Article · Wikipedia archive · Last revised Jun 24, 2026

Omega Ophiuchi

Omega Ophiuchi, which is Latinized from ω Ophiuchi, is a variable star in the equatorial constellation of Ophiuchus, located just to the north of the ecliptic near the western constellation border with Scorpius. It is a white-hued star that is faintly visible to the naked eye with an apparent visual magnitude that fluctuates around 4.45. Parallax measurements indicate it lies at a distance of about 168.6 light years from the Sun. It is drifting further away with a radial velocity of +2.5 km/s.

Last revised
Jun 24, 2026
Read time
≈ 3 min
Length
791 w
Citations
29
Source
Wikipedia ↗
Omega Ophiuchi
Location of ω Ophiuchi (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Ophiuchus
Right ascension 16h 32m 08.19983s1
Declination −21° 27′ 59.0120″1
Apparent magnitude (V) 4.44 – 4.512
Characteristics
Evolutionary stage main sequence3
Spectral type ApSrEuCr4 or A2 VpSrCrEu Ksn35
U−B color index +0.136
B−V color index +0.130±0.0027
Variable type α2 CVn2
Astrometry
Radial velocity (Rv)+2.5±0.78 km/s
Proper motion (μ) RA: +21.121 mas/yr
Dec.: +35.261 mas/yr
Parallax (π)19.34±0.21 mas1
Distance169 ± 2 ly
(51.7 ± 0.6 pc)
Absolute magnitude (MV)0.887
Details9
Mass2.25+0.13
−0.35 M☉
Radius2.97±0.19 R☉
Luminosity34.7±1.6 L☉
Surface gravity (log g)3.85+0.65
−0.69 cgs
Temperature8,150±250 K
Metallicity [Fe/H]−0.4710 dex
Rotation2.3205±0.000211 days
Rotational velocity (v sin i)32.2±1.4 km/s
Age860+150
−110 Myr
Other designations
ω Oph, 9 Ophiuchi, BD−21°4381, FK5 3307, GC 22221, HD 148898, HIP 80975, HR 6153, SAO 18445012
Database references
SIMBADdata

Omega Ophiuchi, which is Latinized from ω Ophiuchi, is a variable star in the equatorial constellation of Ophiuchus, located just to the north of the ecliptic near the western constellation border with Scorpius. It is a white-hued star that is faintly visible to the naked eye with an apparent visual magnitude that fluctuates around 4.45. Parallax measurements indicate it lies at a distance of about 168.6 light years from the Sun. It is drifting further away with a radial velocity of +2.5 km/s.

This is an Ap star with a stellar classification of ApSrEuCr,4 displaying strong abundance anomalies of the elements strontium, europium, and chromium. Abt and Morrell (1995) found a class of A2 Vp(SrCrEu Ksn), showing further an A-type main-sequence star with a Ca II line (singly-ionized calcium) having both sharp and broad components. It is an Alpha2 Canum Venaticorum variable, with its brightness ranging from magnitude 4.44 down to 4.51 over a period of 2.99 days.2 A strong magnetic field has been measured on the surface.11

Omega Ophiuchi is around 860 million years old with a projected rotational velocity of 32.2 km/s9 and a rotation period of 2.3 days.11 It has 2.3 times the mass of the Sun and 3.0 times the Sun's radius. The star is radiating 35 times the luminosity of the Sun from its photosphere at an effective temperature of 8,150 K.9 It is a source of X-ray emission.13

On 9 December 2017, it was occulted by Venus as viewed from the South Atlantic.14: 165 

This star has a significant change in its proper motion measurements over time, suggesting it may be an astrometric binary. The proposed companion is probably a compact star such as a white dwarf.15

References

References

  1. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600. Vizier catalog entry
  2. Samus, N. N.; et al. (2017). "General Catalogue of Variable Stars". Astronomy Reports. 5.1. 61 (1): 80–88. Bibcode:2017ARep...61...80S. doi:10.1134/S1063772917010085. S2CID 125853869.
  3. Abt, Helmut A.; Morrell, Nidia I. (1995). "The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars". Astrophysical Journal Supplement. 99: 135. Bibcode:1995ApJS...99..135A. doi:10.1086/192182.
  4. Houk, Nancy; Smith-Moore, M. (1978). Michigan catalogue of two-dimensional spectral types for the HD stars. Vol. 4. Ann Arbor: Dept. of Astronomy, University of Michigan. Bibcode:1988mcts.book.....H.
  5. Skiff, B. A. (2014). "VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009- )". Vizier Online Data Catalog. Bibcode:2014yCat....1.2023S.
  6. 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.
  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. S2CID 119257644. Vizier catalog entry
  8. Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759–771. arXiv:1606.08053. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065. S2CID 119231169.
  9. Sikora, J.; et al. (February 2019). "A volume-limited survey of mCP stars within 100 pc - I. Fundamental parameters and chemical abundances". Monthly Notices of the Royal Astronomical Society. 483 (2): 2300–2324. arXiv:1811.05633. Bibcode:2019MNRAS.483.2300S. doi:10.1093/mnras/sty3105. S2CID 119089236.
  10. Soubiran, Caroline; Le Campion, Jean-François; Brouillet, Nathalie; Chemin, Laurent (2016). "The PASTEL catalogue: 2016 version". Astronomy & Astrophysics. 591: A118. arXiv:1605.07384. Bibcode:2016A&A...591A.118S. doi:10.1051/0004-6361/201628497. S2CID 119258214.
  11. Sikora, J.; et al. (March 2019). "A volume-limited survey of mCP stars within 100 pc II: rotational and magnetic properties". Monthly Notices of the Royal Astronomical Society. 483 (3): 3127–3145. arXiv:1811.05635. Bibcode:2019MNRAS.483.3127S. doi:10.1093/mnras/sty2895. S2CID 119415579.
  12. "ome Oph". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-10-28.
  13. Schröder, C.; et al. (April 2008). "Magnetic fields in X-ray emitting A-type stars". Contributions of the Astronomical Observatory Skalnaté Pleso. 38 (2): 447–448. arXiv:0712.0173. Bibcode:2008CoSka..38..447S.
  14. Meeus, Jan (2002). "Mutual occultations of planets". More Mathematical Astronomy Morsels (PDF). pp. 174–185. ISBN 0943396743. Archived from the original (PDF) on 2025-04-05. Retrieved 2025-02-21.
  15. Waisberg, Idel; Klein, Ygal; Katz, Boaz (2023). "Binarity and beyond in a stars – I. Survey description and first results of VLTI/GRAVITY observations of VAST targets with high Gaia–Hipparcos accelerations". Monthly Notices of the Royal Astronomical Society. 521 (4): 5232–5254. arXiv:2206.05251. Bibcode:2023MNRAS.521.5232W. doi:10.1093/mnras/stad872.