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DELTA BOO (Delta Bootis). In spite of its prominent positionwithinBootes, the Herdsman, Delta Boocarries no proper name at all, probably because it shines down atus at only faint third -- and nearly fourth -- magnitude (3.47). Bayer seemed to use position in the constellation as much asbrightness in naming by Greek letters, as Delta not only ranksfifth, but is outstripped by brighter Eta Bootis (Muphrid). Delta Boo is a class G (G8)giant located 117 light years away. From its yellow-white 4840Kelvin surface, it shines with the light of 59Suns. It is a rather typical "clump" star, oneof a breed of similar temperature and luminosity that is fusinghelium into carbon in its deep core, one that carries a mass 2.5times that of the Sun. Not all that large for a star called a"giant," its diameter deduced from temperature and luminosity is11.2 solar. It is large and close enough, however, to have had itsangular size determined, from which we come up with a similardiameter 10.4 times solar (from which we then find a somewhatwarmer temperature of 4990 Kelvin). The star stands out in twoways. Its metal content is rather low, only 40 percent that of theSun. As a result it is spectroscopically classified as a "CN weak"star (from the strength of absorptions of cyanogen, CN). Delta Booalso has a well-known 8th magnitude (7.8) class G (G0) dwarfcompanion that is quite similar to the Sun, just a bit warmer (5900Kelvin), 80 percent less luminous, and 87 percent the radius. Thecompanion is a bit warmer than expected for its luminosity, andmight be a marginal "subdwarf," in keeping with the low metalcontent of the main star (the two go together). Since thecompanion is nearly two minutes of arc away from its brighterneighbor, Delta Boo B shows no orbital motion, but since the twostars are moving in lockstep through space, there is no questionabout their connection. At least 3800 Astronomical Units (Earth-Sun distances) apart, the two take at least 120,000 years to orbiteach other. From Delta Boo A, the smaller star would shine 30 orso times brighter than Venus does in our sky, while from Delta BooB, the giant would glow with the light of 2.5 full moons.
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