# Alpha Gruis

Observation data Characteristics Epoch J2000      Equinox J2000 Location of α Gruis (circled) Constellation Grus Right ascension 22h 08m 13.98473s[1] Declination –46° 57′ 39.5078″[1] Apparent magnitude (V) +1.74[2] Spectral type B6 V[3] U−B color index –0.47[2] B−V color index –0.13[2] Radial velocity (Rv) +11.8[4] km/s Proper motion (μ) RA: +126.69[1] mas/yr Dec.: −147.47[1] mas/yr Parallax (π) 32.29 ± 0.21[1] mas Distance 101.0 ± 0.7 ly (31.0 ± 0.2 pc) Mass 4.0[5] M☉ Radius 3.4[6] R☉ Luminosity 263[5] L☉ Surface gravity (log g) 3.76 ± 0.11[7] cgs Temperature 13,920[8] K Metallicity [Fe/H] –0.13 ± 0.02[9] dex Rotational velocity (v sin i) 215[10] km/s Age 0.10[11] Gyr Al Na'ir, CD−47° 14063, FK5 829, GCTP 5339.00, Gl 848.2, HD 209952, HIP 109268, HR 8425, SAO 230992.[12]

Alpha Gruis (α Gruis, α Gru) or Alnair is the brightest star in the southern constellation Grus. Its first depiction in a celestial atlas was in Johann Bayer's Uranometria of 1603.[13]

## Properties

Alpha Gruis has a stellar classification of B6 V,[3] although some sources give it a classification of B7 IV.[14] 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.[14]

The measured angular diameter of this star, after correcting for limb darkening, is 1.02 ± 0.07 mas.[6] 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.[15] 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.[10] This star has around four times the Sun's mass and is radiating 263 times the luminosity of the Sun.[5]

The effective temperature of Alpha Gruis's outer envelope is 13,920 K,[8] giving it the blue-white hue characteristic of B-type stars.[16] The abundance of elements other than hydrogen and helium, what astronomers term the metallicity, is about 74% of the abundance in the Sun.[9] The Spitzer Space Telescope detected an excess of infrared emission from this star, suggesting that a circumstellar disk of dust may be in orbit around it.[11]

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,[17] which is consistent with α Gruis's 100-million-year[11] 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.[17]

## Name

Alpha Gruis has the proper name Alnair or Al Nair (sometimes Al Na'ir), from the Arabic al-nayyir [an-nai:r], meaning "the bright one".[18] It is derived from its Arabic name, al-Nayyir min Dhanab al-ḥūt (al-Janūbiyy), "the Bright (star) belongs to the Tail of (the constellation of) the (Southern) Fish".[18] Confusingly, "Alnair" is also given as the proper name for Zeta Centauri in an astronomical ephemerides in the middle of the 20th century.[19] With β, δ, θ. ι, and λ Gru, Alnair belonged to Piscis Austrinus in traditional Arabic astronomy.[20]

In Chinese, (), meaning Crane, refers to an asterism consisting of α Gruis, β Gruis, ε Gruis, η Gruis, δ Tucanae, ζ Gruis, ι Gruis, θ Gruis, δ2 Gruis and μ1 Gruis.[21] Consequently, α Gruis itself is known as 鶴一 (Hè yī, English: First Star of the Crane).[22] The Chinese name gave rise to another English name, Ke.[23]

## References

1. 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 Johnson, H. L. et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory 4 (99), Bibcode:1966CoLPL...4...99J
3. ^ a b Gray, R. O. et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637
4. ^ Wilson, Ralph Elmer (1953), General catalogue of stellar radial velocities, Carnegie Institution of Washington, Bibcode:1953QB901.W495.....
5. ^ a b c 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
6. ^ a b Richichi; 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
7. ^ 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
8. ^ 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, Bibcode:2009A&A...501..297Z, doi:10.1051/0004-6361/200811147
9. ^ a b Niemczura, E. (June 2003), "Metallicities of the SPB stars from the IUE ultraviolet spectra", Astronomy and Astrophysics 404: 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%.
10. ^ 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
11. ^ a b c 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
12. ^ "LTT 8869 -- High proper-motion Star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2011-12-24
13. ^ Scalzi, John (2008), Rough Guide to the Universe, Penguin, p. 306, ISBN 1-4053-8370-4
14. ^ a b Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x
15. ^ Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library 1 (3 ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R*) is given by:
\begin{align} 2\cdot R_* & = \frac{(10^{-3}\cdot 31\cdot 1.02)\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\ & \approx 6.8\cdot R_{\bigodot} \end{align}
16. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
17. ^ a b Zuckerman, B. et al. (May 2011), "The Tucana/Horologium, Columba, AB Doradus, and Argus Associations: New Members and Dusty Debris Disks", The Astrophysical Journal 732 (2): 61, arXiv:1104.0284, Bibcode:2011ApJ...732...61Z, doi:10.1088/0004-637X/732/2/61
18. ^ a b Kunitzsch, P.; Smart, T. (2006), A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd ed.), Cambridge, MA: Sky Publishing, p. 39, ISBN 978-1-931559-44-7
19. ^ Kunitzsch,, P. (1959), Arabische Sternnamen in Europa, Wiesbaden: Otto Harrassowitz, p. 128
20. ^ Allen, Richard H. (1963). Star Names: Their Lore and Meaning (reprint ed.). New York, NY: Dover Publications Inc. p. 237. ISBN 0-486-21079-0.
21. ^ (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.
22. ^ (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.
23. ^ Richard Hinckley Allen: Star Names — Their Lore and Meaning: Grus