Alpha Serpentis

Coordinates: Sky map 15h 44m 16.07431s, +06° 25′ 32.2633″
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α Serpentis
Serpens Caput IAU.svg
Red circle.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 IIIb CN1[3]
U−B color index +1.248[2]
B−V color index +1.167[2]
Radial velocity (Rv)+2.63[4] km/s
Proper motion (μ) RA: +133.84[1] mas/yr
Dec.: +44.81[1] mas/yr
Parallax (π)44.10 ± 0.19 mas[1]
Distance74.0 ± 0.3 ly
(22.68 ± 0.10 pc)
Absolute magnitude (MV)+0.88±0.03[5]
Mass1.66[6] M
Radius13.48[6] R
Luminosity70[7] L
Surface gravity (log g)2.4[6] cgs
Temperature4,498[8] K
Metallicity [Fe/H]0.189[6] dex
Rotational velocity (v sin i)4.3[8] km/s
Other designations
Unukalhai, Cor Serpentis, Alpha Ser, α Ser, 24 Serpentis, BD+06° 3088, HD 140573, HIP 77070, HR 5854, SAO 121157, ADS 9765, CCDM 15442+0626[9]
Database references

Alpha Serpentis or α Serpentis, formally named Unukalhai (/ˌjuːnək.ælˈh/),[10][11] is a double star in the head (Serpens Caput) of the equatorial constellation of Serpens. 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 the Sun.


Alpha Serpentis is a giant star with a stellar classification of K2IIIbCN1,[3] having consumed the hydrogen at its core and evolved away from the main sequence. It has 1.7 times the mass and 13.5 times the radius of the Sun.[6] The effective temperature of the outer envelope is 4,498 K,[8] giving it an orange hue that is characteristic of a K-type star.[12] It has been classified as a strong CN star, showing a higher than expected strength in the cyanogen bands.[13]

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.[7] 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.


α Serpentis (Latinised to Alpha Serpentis) is the system's Bayer designation.

It bore the traditional names Unukalhai (alternatively spelt Unuk al Hay or Unuk Elhaija) from the Arabic عنق الحيّة ʽunuq al-ḥayyah "the serpent's neck", and Cor Serpentis from the Latin "the Heart of the Serpent". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalogue and standardize proper names for stars. The WGSN approved the name Unukalhai for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[11]

Alpha Serpentis is a member of the indigenous Arabic asterism al-Nasaq al-Yamānī "the Southern Line" of al-Nasaqān "the Two Lines",[15] along with Delta Serpentis, Epsilon Serpentis, Delta Ophiuchi, Epsilon Ophiuchi, Zeta Ophiuchi and Gamma Ophiuchi.[16] According to a 1971 NASA catalogue, al-Nasaq al-Yamānī or Nasak Yamani was the name for two stars: Delta Serpentis as Nasak Yamani I and Epsilon Serpentis as Nasak Yamani II.[17]

In Chinese, 天市右垣 (Tiān Shì Yòu Yuán), meaning Right Wall of Heavenly Market Enclosure, refers to an asterism which represents eleven old states in China and which is marking the right borderline of the enclosure, consisting of Alpha Serpentis, Beta Herculis, Gamma Herculis, Kappa Herculis, Gamma Serpentis, Beta Serpentis, Delta Serpentis, Epsilon Serpentis, Delta Ophiuchi, Epsilon Ophiuchi and Zeta Ophiuchi.[18] Consequently, the Chinese name for Alpha Serpentis itself is 天市右垣七 (Tiān Shì Yòu Yuán qī, English: the Seventh Star of Right Wall of Heavenly Market Enclosure), and represents the state Shu (蜀) (or Shuh)[19][20] (together with Lambda Serpentis in R.H.Allen's works).[16]


  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, S2CID 18759600
  2. ^ a b c d Gutierrez-Moreno, Adelina; et al. (1966), "A System of photometric standards", Publications of the Department of Astronomy University of Chile, Publicaciones Universidad de Chile, Department de Astronomy, 1: 1–17, Bibcode:1966PDAUC...1....1G
  3. ^ a b Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  4. ^ 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, S2CID 17804304
  5. ^ 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, S2CID 2756572
  6. ^ a b c d e Gray, David F.; Kaur, Taranpreet (September 2019). "A Recipe for Finding Stellar Radii, Temperatures, Surface Gravities, Metallicities, and Masses Using Spectral Lines". The Astrophysical Journal. 882 (2): 16. Bibcode:2019ApJ...882..148G. doi:10.3847/1538-4357/ab2fce. S2CID 203078143. 148.
  7. ^ a b Kaler, James B., "UNUKALHAI (Alpha Serpentis)", Stars, University of Illinois, retrieved 2012-01-11
  8. ^ a b c 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, S2CID 121883397
  9. ^ "UNUKALHAI -- Star in double system", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-11
  10. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  11. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016.
  12. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on February 22, 2012, retrieved 2012-01-16
  13. ^ Keenan, Philip C.; et al. (July 1987). "Recognition and classification of strong-CN giants". Publications of the Astronomical Society of the Pacific. 99: 629–636. Bibcode:1987PASP...99..629K. doi:10.1086/132025.
  14. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  15. ^ 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
  16. ^ 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
  17. ^ 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
  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) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2010-08-19 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  20. ^ (in Chinese) English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name Archived 2008-09-24 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.