Theta Ophiuchi

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 (very rarely) occulted by a planet. The next occultation by a planet will be by Mars on 3 October 2078.^[13]

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]
Distance	436 ± 10 ly (134 ± 3 pc)
Absolute magnitude (MV)	−2.4[6]
Details
Mass	8.8±0.2[3] M☉
Radius	6.3[7] R☉
Luminosity	5,000[6] 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	21.3±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
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.^[14] 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^[15]

The primary component of this system is a variable star of the Beta Cephei type with a period of just 3h 22m.^[16] 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.^[17]