Theta Herculis

Theta Herculis

Historical view of the Hercules constellation showing Rukbalgethi Genubi (θ Her) as the "southern knee", right of center
Observation data Epoch J2000      Equinox J2000
Constellation	Hercules
Right ascension	17h 56m 15.18054s[1]
Declination	+37° 15′ 01.9343″[1]
Apparent magnitude (V)	3.851[2]
Characteristics
Spectral type	K1IIaCN2[3]
U−B color index	+1.40 [4]
B−V color index	+1.35[4]
R−I color index	+0.63[5]
Variable type	Irregular (suspected)[6]
Astrometry
Radial velocity (Rv)	–28.32[7] km/s
Proper motion (μ)	RA: 2.67[1] mas/yr Dec.: 6.47[1] mas/yr
Parallax (π)	4.33±0.13 mas[1]
Distance	750 ± 20 ly (231 ± 7 pc)
Absolute magnitude (MV)	−2.71+0.26 −0.23[8]
Details[9]
Mass	5.89±0.07 M☉
Radius	82±2 R☉
Luminosity	2,410±150 L☉
Surface gravity (log g)	1.37±0.04 cgs
Temperature	4,448±23 K
Metallicity [Fe/H]	−0.03±0.15 dex
Rotational velocity (v sin i)	3.4 ± 0.6[8] km/s
Age	130[10] Myr
Other designations
Rukbalgethi Genubi, θ Her, 91 Her, BD+37°2982, FK5 672, GC 24415, HD 163770, HIP 87808, HR 6695, SAO 66485[11]
Database references
SIMBAD	data

Theta Herculis is a single,^[12] variable star in the northern constellation of Hercules. This object is visible to the naked eye as a faint, orange-hued star with an apparent visual magnitude of 3.851.^[2] Based upon parallax measurements, it is located around 750 light years away from the Sun. The star is advancing toward the Earth with a heliocentric radial velocity of –28 km/s.^[7]

This is an aging K-type bright giant with a stellar classification of K1IIaCN2,^[3] where the suffix notation indicates a strong overabundance of CN in the spectrum. The brightness variations of this star was first noticed in 1935 by M. Fedtke and confirmed by Erich Przybyllok and Kurt Walter the same year.^[13] French astronomer Paul Muller then classified Theta Herculis as an irregular variable with a range of magnitudes between 3.7 and 4.1 and a periodicity of roughly 8–9 days.^[6]

The star is about 130 million years old^[10] with 5.89 times the mass of the Sun. With the hydrogen at its core exhausted, the star has expanded to 82 times the Sun's radius. It is radiating 2,400 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,448 K.^[9]

θ Herculis is the Bayer designation, Latinised to Theta Herculis, abbreviated to θ Her or Theta Her.

In the Calendarium of Al Achsasi Al Mouakket, this star was designated Rekbet al Jathih al Aisr, which was translated into Latin as Genu Sinistrum Ingeniculi, meaning the left knee of the kneeling man.^[14] The traditional name Rukbalgethi Genubi that is encountered in various texts is etymologically similar to the stars Ruchbah and Zubenelgenubi, the term "ruchbah" meaning "knee" while "genubi" signifies "southern"—hence the "southern knee", a meaning which can be gleaned more easily by looking at the accompanying constellation map.^[15]

In Chinese, 天紀 (Tiān Jì), meaning Celestial Discipline, refers to an asterism consisting of θ Herculis, ξ Coronae Borealis, ζ Herculis, ε Herculis, 59 Herculis, 61 Herculis, 68 Herculis and HD 160054.^[16] Consequently, the Chinese name for θ Herculis itself is 天紀九 (Tiān Jì jiǔ, English: the Ninth Star of Celestial Discipline.)^[17] It was also Tien Ke, Heaven's Record.^[18]

• Spacedude's Constellation map: Stars in Hercules