THETA DRA (Theta Draconis). The long snaking body of Draco, the Dragon, starts with its tail
between the Dippers. At the southernmost point of its curve north
of Hercules lies Theta Herculis (sadly of no proper name), just a
couple doors east of Thuban, the once and
great pole star of ancient Egypt. Smack at mid-fourth magnitude
(4.01), Theta Dra is actually fairly nearby, only 68 light years
away, and is faint because of relatively low intrinsic luminosity.
This class F (F8) subgiant is not all that much brighter than the
Sun, radiating 8.6 solar luminosities from
its 6290 Kelvin surface. The two combine to yield a radius of 2.5
solar, the same value derived from direct measurement of angular
diameter. Seemingly rather ordinary, with a metal content 85
percent that of the Sun, the star has a couple unusual
characteristics, and a bit of mystery about it too. Theta is a
tight spectroscopic double (one observed through shifts in its
spectrum) with a 3.0708216-day period (known to the second) and a
remarkably high equatorial rotation speed of at least 28 kilometers
per second, which gives it a rotation period less than 4.5 days.
The tilt of the axis cannot be directly found, so we have only
these limits on rotation. More remarkably, the dim companion's
spectrum has been detected through sophisticated infrared
technology, which gives a low orbital eccentricity, a mass ratio of
0.38 (secondary to primary), and masses of 1.21 and 0.46 solar,
making the secondary a dim class M2 dwarf. Evolutionary
considerations show a bit higher mass for the primary, around 1.5
solar, the star nearing the end of its core hydrogen-fusing
lifetime (hence the subgiant category). From the masses and
period, we find an orbital separation of a mere 0.05 Astronomical
Units, just 7 to 8 million kilometers, 13 percent Mercury's
distance from the Sun. The high rotation speed may well be due to
tidal effects that have spun the stars into synchrony (much as one
face of the Moon always points toward Earth). If so, the real
rotation period is the same as the orbital period, 3.07 days, which
in turn means that the axial tilt is about 45 degrees from the line
of sight. Curiously, however, direct measurement of orbital size
(which just gives a lower limit) suggests that the rotation pole
points more toward us than the 45 degree tilt, which in turn makes
for an even higher rotation speed, which is very unusual for such
a cool class F star. In any case, the high rotation speed helps
produce magnetic "chromospheric" activity (the "chromosphere" in
the Sun the layer between the bright surface and the outer hot
corona that is visible during a solar eclipse), though no magnetic
field has actually been detected. Stars are not just little bright
points in the sky -- each has its own distinctive personality.