RHO HER (Rho Herculis). Not one, not two, but probably three, and maybe even four! No question about the two however, as Rho Herculis (just off the northeast corner of the "Keystone" of Hercules, the greatest of Greek mythical heroes) is an attractive fourth magnitude (4.15) double star with two rather similar components separated by an easily resolvable (through the telescope) four seconds of arc. The brighter (just barely at fifth magnitude, 4.56), Rho Her A, is a class B (again just barely, B9.5) giant, while Rho Her B, a class A (A0) dwarf, comes in close to sixth magnitude (5.42). (Careful here: like Kappa Cancri, Rho Her A is the class B star, while Rho Her B is the class A star; isn't astronomy fun?). Though both are really white, juxtaposition enhances, even creates color: Admiral Smythe's 19th century "A Cycle of Celestial Objects" refers to the brighter as "bluish white," the fainter as "pale emerald." There seem to be no actual temperature measures, so we adopt 10,500 and 10,000 for "A" and "B" from their classes. These (needed to account for some ultraviolet light) and a distance of 402 light years yield respective luminosities of 251 and 103 times that of the Sun, radii of 4.8 and 3.4 solar, and masses of 3.2 and 2.9 solar. These in turn show that Rho Her A is not yet a true giant, but more likely a subgiant that has just run out of hydrogen fuel in its nuclear-burning core, the age of the star (and system) about 300 million years. The pair are distinguished more by their rotations. Among such stars, the hotter is usually the faster. But again like Kappa Cnc, while Rho Her A (remember, the class B star) spins at its equator with at a modest 73 kilometers per second (a lower limit), Rho Her A madly rotates at 291 km/s, giving them respective rotation periods of less than 3.3 and 0.6 days. The slower spin of Rho Her A has apparently allowed some diffusion of elements, making it into a "silicon star," while the faster spin of Rho Her B keeps things stirred up. Rho Her A may have a very close companion (called Aa) about which nothing is known, while off in the distance (nearly two minutes of arc away) lies 13th magnitude Rho Her C, which seems to be tracking the bright pair. At that distance it would have to be a class K dwarf with a mass of perhaps 0.6 Suns. Rho Her A and B are separated by at least 500 Astronomical Units, so (ignoring Aa) it must take them at least 4600 years to orbit. If Rho Her C is a true companion, it's at least 14,700 AU distant and takes a good 700,000 years to make a full turn. From Rho Her A or B, C would appear about as bright as our Jupiter, while from C, A and B would rival full and gibbous Moons separated by up to a couple degrees.
Written by Jim Kaler 5/25/07. Return to STARS.