Upsilon Sagittarii

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Upsilon Sagittarii
Sagittarius constellation map.svg
Red circle.svg
Location of υ Sgr (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 19h 21m 43.62284s[1]
Declination −15° 57′ 18.0625″[1]
Apparent magnitude (V) 4.61[2]
Spectral type A2 Ia + B2 Vpe[3]
U−B color index −0.53[2]
B−V color index +0.10[2]
Variable type PV Tel[4] (β Lyr?[5])
Radial velocity (Rv)8.9[6] km/s
Proper motion (μ) RA: 1.34[1] mas/yr
Dec.: −6.25[1] mas/yr
Parallax (π)1.83 ± 0.23[1] mas
Distanceapprox. 1,800 ly
(approx. 550 pc)
Absolute magnitude (MV)−4.13[7]
Primaryυ Sgr1
Companionυ Sgr2
Period (P)137.9 days
Semi-major axis (a)270.8 D
Eccentricity (e)0
Inclination (i)50[9]°
Semi-amplitude (K1)
49.6[10] km/s
Semi-amplitude (K2)
29.7[10] km/s
Mass2.5 + 4[11] M
Radius50[12] R
Diameter50[12] D
Luminosity39,000[9] L
Surface gravity (log g)2.5[3] cgs
Temperature12,300[3] K
Metallicity [Fe/H]−0.2[3] dex
Age52[11] Myr
Other designations
υ Sagittarii, υ Sgr, Upsilon Sgr, 46 Sagittarii, BD−16°5283, FK5 727, GC 26697, HD 181615, HD 181616, HIP 95176, HR 7342, PPM 235885, SAO 162518
Database references

Upsilon Sagittarii (Upsilon Sgr, υ Sagittarii, υ Sgr) is a spectroscopic binary star system in the constellation Sagittarius. Upsilon Sagittarii is the prototypical hydrogen-deficient binary (HdB), and one of only four such systems known. The unusual spectrum of hydrogen-deficient binaries has made stellar classification of Upsilon Sagittarii difficult.


υ Sgr is a binary system with an orbital period of 137.939 days and is approximately 1,672 light years from Earth. The primary star dominates the visible radiation and spectrum, but the secondary is hotter and more massive. There is also a disc of material being stripped from the primary and transferring material to the secondary, but no eclipses[9]

The system is classified as a single-lined spectroscopic binary, but high excitation lines from the secondary can be detected in the ultraviolet.[10] Radial velocity variations were discovered in 1899,[13] The first orbit was calculated in 1914, reasonably close to modern understanding of the system.[14]


The primary component, υ Sagittarii1, appears as an A type supergiant, although published spectral types vary from F2p to B5II.[8] Contrasting components in the spectrum may originate from disc material, polar jets, or the star itself. The low mass and unusual chemical composition are also thought to produce misleading spectral calibrations, with the star not as massive or as luminous as the Ia luminosity class would suggest.[3]

υ Sgr1 is a helium star, almost entirely deficient of hydrogen.[15] It has also been described as a neon star, due to the very high relative levels of that element.[16] It has been stripped of its outer hydrogen layers after it expanded away from the main sequence.[15] It is thought to have originated as a main sequence star with around 8 M, expanded when it exhausted its core hydrogen, and now only 2.5 M remains, highly inflated and giving the appearance of a supergiant star.[11] Other estimates give higher masses, as much as 5.45 M and 8.56 M at the known inclination of 50°.[8]

υ Sgr1 is also classified as an PV Telescopii variable, although it was originally catalogued as an eclipsing binary. It shows apparent magnitude fluctuations between +4.51 and +4.65 with a period of approximately 20 days.[5][17]

The companion, υ Sgr2, is more massive than the supergiant primary, but so dim at visible frequencies as to be undetectable. It is thought to be a B-type main sequence star accreting mass from the primary.[8]


υ Sagittarii has two entries in the Henry Draper Catalogue, HD 181615 and HD 181616.

