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Tau Herculis

Tau Herculis, a name Latinized from τ Herculis, is a variable star in the northern constellationofHercules. 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]

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]

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]}

InChinese, 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]