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Theta Ophiuchi

Theta Ophiuchi
Location of θ Ophiuchi (circled)
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]
Distance436 ± 10 ly
(134 ± 3 pc)
Absolute magnitude (MV)−2.4[6]
Details
Mass8.8±0.2[3] M☉
Radius6.3[7] R☉
Luminosity5,000[6] L☉
Surface gravity (log g)3.950±0.006[8] cgs
Temperature22,260±280[8] K
Metallicity [Fe/H]−0.15±0.12[9] dex
Rotational velocity (v sin i)30[10] km/s
Age21.3±5.8[3] Myr
Other designations
42 Ophiuchi, CD–24 13292, FK5 644, GC 23451, HD 157056, HIP 84970, HR 6453, SAO 185320[11]
Database references
SIMBADdata

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. 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".[12] 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 less frequently occulted by a planet.

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]

A light curve for Theta Ophiuchi, plotted from Hipparcos data[14]

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 nearly nine[3] times the mass of the Sun and more than six[7] times the Sun's radius. Although only 21 million years old,[3] 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 around 5,000 times the luminosity of the Sun from its outer atmosphere at an effective temperature of about 22,260 K,[8] giving it the blue-white hue of a B-type star.[16]

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