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.
Written by Jim Kaler. Return to STARS.