ASELLUS PRIMUS (Theta Bootis). Three stars in far
northwestern Bootes near the end of the
handle of the Big Dipper of
Ursa
Major represent the Herdsman's
upraised fingers.
Oddly, they are given the Latin name "Asellus," meaning "donkeys,"
such inconsistencies not at all uncommon among star names, given
their checkered histories. From east to west, the three are known
as Asellus Primus (first),
Secundus (second) and
Tertius (third).
At mid-fourth magnitude (4.05) the brightest of them, Asellus
Primus is much better known as Theta Bootis, or just Theta Boo.
Stars like Theta Boo have a peculiar fascination about them, as
they are more sunlike than most of those that dot the familiar
constellations, and as few of them are readily visible to the naked
eye. Just a bit more massive, warmer, and brighter than the Sun, Theta Boo is a class F (F7) hydrogen-
fusing dwarf. Located only 48 light years from Earth, the star
shines with a luminosity of just five times that of the Sun, that
and its surface temperature of 6300 Kelvin (just topping the Sun's
5780 K) leading to a radius not quite twice solar, a mass that
falls between 1.25 and 1.5 solar, and an age of a few billion years
(the best estimate about 3 billion, a bit younger than our 5
billion year old Sun). Like the Sun, Theta radiates X-rays that
indicate a hot surrounding corona, though no magnetic field (which
heats the solar corona) has been detected. Even the metal content
is similar to the Sun, just 85 percent less. Quite unlike the Sun,
however, Theta is accompanied by a faint stellar companion 70
seconds of arc away. Though no orbital motion is detected, the two
stars move against the distant stellar background in lockstep, so
they are almost certainly related. From its low luminosity, the
11th magnitude neighbor must be a class M (M2.5) dwarf that carries
no more than a third of a solar mass, the temperature only 3500
Kelvin, the luminosity only about two to three percent that of the
Sun. The true separation is at least 1000 Astronomical Units (25
times Pluto's distance from the Sun), and the orbital period is at
least 25,000 years. From the companion, bright Theta proper would
shine with the light of five full Moons, while from Theta the
little one would appear only with the glow of three Venuses. No
planets have been detected. Given the large separation, the two
will probably separate some time in the distant future, victims of
tides raised by the Galaxy.