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W Sagittarii

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W Sagittarii
Location of W Sgr (circled)
Observation data
Epoch J2000      Equinox ICRS
Constellation Sagittarius
Right ascension 18h 05m 01.22643s[1]
Declination −29° 34′ 48.3222″[1]
Apparent magnitude (V) 4.29 - 5.14[2]
Characteristics
Aa1
Spectral type F4 - G2Ib[2]
U−B color index +0.52[3]
B−V color index +0.78[3]
Variable type δ Cep[2]
Aa2
Spectral type A5V - F5V[4]
Ab
Spectral type A0 V[5]
Astrometry
Radial velocity (Rv)−28.04[6] km/s
Proper motion (μ) RA: +4.372[1] mas/yr
Dec.: −5.588[1] mas/yr
Parallax (π)2.365±0.1765 mas[1]
Distance1,400 ± 100 ly
(420 ± 30 pc)
Absolute magnitude (MV)−3.76[5]
Orbit[4]
PrimaryAa1
NameAa2
Period (P)4.33±0.01 yr
Semi-major axis (a)12.9 ± 0.3"
(5.67 ± 0.13 AU)
Eccentricity (e)0.41 ± 0.02
Inclination (i)7.0 ± 0.8°
Longitude of the node (Ω)68.4 ± 4.0°
Periastron epoch (T)2004.16 ± 0.01
Argument of periastron (ω)
(secondary)		328.0 ± 1.3°
Orbit[5]
PrimaryAa
NameAb
Period (P)172.9 yr
Semi-major axis (a)63 AU
Details
Aa
Mass5.8[5] M☉
Radius61.0[7] R☉
Luminosity2,690[5] L☉
Surface gravity (log g)1.50 - 2.15[8] cgs
Temperature5,380 - 6,474[8] K
Metallicity [Fe/H]+0.02[9] dex
Aa2
Mass1.4 - 2.0[4] M☉
Ab
Mass2.2[5] M☉
Other designations
Dhanusaragra, γ1 Sgr, W Sgr, CD−29 14447, HD 164975, HIP 88567, HR 6742, SAO 186237, ADS 11029, CCDM J18050-2935A
Database references
SIMBADdata

W Sagittarii, also named Dhanusaragra,[10] is a multiple star system star in the constellation Sagittarius, and a Cepheid variable star.

W Sagittarii is an optical line-of-sight companion nearly a degree from the much brighter γ2 Sgr (Alnasl) which marks the nozzle or spout of the teapot asterism.

Nomenclature

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W Sagittarii (abbreviated W Sgr) is this star's variable star designation; it also has the Bayer designation Gamma1 Sagittarii (γ1 Sagittarii, γ1 Sgr).

Dhanuśarāgra (धनुशराग्र), the Arrow, is an Indian star name used since the 14th century for the pair γ1 and γ2 Sgr, the latter having the Arabic name Alnasl.[11] The IAU Working Group on Star Names adopted the name Dhanusaragra for γ1 Sgr Aa1 on 13 August 2026.[10]

System

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W Sgr is listed as component A of a multiple star system catalogued as ADS 11029 and WDS J18050-2935. Components B and C are at 33" and 46" respectively and both are 13th magnitude. They are purely optical companions, not physically associated with W Sgr.[12]

Component A, W Sgr, is itself a triple star system, with the components referred to as W Sgr Aa1, Aa2, and Ab.[13] These have also been referred to as components Aa, Ab, and B respectively.[12] The outer companion Ab has been resolved at a separation of 0.14" and is over 5 magnitudes fainter than the primary supergiant. The inner components can only be identified spectroscopically by their radial velocity variations. The primary is a 6 M☉ yellow supergiant, while the secondary is an early F main sequence star with a mass less than 1.4 M☉.[12]

Variability

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A light curve for W Sagittarii, plotted from TESS data[14]

