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.
Written by Jim Kaler. Return to STARS.