cor Serpentis, 24 Serpentis, HR 5854, HD 140573, SAO 121157, HIP 77070, ADS 9765, CCDM 15442+0626.[8]

Alpha Serpentis (α Serpentis, α Ser) is a double star in the head (Serpens Caput) of the equatorial constellation Serpens. It has the traditional name Unukalhai. With an apparent visual magnitude of 2.6,[2] this star is the brightest in the constellation and it can be viewed with the naked eye from most of the Earth. Parallax measurements yield an estimated distance of about 74 light-years (23 parsecs) from Earth.


Alpha Serpentis is a giant star with a stellar classification of K2 III, having consumed the hydrogen at its core and evolved away from the main sequence. The interferometry-measured angular diameter of this star, after correcting for limb darkening, is 4.85 ± 0.05 mas,[9] which, at its estimated distance, equates to a physical radius of about 12 times the radius of the Sun.[5] The effective temperature of the outer envelope is 4,498 K,[7] giving it an orange hue that is characteristic of a K-type star.[10]

This star is radiating about 38 times the luminosity of the Sun, while a further 32 times the Sun's luminosity is being emitted in the infrared, for 70-fold total.[6] A magnitude +11.8 companion is at an angular separation of 58 arcseconds from Alpha Serpentis, while a 13th magnitude star lies 2.3 arcminutes distant.


Alpha Serpentis has the proper names Unukalhai,[11] from the Arabic عنق الحية ‘Unuq al-Ħayyah "the Serpent's Neck", and Cor Serpentis from the Latin "the Heart of the Serpent". It was a member of indigenous Arabic asterism al-Nasaq al-Yamānī, "the Southern Line" of al-Nasaqān "the Two Lines".,[12] along with δ Ser (Qin, Tsin), ε Ser (Ba, Pa), δ Oph (Yed Prior), ε Oph (Yed Posterior), ζ Oph (Han) and γ Oph (Tsung Ching).[13]

According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, al-Nasaq al-Yamānī or Nasak Yamani were the title for two stars :δ Ser as Nasak Yamani I and ε Ser as Nasak Yamani II (exclude this star, δ Oph, ε Oph, ζ Oph and γ Oph)[14]

In Chinese, 天市右垣 (Tiān Shì Yòu Yuán), meaning Right Wall of Heavenly Market Enclosure, refers to an asterism which is represent eleven old states in China which is marking the right borderline of the enclosure, consisting of α Serpentis, β Herculis, γ Herculis, κ Herculis, γ Serpentis, β Serpentis, δ Serpentis, ε Serpentis, δ Ophiuchi, ε Ophiuchi and ζ Ophiuchi.[15] Consequently, α Serpentis itself is known as 天市右垣七 (Tiān Shì Yòu Yuán qī, English: the Seventh Star of Right Wall of Heavenly Market Enclosure), represent the state Shu (蜀) (or Shuh)[16][17](together with λ Ser in R.H.Allen's works).[13]


  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 d Gutierrez-Moreno, Adelina et al. (1966), A System of photometric standards 1, Publicaciones Universidad de Chile, Department de Astronomy, pp. 1–17, Bibcode:1966PDAUC...1....1G 
  3. ^ Famaey, B. et al. (January 2005), "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters", Astronomy and Astrophysics 430 (1): 165–186, arXiv:astro-ph/0409579, Bibcode:2005A&A...430..165F, doi:10.1051/0004-6361:20041272 
  4. ^ Carney, Bruce W. et al. (March 2008), "Rotation and Macroturbulence in Metal-Poor Field Red Giant and Red Horizontal Branch Stars", The Astronomical Journal 135 (3): 892–906, arXiv:0711.4984, Bibcode:2008AJ....135..892C, doi:10.1088/0004-6256/135/3/892 
  5. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1 . The radius (R*) is given by:
    \begin{align} 2\cdot R_*
 & = \frac{(10^{-3}\cdot 22.7\cdot 4.85)\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\
 & \approx 23.7\cdot R_{\bigodot}
  6. ^ a b Kaler, James B., "UNUKALHAI (Alpha Serpentis)", Stars (University of Illinois), retrieved 2012-01-11 
  7. ^ a b c d e Massarotti, Alessandro et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209 
  8. ^ "UNUKALHAI -- Star in double system", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2012-01-11 
  9. ^ Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics 431: 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039 
  10. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16 
  11. ^ Also spelt Unuk al Hay or Unuk Elhaija.
  12. ^ Kunitzsch, P.; Smart, T. (2006), A Dictionary of Modern Star names: A Short Guide to 254 Star names and Their Derivations (Second Revised ed.), Cambridge, MA: Sky Publishing, p. 31, ISBN 1-931559-44-9 
  13. ^ a b Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York, NY: Dover Publications Inc, p. 243, ISBN 0-486-21079-0, retrieved 2010-12-12 
  14. ^ Jack W. Rhoads - Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars, Jet Propulsion Laboratory, California Institute of Technology; November 15, 1971
  15. ^ (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.
  16. ^ (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.
  17. ^ (Chinese) English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name, Hong Kong Space Museum. Accessed on line November 23, 2010.