Tau Herculis, also named Asuusiha (), 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. The star is located at a distance of approximately 307 light-years from the Sun based on parallax, but it is drifting closer with a radial velocity of −16 km/s.
Nomenclature Tau Herculis, Latinized from τ Herculis and abbreviated Tau Her or τ Her, is the star's Bayer designation.
In Chinese astronomy, (), meaning Seven Excellencies, refers to an asterism consisting of τ Herculis, 42 Herculis, φ Herculis, χ Herculis, ν1 Boötis, μ1 Boötis and δ Boötis. 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, . Consequently, the Chinese name for τ Herculis itself is (, ). This star is also part of the Manchu constellation Asu Usiha (), representing a net used to hunt sables. The IAU Working Group on Star Names adopted the name Asuusiha for τ Herculis on 14 May 2026.
Properties
The stellar classification of Tau Herculis is B5 IV, and it serves as a standard spectrum in the modern Morgan–Keenan (MK) classification. It is estimated to be just 26 million years old with a relatively low projected rotational velocity of 32 km/s. 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. The abundance of most heavier elements in this star are about 85% of those in the Sun. The star has four times the mass of the Sun and around 3.8 times the Sun's radius. On average, it is radiating 574 times the luminosity of the Sun from its photosphere at an effective temperature of 15,615 K.
During the Hipparcos mission, Tau Herculis 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; the brightness of Tau Herculis varies by 0.03 magnitude over a period of 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.
Pole star
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.
Preceded byPole StarSucceeded byIota Herculis18,400 ADAlpha Draconis
References AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 26 日