DELTA BOO (Delta Bootis). In spite of its prominent position within Bootes, the Herdsman, Delta Boo carries no proper name at all, probably because it shines down at us at only faint third -- and nearly fourth -- magnitude (3.47). Bayer seemed to use position in the constellation as much as brightness in naming by Greek letters, as Delta not only ranks fifth, 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 4840 Kelvin surface, it shines with the light of 59 Suns. It is a rather typical "clump" star, one of a breed of similar temperature and luminosity that is fusing helium into carbon in its deep core, one that carries a mass 2.5 times that of the Sun. Not all that large for a star called a "giant," its diameter deduced from temperature and luminosity is 11.2 solar. It is large and close enough, however, to have had its angular size determined, from which we come up with a similar diameter 10.4 times solar (from which we then find a somewhat warmer temperature of 4990 Kelvin). The star stands out in two ways. Its metal content is rather low, only 40 percent that of the Sun. As a result it is spectroscopically classified as a "CN weak" star (from the strength of absorptions of cyanogen, CN). Delta Boo also has a well-known 8th magnitude (7.8) class G (G0) dwarf companion that is quite similar to the Sun, just a bit warmer (5900 Kelvin), 80 percent less luminous, and 87 percent the radius. The companion is a bit warmer than expected for its luminosity, and might be a marginal "subdwarf," in keeping with the low metal content of the main star (the two go together). Since the companion is nearly two minutes of arc away from its brighter neighbor, Delta Boo B shows no orbital motion, but since the two stars are moving in lockstep through space, there is no question about their connection. At least 3800 Astronomical Units (Earth- Sun distances) apart, the two take at least 120,000 years to orbit each other. From Delta Boo A, the smaller star would shine 30 or so times brighter than Venus does in our sky, while from Delta Boo B, the giant would glow with the light of 2.5 full moons.
Written by Jim Kaler. Return to STARS.