Alpha Gruis

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Alpha Gruis
Grus IAU.svg
Red circle.svg
Location of α Gruis (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Grus
Pronunciation /ælˈnɛər/[1]
Right ascension 22h 08m 13.98473s[2]
Declination –46° 57′ 39.5078″[2]
Apparent magnitude (V) +1.74[3]
Spectral type B6 V[4]
U−B color index –0.47[3]
B−V color index –0.13[3]
Radial velocity (Rv)+11.8[5] km/s
Proper motion (μ) RA: +126.69[2] mas/yr
Dec.: −147.47[2] mas/yr
Parallax (π)32.29 ± 0.21 mas[2]
Distance101.0 ± 0.7 ly
(31.0 ± 0.2 pc)
Absolute magnitude (MV)−0.72[6]
Mass4.0[7] M
Radius3.4[8] R
Luminosity520[9] L
Surface gravity (log g)3.76±0.11[10] cgs
Temperature13,920[11] K
Metallicity [Fe/H]−0.13±0.02[12] dex
Rotational velocity (v sin i)215[13] km/s
Age100[14] Myr
Other designations
Al Na'ir, α Gru, CD−47°14063, FK5 829, GJ 848.2, HD 209952, HIP 109268, HR 8425, SAO 230992[15]
Database references

Alpha Gruis is the brightest star in the southern constellation of Grus. It is officially named Alnair;[1] Alpha Gruis is the star's Bayer designation, which is Latinized from α Gruis and abbreviated α Gru. With an magnitude of 1.7, it is one of the brightest stars in the sky and one of the fifty-eight stars selected for celestial navigation. Alpha Gruis is a single, B-type main-sequence star located at a distance of 31 pc.


α Gruis (Latinised to Alpha Gruis) is the star's Bayer designation. (Its first depiction in a celestial atlas was in Johann Bayer's Uranometria of 1603.[16])

It bore the traditional name Alnair or Al Nair (sometimes Al Na'ir in lists of stars used by navigators),[17] from the Arabic al-nayyir "the bright one", itself derived from its Arabic name, al-nayyir min dhanab al-ḥūt (al-janūbiyy), "the bright one from the (southern) fish's tail" (see Aldhanab).[18] Confusingly, Alnair was also given as the proper name for Zeta Centauri in an astronomical ephemerides in the middle of the 20th century.[19] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[20] to catalog and standardize proper names for stars. The WGSN approved the name Alnair for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[21]

Along with Beta Gruis, Delta Gruis, Theta Gruis, Iota Gruis, and Lambda Gruis, Alpha Gruis belonged to Piscis Austrinus in traditional Arabic astronomy.[22]

In Chinese, (), meaning Crane, refers to an asterism consisting of Alpha Gruis, Beta Gruis, Delta2 Gruis, Epsilon Gruis, Zeta Gruis, Eta Gruis, Iota Gruis, Theta Gruis, Mu1 Gruis and Delta Tucanae.[23] Consequently, Alpha Gruis itself is known as 鶴一 (Hè yī, English: First Star of the Crane).[24] The Chinese name gave rise to another English name, Ke.[25]


Alpha Gruis has a stellar classification of B6 V,[4] although some sources give it a classification of B7 IV.[26] The first classification indicates that this is a B-type star on the main sequence of stars that are generating energy through the thermonuclear fusion of hydrogen at the core. However, a luminosity class of 'IV' would suggest that this is a subgiant star; meaning the supply of hydrogen at its core is becoming exhausted and the star has started the process of evolving away from the main sequence. It has no known companions.[26]

The measured angular diameter of this star, after correcting for limb darkening, is 1.02 ± 0.07 mas.[8] At a parallax-measured distance of 101 light-years (31 parsecs) from Earth, this yields a physical size of 3.4 times the radius of the Sun.[27] It is rotating rapidly, with a projected rotational velocity of about 215 km/s providing a lower bound for the rate of azimuthal rotation along the equator.[13] This star has around four times the Sun's mass and is radiating roughly 520 times the luminosity of the Sun.[9]

The effective temperature of Alpha Gruis's outer envelope is 13,920 K,[11] giving it the blue-white hue characteristic of B-type stars.[28] The abundance of elements other than hydrogen and helium, what astronomers term the metallicity, is about 74% of the abundance in the Sun.[12]

Based on the estimated age and motion, it may be a member of the AB Doradus moving group that share a common motion through space. This group has an age of about 70 million years,[29] which is consistent with α Gruis's 100-million-year[14] estimated age (allowing for a margin of error). The space velocity components of this star in the Galactic coordinate system are [U, V, W] = [–7.0 ± 1.1, –25.6 ± 0.7, –15.5 ± 1.4] km/s.[29]


  1. ^ a b 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.
  2. ^ 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.
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  6. ^ Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters. 38 (5): 331. arXiv:1108.4971. Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. S2CID 119257644.
  7. ^ Malagnini, M. L.; Morossi, C. (November 1990). "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars". Astronomy and Astrophysics Supplement Series. 85 (3): 1015–1019. Bibcode:1990A&AS...85.1015M.
  8. ^ a b Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005). "CHARM2: An updated Catalog of High Angular Resolution Measurements". Astronomy and Astrophysics. 431 (2): 773–777. Bibcode:2005A&A...431..773R. doi:10.1051/0004-6361:20042039.
  9. ^ a b McCarthy, K.; White, R. J. (June 2012). "The Sizes of the Nearest Young Stars". The Astronomical Journal. 143 (6): 134. arXiv:1201.6600. Bibcode:2012AJ....143..134M. doi:10.1088/0004-6256/143/6/134. S2CID 118538522.
  10. ^ Fitzpatrick, Edward L.; Massa, Derck (November 1999). "Determining the Physical Properties of the B Stars. I. Methodology and First Results". The Astrophysical Journal. 525 (2): 1011–1023. arXiv:astro-ph/9906257. Bibcode:1999ApJ...525.1011F. doi:10.1086/307944. S2CID 11704765.
  11. ^ a b Zorec, J.; et al. (July 2009). "Fundamental parameters of B supergiants from the BCD system. I. Calibration of the (λ_1, D) parameters into Teff". Astronomy and Astrophysics. 501 (1): 297–320. arXiv:0903.5134. Bibcode:2009A&A...501..297Z. doi:10.1051/0004-6361/200811147. S2CID 14969137.
  12. ^ a b Niemczura, E. (June 2003). "Metallicities of the SPB stars from the IUE ultraviolet spectra". Astronomy and Astrophysics. 404 (2): 689–700. Bibcode:2003A&A...404..689N. doi:10.1051/0004-6361:20030546.. The fractional abundance relative to the Sun is given by:
    10−0.13 = 0.74, or 74%.
  13. ^ a b Dachs, J.; et al. (March 1981). "Photoelectric scanner measurements of Balmer emission line profiles for southern Be stars. II - A survey for variations". Astronomy and Astrophysics Supplement Series. 43: 427–453. Bibcode:1981A&AS...43..427D.
  14. ^ a b Su, K. Y. L.; et al. (December 2006). "Debris Disk Evolution around A Stars". The Astrophysical Journal. 653 (1): 675–689. arXiv:astro-ph/0608563. Bibcode:2006ApJ...653..675S. doi:10.1086/508649. S2CID 14116473.
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  19. ^ Kunitzsch, P. (1959). Arabische Sternnamen in Europa. Wiesbaden: Otto Harrassowitz. p. 128.
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