ASELLUS SECUNDUS (Iota Bootis). Way way up in the northwest corner of Bootes, five or so degrees to the northeast of Alkaid (Eta Ursae Majoris) at the end of the handle of the Big Dipper, lie three stars that that represent the outstretched fingers of the Herdsman, who follows the Great Bear around the pole. From east to west, they are Theta, Iota, and Kappa Boo. The first and third are fourth magnitude, Iota fifth (4.75). One would hardly expect them to have proper names when brighter stars lack them. But they do, again in order Asellus Primus, Asellus Secundus, and Asellus Tertius. Having nothing whatever to do fingers or any other part of Bootes, the names, from Latin, refer to first, second, and third donkeys, more specifically to donkey colts. According to Allen, they were applied by Bayer; one wonders what the great mapmaker had in mind. Donkeys make sense in central Cancer, where two Aselli (Gamma and Delta Cnc) flank the Preasepe (Beehive) cluster, which evokes a manger. But hardly in Bootes. Nevertheless, there they are. Connecting the three with central Bootes is Lambda Boo, which has one of the odder chemical compositions found among stars and is the prototype for its class. Follow the trio back through Alkaid, and they point at the Whirlpool Galaxy, Messier 51, in Canes Venatici (the Hunting Dogs).

But what about the star itself? Iota Boo is a class A (A9) hydrogen-fusing dwarf 94.8 light years away (good to half a light year) with a temperature of 7730 Kelvin. With little correction for ultraviolet or infrared radiation, "Secundus" shines with the light of 8.2 times that of the Sun, from which we derive a radius of 1.6 times solar. A relatively fast spinner, at least 137 kilometers per second at the equator (which keeps the chemical elements from separating into odd patterns), the star completes a rotation in under 0.6 days. Theory gives it a mass 1.7 times that of the Sun and suggests that the star is relatively young and has most of its hydrogen-fusing lifetime of 1.8 or so billion years left to go. Of special interest, it's a Delta Scuti type variable, oscillating in brightness by a percent or so with at least two separate pulsation periods of 38.22 and 30.55 minutes (the exact figures somewhat elusive). While touted as a triple star, Iota Boo C has moved 8 seconds of arc relative to A in just under a century. The apparent pairing is clearly just a line of sight coincidence. That leaves us with Iota B, which since 1779 has changed its position by a mere 0.2 second of arc and now stands at 38.8 seconds away from A, making Iota Boo a real double. From its brightness and distance from us, Iota Boo B is a K0 dwarf with a mass of about 0.5 Suns. Writing from the nineteenth century, Smythe and Chambers called them pale yellow and creamy white: not their real colors, as doubles fool the eye. At a physical distance from Iota A of at least 1130 Astronomical Units, B would take at least 24,000 years to make a complete orbit. Iota Boo A is near the mass above which magnetic cycles disappear, so X-ray emission is probably coming from cooler, lower mass Iota B.
Written byJim Kaler 6/13/14. Return to STARS.