Gamma2 Sagittarii

Gamma² Sagittarii (γ² Sagittarii, abbreviated Gamma² Sgr, γ² Sgr), formally named Alnasl /ælˈnæzəl/,[9] is a 3rd-magnitude star in the zodiac constellation of Sagittarius. The location of this star forms the tip of the arrow in the bow of Sagittarius the Centaur. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 96.9 light-years (29.7 parsecs) from the Sun. It has an apparent visual magnitude of +2.98,[2] making it the seventh-brightest star in the constellation.

For other stars with this Bayer designation, see γ Sagittarii.
γ² Sagittarii
Location of γ2 Sagittarii (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Sagittarius
Right ascension 18h 05m 48.48810s[1]
Declination –30° 25′ 26.7235″[1]
Apparent magnitude (V) +2.98[2]
Characteristics
Spectral type K1 III[2]
U−B color index +0.77[3]
B−V color index +1.01[3]
Astrometry
Radial velocity (Rv)22.0[2] km/s
Proper motion (μ) RA: –53.92[1] mas/yr
Dec.: -180.90[1] mas/yr
Parallax (π)33.67 ± 0.18[1] mas
Distance96.9 ± 0.5 ly
(29.7 ± 0.2 pc)
Absolute magnitude (MV)+0.62[4]
Details
Radius12[5] R☉
Luminosity68[4] L☉
Surface gravity (log g)2.72[6] cgs
Temperature4,760[6] K
Metallicity [Fe/H]–0.36[6] dex
Other designations
Alnasl, Nushaba,[7] Awal al Warida, γ Sagittarii, γ Sgr, Gamma2 Sgr, 10 Sagittarii, CPD-30  5241, FK5 679, GC 24632, HD 165135, HIP 88635, HR 6746, PPM 297231, SAO 209696[8]
Database references
SIMBADdata

It forms part of a double star along with a fainter optical companion designated Gamma¹ Sagittarii located about 50 arcminutes north of this star. The latter is a magnitude 4.7 Cepheid variable star that also has the variable star designation W Sagittarii.[10][11]

Nomenclature

γ² Sagittarii (Latinised to Gamma² Sagittarii) is the star's Bayer designation.

It bore the traditional names Alnasl (alternatively Nasl, El Nasl), Nushaba (Nash) and Awal al Warida. Alnasl is derived from the Arabic النصل al-naşl and Nushaba is derived from the Arabic Zujj al-Nashshaba, both meaning "arrowhead".[12][13] In the catalogue of stars in the calendarium of Al Achsasi al Mouakket, this star was designated Awal al Waridah, meaning "first [star] of the [ostrich] going down to the water", from the Arabic النعامة الواردة Al Naʽāma al Wārida, the name of the asterism consisting of this star, Delta Sagittarii, Epsilon Sagittarii and Eta Sagittarii. This ostrich was thought of as going down to the river (the Milky Way) to drink, and another ostrich (σ, φ, τ, and ζ, al Sadira) was thought of as coming back up.[14][13]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[15] to catalogue and standardize proper names for stars. The WGSN approved the name Alnasl for this star on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[9]

Gamma² Sagittarii, together with Delta Sagittarii, Epsilon Sagittarii, Zeta Sagittarii, Lambda Sagittarii, Sigma Sagittarii, Tau Sagittarii and Phi Sagittarii, comprise the Teapot asterism.[16]

Together with Delta Sagittarii and Epsilon Sagittarii, Gamma Sagittarii formed the Akkadian Sin-nun‑tu, or Si-nu-nu‑tum 'the Swallow'.[13]

In Chinese, 箕 (Jī), meaning Winnowing Basket, refers to an asterism consisting of Gamma Sagittarii, Delta Sagittarii, Epsilon Sagittarii and Eta Sagittarii. Consequently, Gamma Sagittarii itself is known as 箕宿一 (Jī Sù yī, English: the First Star of Winnowing Basket.)[17]

Properties

A stellar classification of K1 III reveals that this is a giant star,[2] having expanded to an estimated 12 times the Sun's radius.[5] This means it has exhausted the hydrogen in its core and evolved away from the main sequence. The abundance of elements other than hydrogen and helium in this star, what astronomers term the star's metallicity,[6] is lower than in the Sun. Gamma² Sagittarii has an effective temperature of 4,760 K,[6] compared to 5,778 K for the Sun.[18] It is this lower temperature that gives Gamma² Sagittarii the orange hue that is a characteristic of K-type stars.

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, S2CID 18759600
  2. Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, Astronomisches Rechen-Institut Heidelberg, 35 (35): 1, Bibcode:1999VeARI..35....1W
  3. Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J
  4. 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.
  5. Pasinetti-Fracassini, L. E.; et al. (February 2001), "Catalogue of Stellar Diameters (CADARS)", Astronomy and Astrophysics, 367 (2): 521–524, arXiv:astro-ph/0012289, Bibcode:2001A&A...367..521P, doi:10.1051/0004-6361:20000451, S2CID 425754
  6. McWilliam, Andrew (December 1990), "High-resolution spectroscopic survey of 671 GK giants. I - Stellar atmosphere parameters and abundances", Astrophysical Journal Supplement Series, 74: 1075–1128, Bibcode:1990ApJS...74.1075M, doi:10.1086/191527
  7. Olcott, William Tyler (1907), A field book of the stars (2nd ed.), G. P. Putnam's sons, p. 136, Bibcode:1907fbs..book.....O
  8. "gam Sgr -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-05
  9. "Naming Stars". IAU.org. Retrieved 16 December 2017.
  10. Kaler, James B., "ALNASL (Gamma-2 and Gamma-1=W Sagittarii)", Stars, University of Illinois, retrieved 2012-01-05
  11. Burnham, Robert (1978), Burnham's celestial handbook: an observer's guide to the universe beyond the solar system, Dover books explaining science, 3 (2nd ed.), Courier Dover Publications, p. 1560, ISBN 0-486-23673-0
  12. Ridpath, Ian (1989), Star tales, James Clarke & Co., p. 113, ISBN 0-7188-2695-7
  13. Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 355. ISBN 0-486-21079-0. Retrieved 2012-09-04.
  14. Knobel, E. B. (June 1895). "On a Catalogue of Stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society. 55 (8): 435. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
  15. IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  16. "Teapot". constellation-guide.com. Retrieved 2017-05-13.
  17. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日
  18. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-10, retrieved 2012-01-16
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