Tau Herculis

Tau Herculis, a name Latinized from τ Herculis, is a variable star in the northern constellation of Hercules. It has a blue-white hue and is visible to the naked eye at night with an apparent visual magnitude that fluctuates around 3.91.^[2] The star is located at a distance of approximately 307 light years from the Sun based on parallax,^[1] but it is drifting closer with a radial velocity of −16 km/s.^[2]

A light curve for Tau Herculis, plotted from TESS data^[11]

Tau Herculis

Historical depiction of the constellation Hercules with τ, bottom left, as the "northern knee"
Observation data Epoch J2000      Equinox J2000
Constellation	Hercules
Right ascension	16h 19m 44.43666s[1]
Declination	46° 18′ 48.1123″[1]
Apparent magnitude (V)	3.91[2] 3.83 to 3.86[3]
Characteristics
Evolutionary stage	Main sequence[4]
Spectral type	B5 IV[5]
U−B color index	−0.569[2]
B−V color index	−0.151±0.009[2]
Variable type	SPB[3]
Astrometry
Radial velocity (Rv)	−15.5±0.5[2] km/s
Proper motion (μ)	RA: −13.33[1] mas/yr Dec.: 38.48[1] mas/yr
Parallax (π)	10.61±0.11 mas[1]
Distance	307 ± 3 ly (94.3 ± 1.0 pc)
Absolute magnitude (MV)	−0.96[2]
Details
Mass	4.01[6] M☉
Radius	3.55±0.19 R☉[7] 3.80±0.25[8] R☉
Luminosity	574[2] L☉
Surface gravity (log g)	4.02±0.05[7] cgs
Temperature	15,615±301[7] K
Metallicity [Fe/H]	0.15[9] dex
Rotational velocity (v sin i)	32[6] km/s
Age	26[6] Myr
Other designations
Rukbalgethi Shemali, τ Her, 22 Her, BD+46°2169, FK5 608, GC 21987, HD 147394, HIP 79992, HR 6092, SAO 46028, CCDM J16197+4619A[10]
Database references
SIMBAD	data

The stellar classification of Tau Hercules is B5 IV,^[5] and it serves as a standard spectrum in the modern Morgan–Keenan (MK) classification.^[12] It is estimated to be just 26 million years old with a relatively low projected rotational velocity of 32 km/s.^[6] Slowly rotating B-type stars are often chemically peculiar, so the mostly normal spectra of this star suggests we may be viewing it from near pole-on.^[13] The abundance of most heavier elements in this star are about 85% of those in the Sun.^[14] The star has four times the mass of the Sun^[6] and around 3.8^[8] times the Sun's radius. On average, it is radiating 574^[2] times the luminosity of the Sun from its photosphere at an effective temperature of 15,615 K.^[7]

During the Hipparcos mission,^[4] Tau Hercules was discovered to be a variable star of the slowly pulsating B-type. These are mid-B main sequence stars that vary with a period of about a day;^[4] the brightness of Tau Hercules varies by 0.03 magnitude^[3] over a period of 1.24970±0.00008 days. The radial velocity of the star varies at a different rate than the photometric period, with the object showing both radial and non-radial pulsation modes.^[4][15]

Historical significance and etymology

The path of the north celestial pole among the stars due to the precession.

Tau Herculis is located within 1° of the precessional path traced across the celestial sphere by the Earth's North pole. It could have served the northern pole star around the year 7400 BCE, a phenomenon which is expected to reoccur in the year 18,400 due to precession.^[13]

Preceded by	Pole Star	Succeeded by
Iota Herculis	18,400 AD	Alpha Draconis

Its traditional name, Rukbalgethi Shemali, is of Arabic origin and shares certain etymological characteristics with the stars Ruchbah and Zubeneschamali, signifying Hercules' "northern knee".^{[16][better source needed]}

In Chinese, 七公 (Qī Gōng), meaning Seven Excellencies, refers to an asterism consisting of τ Herculis, 42 Herculis, φ Herculis, χ Herculis, ν¹ Boötis, μ¹ Boötis and δ Boötis.^[17] Consequently, the Chinese name for τ Herculis itself is 七公二 (Qī Gōng èr, English: the Second Star of Seven Excellencies.)^[18]