9 Sagittarii (9 Sgr) is a massive binary star in the constellation Sagittarius. It has an apparent magnitude of 5.97.

9 Sagittarii
Sagittarius constellation map.svg
Red circle.svg
Location of 9 Sagittarius (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension  18h 03m 52.44466s[1]
Declination −24° 21′ 38.6269″[1]
Apparent magnitude (V) 5.97[2]
Spectral type O4V((f))z[3] (O3.5V((f+)) + O5–5.5V((f))[4])
U−B color index −0.89[2]
B−V color index 0.00[2]
Radial velocity (Rv)10.70[5] km/s
Proper motion (μ) RA: 0.19[1] mas/yr
Dec.: −1.49[1] mas/yr
Parallax (π)0.49 ± 0.40[1] mas
Distance1,790[6] pc
Absolute magnitude (MV)−6.20[4]
Period (P)3,327[6]
(9.1[6] yr)
Semi-major axis (a)4,100 R[7]
Eccentricity (e)0.702 ± 0.010[6]
Inclination (i)45[6]°
Semi-amplitude (K1)
25.7 ± 0.6[4] km/s
Semi-amplitude (K2)
38.8 ± 0.9[4] km/s
9 Sgr A
Mass55 M
Radius13.1 R
Luminosity562,000 L
Surface gravity (log g)3.92 cgs
Temperature43,850 K
Rotational velocity (v sin i)87 km/s
9 Sgr B
Mass36 M
Radius11.2 R
Luminosity316,000 L
Surface gravity (log g)3.92 cgs
Temperature40,850 K
Rotational velocity (v sin i)57 km/s
Other designations
9 Sgr, HR 6736, HIP 88469, HD 164794, CD−24°13814, 2MASS J18035245-2421386, WDS J18039-2422
Database references


9 Sgr is the brightest star in the image, just left of the intense Hourglass Nebula core of the Lagoon Nebula. NGC 6530 is the scattering of stars on the left.

9 Sgr is a naked eye star lying in the direction of the Lagoon Nebula (Messier 8), very close to the central condensation of the Hourglass Nebula around Herschel 36. It was not catalogued by Bayer with a Greek letter, but was listed by Flamsteed as number 9 in the constellation Sagittarius. It lies close to the open cluster NGC 6530. 9 Sgr is the main source of ionisation for much of the visible nebulosity in the region, although the young O star Herschel 36 ionises the dense Hourglass Nebula region.[8]

9 Sgr itself is surrounded by an ionised HII region about 30 light years across including the reflection nebulae NGC 6523 and NGC 6533. This ionised region lies in front of a denser molecular cloud.[8]

The distances to 9 Sgr, M8, and NGC 6530 are uncertain, but generally estimated to be between 1,200 and 1,800 parsecs. Recent studies derive a distance around 1,250 pc for the M8 region.[8] Erosion of the front of the molecular cloud apparently caused by 9 Sgr suggests that it lies in front of the cloud, but studies of 9 Sgr as a binary star give a distance of 1,790 pc.[4][9]


9 Sagittarii is a binary system with the longest known period for a pair of class O stars at 9.1 years. The orbit is eccentric and the separation between the stars varies from 11 AU to 27 AU. The large separation means that the stellar winds of the two stars do not impact strongly and so the pair are not a strong source of x-rays. Lack of x-rays, low orbital velocities, and similar spectral types merging to a combined spectrum of O4V, mean that 9 Sgr was only confirmed to be a binary in 2012.[4] Earlier clues such as nonthermal radio emission and periodic spectral line profile variations had prompted the detail search for a companion.[10][11]

The two stars have been resolved using the Hubble Space Telescope Fine Guidance Sensor and with ground-based interferometry using the Very Large Telescope PIONIER instrument.[12][13]


Both components of 9 Sgr are massive main sequence class O stars, and both have temperatures above 40,000 K. Each is several hundred thousand times the luminosity of the sun. As main sequence stars, they are still fusing hydrogen in their cores, although strong convection have enriched the surface with nitrogen and helium which are visible in the spectrum. The powerful stellar winds produce emission lines in their spectra.[4]


