Iota Boötis
Iota Boötis is a wide binary star system in the constellation Boötes, approximately 96 light-years from Earth. Its name is a Bayer designation that is Latinized from ι Boötis, and abbreviated Iota Boo or ι Boo. The brighter component has the traditional name Asellus Secundus, pronounced, which is Latin for "second donkey colt", and the Flamsteed designation 21 Boötis. It is faintly visible to the naked eye with a typical apparent visual magnitude of +4.75. Based on parallax measurements, it is located at a distance of from the Earth. The star is drifting closer to the Sun with a radial velocity of −19 km/s.
The companion is HD 234121, a K-type main-sequence star at an angular distance of 38.6 arcseconds; easily separated with binoculars.
Components
The primary component is a white hued A-type main-sequence star with a stellar classification of A7V. It is classified as a Delta Scuti-type variable star and its brightness varies from magnitude +4.73 to +4.78 with a stable period of 38 minutes. This star is 785 million years old and has a high rate of spin, showing a projected rotational velocity of 144 km/s. It has 1.8 times the mass of the Sun and 1.7 times the Sun's radius. Iota Boötis is radiating 8.8 times the luminosity of the Sun from its photosphere at an effective temperature of 7,764 K.The common proper motion companion, HD 234121, is a magnitude 7.3 main-sequence star belonging to spectral class K0V. It has a projected separation from ι Boo of. HD 234121 has a mass of, a luminosity of, a temperature of, and a radius of.
The Washington Double Star Catalog lists a third component, a 14th-magnitude star at 90 arcseconds, but it is an unrelated background star.
Nomenclature
This star, along with the other Aselli and λ Boo, were Aulād al Dhiʼbah, "the Whelps of the Hyenas".In Chinese, 天槍, meaning Celestial Spear, refers to an asterism consisting of ι Boötis, κ2 Boötis and θ Boötis. Consequently, the Chinese name for ι Boötis itself is 天槍二.