In Chinese, (Jiàn), meaning Establishment, refers to an asterism consisting of υ Sagittarii, ξ² Sagittarii, ο Sagittarii, π Sagittarii, 43 Sagittarii and ρ¹ Sagittarii . Consequently, υ Sagittarii itself is known as 建六 (Jiàn liù, English: the Sixth Star of Establishment.)[18]


  1. 1.0 1.1 1.2 1.3 1.4 Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653. arXiv:0708.1752. Bibcode 2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. 2.0 2.1 2.2 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.
  3. 3.0 3.1 3.2 3.3 3.4 Kipper, Tõnu; Klochkova, Valentina G. (2012). "High-Resolution Spectroscopy of the Hydrogen-Deficient Binary Upsilon Sgr". Baltic Astronomy 21: 219. Bibcode 2012BaltA..21..219K. doi:10.1515/astro-2017-0382.
  4. Malcolm, G. J.; Bell, S. A. (1986). "Evidence for pulsation in the hydrogen-deficient binary Upsilon Sagittarii". Monthly Notices of the Royal Astronomical Society 222 (3): 543. Bibcode 1986MNRAS.222..543M. doi:10.1093/mnras/222.3.543.
  5. 5.0 5.1 Samus, N. N.Expression error: Unrecognized word "etal". (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.
  6. Wilson, Ralph Elmer (1953). "General catalogue of stellar radial velocities". Washington. Bibcode 1953GCRV..C......0W.
  7. 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.
  8. 8.0 8.1 8.2 8.3 Koubský, P.; Harmanec, P.; Yang, S.; Netolický, M.; Škoda, P.; Šlechta, M.; Korčáková, D. (2006). "Properties and nature of Be stars. 25. A new orbital solution and the nature of a peculiar emission-line binary υ Sagittarii". Astronomy and Astrophysics 459 (3): 849. Bibcode 2006A&A...459..849K. doi:10.1051/0004-6361:20065274.
  9. 9.0 9.1 9.2 Netolický, M.; Bonneau, D.; Chesneau, O.; Harmanec, P.; Koubský, P.; Mourard, D.; Stee, P. (2009). "The circumbinary dusty disk around the hydrogen-deficient binary star υ Sagittarii". Astronomy and Astrophysics 499 (3): 827. Bibcode 2009A&A...499..827N. doi:10.1051/0004-6361/200811192.
  10. 10.0 10.1 10.2 Dudley, R. E.; Jeffery, C. S. (1990). "The Mass Ratio of Upsilon-Sagittarii from Ultraviolet Radial Velocities". Monthly Notices of the Royal Astronomical Society 247: 400. Bibcode 1990MNRAS.247..400D.
  11. 11.0 11.1 11.2 Leushin, V. V. (2001). "Atmospheric Iron Abundance in the Primary Component of upsilon Sgr". Astronomy Letters 27 (10): 634. Bibcode 2001AstL...27..634L. doi:10.1134/1.1404457.
  12. Bonneau, D.; Chesneau, O.; Mourard, D.; Bério, Ph.; Clausse, J. M.; Delaa, O.; Marcotto, A.; Perraut, K. et al. (2011). "A large Hα line forming region for the massive interacting binaries β Lyrae and υ Sagitarii". Astronomy & Astrophysics 532: A148. Bibcode 2011A&A...532A.148B. doi:10.1051/0004-6361/201116742.
  13. Campbell, W. W. (1895). "Stars whose spectra contain both bright and dark hydrogen lines". Astrophysical Journal 2: 177. Bibcode 1895ApJ.....2..177C. doi:10.1086/140127.
  14. Wilson, R. E. (1915). "The orbit of the spectroscopic binary upsilon Sagittarii". Lick Observatory Bulletin 8: 132. Bibcode 1915LicOB...8..132W. doi:10.5479/ADS/bib/1915LicOB.8.132W.
  15. 15.0 15.1 Leushin, V. V.; Snezhko, L. I.; Chuvenkov, V. V. (1998). "History of the chemical evolution of the primary component of the binary system υ Sagittarius". Astronomy Letters 24: 39. Bibcode 1998AstL...24...39L.
  16. Leushin, V. V. (2000). "The primary component of υ Sagittarius is a neon star". Bulletin of the Special Astrophysical Observatory. Russian Academy of Sciences 50: 60. Bibcode 2000BSAO...50...60L.
  17. Jeffery, C. Simon (2008). "Variable Star Designations for Extreme Helium Stars". Information Bulletin on Variable Stars 5817: 1. Bibcode 2008IBVS.5817....1J.
  18. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日

Further reading

  • Dudley, R. E., Jeffery, C. S., 1990. Mon. Not. R. astr. Soc. 247, 400