  1. ^ a b c d e 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. ^ a b c 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. ^ Sota, A.; Maíz Apellániz, J.; Morrell, N. I.; Barbá, R. H.; Walborn, N. R.; Gamen, R. C.; Arias, J. I.; Alfaro, E. J. (2014). "The Galactic O-Star Spectroscopic Survey (GOSSS). II. Bright Southern Stars". The Astrophysical Journal Supplement. 211: 10. arXiv:1312.6222. Bibcode:2014ApJS..211...10S. doi:10.1088/0067-0049/211/1/10.
  4. ^ a b c d e f g Rauw, G.; Sana, H.; Spano, M.; Gosset, E.; Mahy, L.; De Becker, M.; Eenens, P. (2012). "9 Sagittarii: Uncovering an O-type spectroscopic binary with an 8.6 year period". Astronomy & Astrophysics. 542: A95. arXiv:1205.3314. Bibcode:2012A&A...542A..95R. doi:10.1051/0004-6361/201219254.
  5. ^ Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759. arXiv:1606.08053. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065.
  6. ^ a b c d e Blomme, R.; Volpi, D. (2014). "Non-thermal radio emission from O-type stars. V. 9 Sagittarii". Astronomy & Astrophysics. 561: A18. arXiv:1310.3997. Bibcode:2014A&A...561A..18B. doi:10.1051/0004-6361/201322679.
  7. ^ a b Krtička, J.; Kubát, J.; Krtičková, I. (2015). "X-ray irradiation of the winds in binaries with massive components". Astronomy & Astrophysics. 579: A111. arXiv:1505.03411. Bibcode:2015A&A...579A.111K. doi:10.1051/0004-6361/201525637.
  8. ^ a b c Tothill, N. F. H.; Gagné, M.; Stecklum, B.; Kenworthy, M. A. (2008). "The Lagoon Nebula and its Vicinity". Handbook of Star Forming Regions: 533. arXiv:0809.3380. Bibcode:2008hsf2.book..533T.
  9. ^ Kalari, V. M.; Vink, J. S.; Drew, J. E.; Barentsen, G.; Drake, J. J.; Eislöffel, J.; Martín, E. L.; Parker, Q. A.; Unruh, Y. C.; Walton, N. A.; Wright, N. J. (2015). "Classical T Tauri stars with VPHAS+ - I. H α and u-band accretion rates in the Lagoon Nebula M8". Monthly Notices of the Royal Astronomical Society. 453: 1026. arXiv:1507.06786. Bibcode:2015MNRAS.453.1026K. doi:10.1093/mnras/stv1676. hdl:10722/218808.
  10. ^ Abbott, D. C.; Bieging, J. H.; Churchwell, E. (1984). "The detection of variable, nonthermal radio emission from two O type stars". Astrophysical Journal. 280: 671. Bibcode:1984ApJ...280..671A. doi:10.1086/162040.
  11. ^ Rauw, G.; Blomme, R.; Waldron, W. L.; Corcoran, M. F.; Pittard, J. M.; Pollock, A. M. T.; Runacres, M. C.; Sana, H.; Stevens, I. R.; Van Loo, S. (2002). "A multi-wavelength investigation of the non-thermal radio emitting O-star 9 Sgr". Astronomy and Astrophysics. 394 (3): 993. Bibcode:2002A&A...394..993R. doi:10.1051/0004-6361:20020926.
  12. ^ Aldoretta, E. J.; Caballero-Nieves, S. M.; Gies, D. R.; Nelan, E. P.; Wallace, D. J.; Hartkopf, W. I.; Henry, T. J.; Jao, W.-C.; Maíz Apellániz, J.; Mason, B. D.; Moffat, A. F. J.; Norris, R. P.; Richardson, N. D.; Williams, S. J. (2015). "The Multiplicity of Massive Stars: A High Angular Resolution Survey with the Guidance Sensor". The Astronomical Journal. 149: 26. arXiv:1410.0021. Bibcode:2015AJ....149...26A. doi:10.1088/0004-6256/149/1/26.
  13. ^ Sana, H.; Le Bouquin, J.-B.; Lacour, S.; Berger, J.-P.; Duvert, G.; Gauchet, L.; Norris, B.; Olofsson, J.; Pickel, D.; Zins, G.; Absil, O.; De Koter, A.; Kratter, K.; Schnurr, O.; Zinnecker, H. (2014). "Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection". The Astrophysical Journal Supplement Series. 215: 15. arXiv:1409.6304. Bibcode:2014ApJS..215...15S. doi:10.1088/0067-0049/215/1/15.