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.