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]

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

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]

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]

References

На других языках

[es] Tau Herculis

[ru] Тау Геркулеса