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Alpha Gruis

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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 1.74, 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.

Alpha Gruis
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]
Characteristics
Spectral type B6 V[4]
U−B color index −0.47[3]
B−V color index −0.13[3]
Astrometry
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)−0.721±0.031[6]
Details
Mass3.82[7] M☉
Radius3.91[8][a] R☉
Luminosity520[8] L☉
Surface gravity (log g)3.76±0.11[9] cgs
Temperature14,245±484[7] K
Metallicity [Fe/H]−0.13±0.02[10] dex
Rotational velocity (v sin i)215[11][7] km/s
Age100[12] Myr
Other designations
Al Na'ir, α Gru, CD−47°14063, FK5 829, GJ 848.2, HD 209952, HIP 109268, HR 8425, SAO 230992[13]
Database references
SIMBADdata

Nomenclature

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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.[14])

It bore the traditional name Alnair or Al Nair (sometimes Al Na'ir in lists of stars used by navigators),[15] 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).[16] Confusingly, Alnair was also given as the proper name for Zeta Centauri in an astronomical ephemerides in the middle of the 20th century.[17] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[18] 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.[19]

Along with Beta Gruis, Delta Gruis, Theta Gruis, Iota Gruis, and Lambda Gruis, Alpha Gruis belonged to Piscis Austrinus in traditional Arabic astronomy.[20]

In Chinese, 鶴 (Hè), 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.[21] Consequently, Alpha 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]

Properties

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Alpha Gruis has a stellar classification of B6 V,[4] although some sources give it a classification of B7 IV.[24] 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.[24]

The measured angular diameter of this star, after correcting for limb darkening, is 1.17 mas.[8] At the Alnair's distance from Earth of 101 light-years (31 parsecs) from Earth, this yields a physical size of 3.9 times the radius of the Sun.[a] 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.[11] This star has around four times the Sun's mass[7] and is radiating roughly 520 times the luminosity of the Sun.[8]

The effective temperature of Alnair's outer envelope is 14,245 K,[7] giving it the blue-white hue characteristic of B-type stars.[25] The abundance of elements other than hydrogen and helium, what astronomers term the metallicity, is about 74% of the abundance in the Sun.[10]

Based on the estimated age and motion, it is a member of the AB Doradus moving group that share a common motion through space.[6] This group has an age of about 70 million years,[26] which is consistent with α Gruis's 100-million-year[12] 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.[26]

Notes

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  1. ^ a b 0.00117 arcseconds*31.1 pc = 0.036387 AU (diameter). Should be multiplied by 107.5 to convert from AU to R☉.

References

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  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.
  3. ^ a b c Hoffleit, D.; Warren, W. H. Jr. "HR 8425, database entry". The Bright Star Catalogue (5th Revised (Preliminary) ed.). CDS. Retrieved 5 September 2015.
  4. ^ 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. S2CID 119476992.
  5. ^ Wilson, Ralph Elmer (1953). "General catalogue of stellar radial velocities". Washington: 0. Bibcode:1953GCRV..C......0W.
  6. ^ a b Bell, Cameron P. M.; Mamajek, Eric E.; Naylor, Tim (2015). "A self-consistent, absolute isochronal age scale for young moving groups in the solar neighbourhood". Monthly Notices of the Royal Astronomical Society. 454 (1): 593. arXiv:1508.05955. Bibcode:2015MNRAS.454..593B. doi:10.1093/mnras/stv1981.
  7. ^ a b c d e David, Trevor J.; Hillenbrand, Lynne A. (2015). "The Ages of Early-type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets". The Astrophysical Journal. 804 (2): 146. arXiv:1501.03154. Bibcode:2015ApJ...804..146D. doi:10.1088/0004-637X/804/2/146.
  8. ^ a b c d 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.
  9. ^ 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.
  10. ^ 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%.
  11. ^ 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.
  12. ^ 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.
  13. ^ "LTT 8869 -- High proper-motion Star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2011-12-24.
  14. ^ Scalzi, John (2008). Rough Guide to the Universe. Penguin. p. 306. ISBN 978-1-4053-8370-7.
  15. ^ Bowditch, LL.D., Nathaniel (2002) [1802]. "15: Navigational Astronomy" (PDF). The American Practical Navigator: An Epitome of Navigation. Bethesda, MD: National Imagery and Mapping Agency. p. 248. ISBN 978-0-939837-54-0. Retrieved 2016-09-20.
  16. ^ 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.
  17. ^ Kunitzsch, P. (1959). Arabische Sternnamen in Europa. Wiesbaden: Otto Harrassowitz. p. 128.
  18. ^ "IAU Working Group on Star Names (WGSN)". International Astronomical Union. Retrieved 22 May 2016.
  19. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
  20. ^ Allen, Richard H. (1963). Star Names: Their Lore and Meaning (reprint ed.). New York, NY: Dover Publications Inc. p. 237. ISBN 978-0-486-21079-7.
  21. ^ 陳久金 (2005). 中國星座神話 (in Chinese). 五南圖書出版股份有限公司. ISBN 978-986-7332-25-7.
  22. ^ "香港太空館 - 研究資源 - 亮星中英對照表" (in Chinese). Hong Kong Space Museum. Archived from the original on 2008-10-25. Retrieved 2019-01-15.
  23. ^ Richard Hinckley Allen. "Star Names — Their Lore and Meaning: Grus". Retrieved 2019-01-15.
  24. ^ 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. S2CID 14878976.
  25. ^ "The Colour of Stars". Australia Telescope, Outreach and Education. Commonwealth Scientific and Industrial Research Organisation. December 21, 2004. Archived from the original on March 18, 2012. Retrieved 2012-01-16.
  26. ^ 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. S2CID 62797470.
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