TAU HER (Tau Herculis). Rather far off the Milky Way for a hot class B star (a B5 subgiant, but see below), Tau Herculis is Hercules' most northerly Greek-letter-named star, beating Iota Her by just over a quarter of a degree, although at fourth magnitude (3.89) it does not make much of an impact on the constellation. Yet it stands out in a variety of ways. We honor the navigation stars that lie at the celestial poles, in particular Polaris, which is less than a degree from the sky's northern point of rotation. But because of the 26,000 year precession (wobble) of the Earth's axis, 'twas not always so. The last good one, as often pointed out, was Thuban in Draco, which reigned during the times of ancient Egypt around 2700 BC. But before that, we had Tau Her to follow, as it presented itself as a truly fine pole star again less than a degree from the turning North Celestial Pole around 7400 BC, during mesolithic times just after the last ice age. Did the people of the times follow it? Probably, but we'll never know. Look for it again at the pole in the year 18,400.

A well-determined temperature of 15,000 Kelvin is consistent with its class. Lying 315 light years away (which accounts for the star's relative faintness), Tau Her shines away with the light of 700 Suns, from which we derive a radius of 4.0 times solar, and a rotation period (from a projected equatorial velocity of 27 kilometers per second) less than 7.4 days. Given that class B stars are known for their fast rotations, and that slow rotators are often chemically peculiar due to settling of elements (and that Tau Her's composition is normal), the longer period suggests that the pole probably more or less directed at us. Not really a subgiant, Tau her is actually a relatively massive dwarf of 4.9 solar masses that is a half to two-thirds of the way through its hydrogen-fusing lifetime of 100 million years. As are a number of stars of its class, it is not completely stable, and is considered to be a "slowly pulsating B (SPB) star" with a five or so percent variation over two periods of 1.25 and 1.28 days. Not quite 7 seconds of arc away lies a 15th magnitude companion with a common motion, suggesting that the two really belong together. If so, it's an M2 dwarf at least 650 Astronomical Units away that takes some 6700 years to orbit and that from Tau Her proper would shine with the light of only seven times that of our Venus.
Written by Jim Kaler 6/19/09. Return to STARS.