Theta Ophiuchi
Star in the constellation Ophiuchus
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Theta Ophiuchi, Latinized from θ Ophiuchi, is a multiple star system in the equatorial constellation of Ophiuchus. It lies on the "right foot" of the serpent-bearer, just southwest of Kepler's Star, the nova of 1604. This star has an apparent visual magnitude of +3.3,[6] making it readily visible to the naked eye. Based upon parallax measurements from the Hipparcos mission, it is roughly 436 light-years (134 parsecs) from Earth.[1] It is 1.8 degrees south of the ecliptic and therefore subject to lunar occultations and (very rarely) occulted by a planet. The next occultation by a planet will be by Mars on 3 October 2078.[12]
 | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ophiuchus |
| Right ascension | 17h 22m 00.57935s[1] |
| Declination | −24° 59′ 58.3670″[1] |
| Apparent magnitude (V) | 3.25 - 3.31[2] |
| Characteristics | |
| Spectral type | B2 IV[3] |
| U−B color index | −0.86[4] |
| B−V color index | −0.23[4] |
| Variable type | β Cep[2] |
| Astrometry | |
| Radial velocity (Rv) | −2[5] km/s |
| Proper motion (μ) | RA: −7.37[1] mas/yr Dec.: −23.94[1] mas/yr |
| Parallax (π) | 7.48±0.17 mas[1] |
| Distance | 436 ± 10 ly (134 ± 3 pc) |
| Absolute magnitude (MV) | −2.4[6] |
| Details | |
| θ Oph Aa | |
| Mass | 8.0–8.64[7] M☉ |
| Radius | 4.98–5.23[7] R☉ |
| Luminosity | 4,740–6,230[7] L☉ |
| Surface gravity (log g) | 3.950±0.006[8] cgs |
| Temperature | 22,260±280[8] K |
| Metallicity [Fe/H] | −0.15±0.12[9] dex |
| Rotational velocity (v sin i) | 30[10] km/s |
| Age | 15.8–24.0[7] Myr |
| θ Oph B | |
| Mass | 5.4[7] M☉ |
| Radius | 3.03–3.12[7] R☉ |
| Luminosity | 738–844[7] L☉ |
| Surface gravity (log g) | 2.87–2.93[7] cgs |
| Temperature | 17,700[7] K |
| Rotational velocity (v sin i) | 150[7] km/s |
| Age | 18.4–20.2[7] Myr |
| Other designations | |
| θ Ophiuchi, 42 Ophiuchi, CD−24 13292, FK5 644, GC 23451, HD 157056, HIP 84970, HR 6453, SAO 185320[11] | |
| Database references | |
| SIMBAD | data |
Theta Ophiuchi appears to be a triple star system. The brightest component is a spectroscopic binary with an orbital period of 56.71 days and an eccentricity of 0.17. The third component is 5.5 magnitude star with a stellar classification of B5. Its angular separation from the binary pair is 0.15 arcseconds.[13] This system is a proper motion member of the Upper Scorpius sub-group in the Scorpius–Centaurus OB association, the nearest such co-moving association of massive stars to the Sun.[6]
The primary component of this system is a variable star of the Beta Cephei type with a period of just 3h 22m.[15] It has an estimated 8.0 to 8.6 times the mass of the Sun and 4.98 to 5.23 times the Sun's radius. Although only 15.8 to 24 million years old,[7] it has begun to evolve away from the main sequence and has become a subgiant star with a stellar classification of B2 IV.[3] This massive star is radiating between 4,740 and 6,230 times the luminosity of the Sun from its photosphere[7] at an effective temperature of about 22,260 K,[8] giving it the blue-white hue of a B-type star.[16]
According to Richard H. Allen's Star Names: Their Lore and Meaning (1899), θ Oph together with ξ Oph formed the Sogdian Wajrik "the Magician", the Khorasmian Markhashik "the Serpent-bitten" and with η Oph the Coptic Tshiō, "the Snake", and Aggia, "the Magician".[17] With ζ Oph, η Oph, and ξ Oph it formed the Persian Garafsa, the Serpent-tamer;[17] a 1971 NASA list of star names includes the name Garafsa for θ Oph.[18]