ZETA HER (Zeta Herculis). With the exception of the brightest
stars, star names were handed out more by position than brightness,
as attested to by Zeta Herculis. At bright third magnitude (2.81),
just barely the second brightest star in the constellation Hercules (right behind Kornephoros, Beta Herculis), Zeta
Herculis was ignored by the ancients. Even Bayer rather ignored it
by giving it the sixth letter in the Greek
alphabet, the Alpha designation going to faint-third-magnitude
Rasalgethi clearly because of its position in the Hero's head. In
spite of the star's lack of public prominence, it has a lot to
recommend it. Zeta Her is actually double, a modestly bright third
magnitude (2.90) star orbited by a sixth magnitude
(5.53) companion only a second of
arc or so away. The brighter star, Zeta Her A, is a class G (G0)
subgiant with the same temperature (5780 Kelvin) as the Sun (which
is a G2 star). With a mass some 50 percent greater than the
Sun, however, and beginning its evolution
toward gianthood (its core hydrogen fusion likely shut down), Zeta
Her A is 6 times more luminous than the Sun with a radius 2.5 times
as large. Nevertheless, the star gives a good idea of what the Sun
would look like from a great distance, in Zeta Her's case 35 light
years. The companion (Zeta Her B), a cooler class G (G7) hydrogen-fusing dwarf
with a luminosity only 65 percent that of the Sun and a mass about
85 percent solar, orbits with a period of 34.5 years at a mean
distance of 15 Astronomical Units (over 50 percent farther than
Saturn is from the Sun). A rather high eccentricity takes the two
as far apart as 21 AU and as close as 8 AU. Under such conditions,
planets would very likely be impossible. Astronomers have
identified a number of extended "moving groups" of stars that seem
to have some common origin (the most famed the stars that are
related to the
Ursa Major cluster). The "Zeta Herculis moving
group," of which the star is the leader, contains stars as far
removed as Perseus, Lupus, and Octans,
the dim constellation that surrounds
the South Celestial Pole. Zeta Her's
velocity relative to the Sun is high, 76 kilometers per second, five times
normal.