Gamma2 Sagittarii

For other stars with this Bayer designation, see γ Sagittarii.

γ^2 Sagittarii

Location of γ^2 Sagittarii (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Sagittarius
Right ascension	18^h 05^m 48.484^s[1]
Declination	30° 25′ 26.50″[1]
Apparent magnitude (V)	+2.96[2]
Characteristics
Evolutionary stage	K-type giant
Spectral type	K0III[2]
U−B color index	+0.77[3]
B−V color index	+1.006[2]
Astrometry
Radial velocity (Rv)	22.0[4] km/s
Proper motion (μ)	RA: −48.839 mas/yr[1] Dec.: −204.86 mas/yr[1]
Parallax (π)	30.6791 ± 0.4451 mas[2]
Distance	106.2+1.6 −1.5 ly (32.58+0.48 −0.47 pc)[2]
Absolute magnitude (MV)	+0.395[2]
Details[2]
Mass	1.98±0.22 M☉
Radius	12.07±0.27 R☉
Luminosity	73.45±2.42 L☉
Surface gravity (log g)	2.712±0.092 cgs
Temperature	4,864±36 K
Metallicity [Fe/H]	−0.153±0.028 dex
Other designations
Alnasl, Nushaba,[5] Awal al Warida, γ Sagittarii, γ Sgr, Gamma^2 Sgr, 10 Sagittarii, CPD−30 5241, FK5 679, GC 24632, HD 165135, HIP 88635, HR 6746, PPM 297231, SAO 209696[6]
Database references
SIMBAD	data

Gamma² Sagittarii (γ² Sagittarii, abbreviated Gamma² Sgr, γ² Sgr), formally named Alnasl /ælˈnæzəl/,^[7] is a 3rd-magnitude star in the zodiac constellation of Sagittarius. The location of this star is in the handle of the Bow of Sagittarius the Centaur. It is approximately 32.6 parsecs (106 light-years) from the Sun and has an apparent visual magnitude of +2.96,^[2] making it the seventh-brightest star in the constellation.

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.^[8][9]

Nomenclature

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

It bore the traditional names Alnasl (alternatively Nasl, El Nasl, "al 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".^[10][11] 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.^[12][11]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[13] 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.^[7]

Gamma² Sagittarii, together with Delta Sagittarii, Epsilon Sagittarii, Zeta Sagittarii, Lambda Sagittarii, Sigma Sagittarii, Tau Sagittarii and Phi Sagittarii, comprise the Teapot asterism.^[14]

Together with Delta Sagittarii and Epsilon Sagittarii, Gamma² Sagittarii formed the Akkadian Sin-nun‑tu, or Si-nu-nu‑tum 'the Swallow'.^[11]

In Chinese, 箕 (Jī), meaning Winnowing Basket, refers to an asterism consisting of Gamma Sagittarii, Delta Sagittarii, Epsilon Sagittarii and Eta Sagittarii. Consequently, Gamma² Sagittarii itself is known as 箕宿一 (Jī Sù yī, English: the First Star of Winnowing Basket.)^[15]

Properties

A stellar classification of K0 III reveals that this is a giant star, having expanded to an estimated 12 times the Sun's radius.^[2] This means it has exhausted the hydrogen in its core and evolved away from the main sequence. Gamma² Sagittarii is two times more massive than the Sun and is emitting 73 times more luminosity.^[2] The abundance of elements other than hydrogen and helium in this star, what astronomers term the star's metallicity, is lower than in the Sun.^[2] Gamma² Sagittarii has an effective temperature of 4,864 K,^[2] compared to 5,772 K for the Sun. It is this lower temperature that gives Gamma² Sagittarii the orange hue that is a characteristic of K-type stars.

References

1. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
2. Ottoni, G.; Udry, S.; Ségransan, D.; Buldgen, G.; Lovis, C.; Eggenberger, P.; Pezzotti, C.; Adibekyan, V.; Marmier, M.; Mayor, M.; Santos, N. C.; Sousa, S. G.; Lagarde, N.; Charbonnel, C. (2022-01-01), "CORALIE radial-velocity search for companions around evolved stars (CASCADES). I. Sample definition and first results: Three new planets orbiting giant stars", Astronomy and Astrophysics, 657: A87, arXiv:2201.01528, Bibcode:2022A&A...657A..87O, doi:10.1051/0004-6361/202040078, ISSN 0004-6361 Alnasl's database entry at VizieR.
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. Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, 35 (35), Astronomisches Rechen-Institut Heidelberg: 1, Bibcode:1999VeARI..35....1W
5. Olcott, William Tyler (1907), A field book of the stars (2nd ed.), G. P. Putnam's sons, p. 136, Bibcode:1907fbs..book.....O
6. "gam Sgr -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-05
7. Naming Stars, IAU.org, retrieved 16 December 2017
8. Kaler, James B., "ALNASL (Gamma-2 and Gamma-1=W Sagittarii)", Stars, University of Illinois, retrieved 2012-01-05
9. Burnham, Robert (1978), Burnham's celestial handbook: an observer's guide to the universe beyond the solar system, Dover books explaining science, vol. 3 (2nd ed.), Courier Dover Publications, p. 1560, ISBN 0-486-23673-0
10. Ridpath, Ian (1989), Star tales, James Clarke & Co., p. 113, ISBN 0-7188-2695-7
11. 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
12. 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
13. IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016
14. Teapot, constellation-guide.com, retrieved 2017-05-13
15. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日 Archived 2011-05-22 at the Wayback Machine