Tau Herculis


Tau Herculis, Latinized from τ Herculis, is a fourth-magnitude star in the constellation Hercules. It is a blue subgiant star, seven hundred times more luminous than the Sun.

Properties

Tau Herculis has a stellar classification B5IV. Its mass is 4.9 times solar. Though its apparent magnitude is only 3.89, like all B-class stars, it is very luminous, boasting a total bolometric luminosity that is 700 times solar. Hipparcos estimated its distance at roughly 96 parsecs from Earth, or 310 ± 20 light years away.

Pole Star

Tau Herculis is a visible star located within 1° of the precessional path traced across the celestial sphere by the Earth's North pole. It was the northern pole star around the year 7400 BC, a phenomenon which is expected to reoccur in the year 18,400 due to precession.
The current pole star is, of course, Polaris. Among the 14 stars that could be reasonably considered eventual northern pole stars, Tau Herculis is the dimmest, but only Polaris itself and Thuban are closer to the precessional path.
Preceded byPole StarSucceeded by
Iota Herculis18,400 ADEdasich

Etymology

Its traditional name, Rukbalgethi Shemali, is of Arabic origin and shares certain etymological characteristics with the stars Ruchbah and Zubeneschamali, signifying Hercules' "northern knee".
In Chinese, 七公, meaning Seven Excellencies, refers to an asterism consisting of τ Herculis, 42 Herculis, φ Herculis, χ Herculis, ν1 Boötis, μ1 Boötis and δ Boötis. Consequently, the Chinese name for τ Herculis itself is 七公二