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.