In early June of 1866, Johann Friedrich Julius Schmidt discovered that the star is a variable star.[15] The supergiant component W Sgr Aa1 pulsates regularly between magnitudes 4.3 and 5.1 every 7.59 days. During the pulsations, that temperature and spectral type also vary. It is classified as a Classical Cepheid (δ Cephei) variable.[5]

References

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  1. 1 2 3 4 5 Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
  2. 1 2 3 Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  3. 1 2 Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D.
  4. 1 2 3 Benedict, G. Fritz; McArthur, Barbara E.; Feast, Michael W.; Barnes, Thomas G.; Harrison, Thomas E.; Patterson, Richard J.; Menzies, John W.; Bean, Jacob L.; Freedman, Wendy L. (2007). "Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations". The Astronomical Journal. 133 (4): 1810. arXiv:astro-ph/0612465. Bibcode:2007AJ....133.1810B. doi:10.1086/511980. S2CID 16384267.
  5. 1 2 3 4 5 6 7 Evans, Nancy Remage; Bond, Howard E.; Schaefer, Gail H.; Mason, Brian D.; Karovska, Margarita; Tingle, Evan (2013). "Binary Cepheids: Separations and Mass Ratios in 5 M ⊙ Binaries". The Astronomical Journal. 146 (4): 93. arXiv:1307.7123. Bibcode:2013AJ....146...93E. doi:10.1088/0004-6256/146/4/93. S2CID 34133110.
  6. ↑ Pourbaix, D.; Tokovinin, A. A.; Batten, A. H.; Fekel, F. C.; Hartkopf, W. I.; Levato, H.; Morrell, N. I.; Torres, G.; Udry, S. (2004). "SB9: The ninth catalogue of spectroscopic binary orbits". Astronomy and Astrophysics. 424 (2): 727–732. arXiv:astro-ph/0406573. Bibcode:2004A&A...424..727P. doi:10.1051/0004-6361:20041213. S2CID 119387088.
  7. ↑ Groenewegen, M. A. T. (2007). "The projection factor, period-radius relation, and surface-brightness colour relation in classical cepheids". Astronomy and Astrophysics. 474 (3): 975–981. Bibcode:2007A&A...474..975G. doi:10.1051/0004-6361:20078225.
  8. 1 2 Luck, R. E.; Andrievsky, S. M. (2004). "Phase-dependent Variation of the Fundamental Parameters of Cepheids. I. Periods from 6 to 10 Days". The Astronomical Journal. 128 (1): 343–356. Bibcode:2004AJ....128..343L. doi:10.1086/420991.
  9. ↑ Marsakov, V. A.; Koval', V. V.; Kovtyukh, V. V.; Mishenina, T. V. (2013). "Properties of the population of classical Cepheids in the Galaxy". Astronomy Letters. 39 (12): 851. Bibcode:2013AstL...39..851M. doi:10.1134/S1063773713120050. S2CID 119788977.
  10. 1 2 "IAU Catalog of Star Names". Retrieved 16 August 2026.
  11. ↑ Shylaja; Hoffmann, Susanne M. "Dhanuśarāgra (धनुशराग्र)". All Skies Encyclopaedia. IAU Working Group on Star Names. Retrieved 16 August 2026.
  12. 1 2 3 Evans, Nancy Remage; Massa, Derck; Proffitt, Charles (2009). "Massive Star Multiplicity: The Cepheid W Sgr". The Astronomical Journal. 137 (3): 3700. arXiv:0902.3281. Bibcode:2009AJ....137.3700E. doi:10.1088/0004-6256/137/3/3700. S2CID 3112765.
  13. ↑ Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466–3471. Bibcode:2001AJ....122.3466M. doi:10.1086/323920.
  14. ↑ "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.
  15. ↑ Schönfeld, E. "Bemerkungen und Nachtrage zum Catalog von veranderlichen Sternen (in German)" (PDF). zobodat. Oberösterreichische Landesmuseen. Retrieved 20 September 2025.
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