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.
  | |
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Sagittarius |
| Right ascension | 18h 05m 48.484s[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 | |
| Database references | |
| SIMBAD | data |
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
editγ2 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
editA 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
edit- ^ a b c 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.
- ^ a b c d e f g h i j k l 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.
- ^ 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
- ^ 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
- ^ Olcott, William Tyler (1907), A field book of the stars (2nd ed.), G. P. Putnam's sons, p. 136, Bibcode:1907fbs..book.....O
- ^ "gam Sgr -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-05
- ^ a b Naming Stars, IAU.org, retrieved 16 December 2017
- ^ Kaler, James B., "ALNASL (Gamma-2 and Gamma-1=W Sagittarii)", Stars, University of Illinois, retrieved 2012-01-05
- ^ 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
- ^ Ridpath, Ian (1989), Star tales, James Clarke & Co., p. 113, ISBN 0-7188-2695-7
- ^ a b c 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
- ^ 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
- ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016
- ^ Teapot, constellation-guide.com, retrieved 2017-05-13
- ^ (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