Epoch J2000 Equinox J2000
|Right ascension||15h 44m 16.07431s|
|Declination||+06° 25′ 32.2633″|
|Apparent magnitude (V)||2.623|
|Spectral type||K2 III|
|U−B color index||+1.248|
|B−V color index||+1.167|
|Radial velocity (Rv)||+2.63 km/s|
|Proper motion (μ)||RA: +133.84 mas/yr
Dec.: +44.81 mas/yr
|Parallax (π)||44.10 ± 0.19 mas|
|Distance||74.0 ± 0.3 ly
(22.68 ± 0.10 pc)
|Absolute magnitude (MV)||+0.88 ± 0.03|
|Surface gravity (log g)||2.5 cgs|
|Metallicity [Fe/H]||0.03 dex|
|Rotational velocity (v sin i)||4.3 km/s|
Alpha Serpentis (α Serpentis, abbreviated Alpha Ser, α Ser), also named Unukalhai, is a double star in the head (Serpens Caput) of the equatorial constellation of Serpens. With an apparent visual magnitude of 2.6, 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 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, which, at its estimated distance, equates to a physical radius of about 12 times the radius of the Sun. The effective temperature of the outer envelope is 4,498 K, giving it an orange hue that is characteristic of a K-type star.
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. 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.
It bore the traditional names Unukalhai (alternatively spelt Unuk al Hay or Unuk Elhaija) from the Arabic عنق الحيّة ‘Unuq al-Ḥayyati "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) 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.
Alpha Serpentis is a member of the indigenous Arabic asterism al-Nasaq al-Yamānī "the Southern Line" of al-Nasaqān "the Two Lines", along with Delta Serpentis, Epsilon Serpentis, Delta Ophiuchi, Epsilon Ophiuchi, Zeta Ophiuchi and Gamma Ophiuchi. 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.
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. Consequently, Alpha 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), and represents the state Shu (蜀) (or Shuh) (together with Lambda Serpentis in R.H.Allen's works).
- van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv: , Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357
- 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
- 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: , Bibcode:2005A&A...430..165F, doi:10.1051/0004-6361:20041272
- 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: , Bibcode:2008AJ....135..892C, doi:10.1088/0004-6256/135/3/892
- 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:
- Kaler, James B., "UNUKALHAI (Alpha Serpentis)", Stars, University of Illinois, retrieved 2012-01-11
- 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
- "UNUKALHAI -- Star in double system", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-11
- "IAU Catalog of Star Names". Retrieved 28 July 2016.
- 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
- "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, retrieved 2012-01-16
- IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
- 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
- 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
- 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
- (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.
- (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.
- (Chinese) English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name, Hong Kong Space Museum. Accessed on line November 23, 2010.