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).
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