68 HER (68 Herculis). Two heroes rule the northern sky, Perseus in northern-hemisphere autumn and winter, Hercules in spring and summer. Curiously, Hercules' best-known star, Rasalgethi, the Alpha star, is not the brightest, probably because it is so far south of the main figure, where it makes a nodding acquaintance with Rasalhague in Ophiuchus. Perhaps best known for its great globular cluster, Messier 13, Hercules is also host to a number of interesting fainter stars that carry Flamsteed numbers and that include the planet-holding class K dwarf 14 Herculis, the semi-regular giant variable 30 Herculis, and the remarkable pre-planetary nebula star 89 Her. Here's one more, an unusual mass-transferring hot binary, 68 Herculis. Fifth magnitude (4.82), the star lies just east of Hercules' "Keystone." It's odd on several levels. For some strange reason, it's better known in the literature as "u Herculis," such lower-case Roman letters used by Bayer when he ran out of Greek ones and hardly ever used today. The name sets up confusion with upper-case "U" Herculis, which is a ninth magnitude Mira- type deep red giant variable. They could not be more different. 68 ("u") Her is a hot class B binary (B1.5 dwarf plus B5 "giant," but not really) 666 light years away (give or take 57) rather well off the plane of the Milky Way. Moreover, it's an eclipsing double something in the mold of Algol, in which one of the stars, the cooler secondary, is transferring mass to the brighter primary. Twice each orbital period of 2.05 days, one gets partially in front of the other, the deeper of the eclipses, around 0.7 magnitudes, occurring when the dimmer secondary cuts across the brighter.
68 Her The partial eclipse of the binary 68 (u) Herculis. The deeper of the two eclipses occurs when the fainter secondary gets partially in front of the brighter primary and vice versa. The brightness is not constant outside of eclipse because of the changing shapes projected to the observer and because the light of the brighter star reflects off the surface of the fainter. (From R. W. Hilditch in The Observatory.)

The other eclipse is about half as deep. With temperatures of around 19,000 and 11,000 Kelvin, the two shine with luminosities of 1300 and 160 times that of the Sun. The orbital data and light variation give masses of 6.8 and 2.8 Suns for the two, Kepler's Laws then yielding a separation of only 0.07 Astronomical Units, or just 14 times the solar radius, about triple the stellar radii. Evolutionary theory gives similar masses. As befits such a close pairing, the stars are rotating with speeds of at least 145 and 105 kilometers per second. The fainter secondary is filling its zero-gravity tidal surface such that its mass is leaking out through a teardrop shaped point and falling toward the primary. The secondary, apparently farther along the evolutionary trail, should be the more massive, but is slowly being consumed. On top of all that, the brighter primary may be a subtle Beta Cephei type variable. Single-star evolution is difficult enough to deal with, but when you put two stars together that toss mass back and forth, the rules all change and you can get a remarkable variety of individual cases that defy easy categorization. (This story taken largely from papers by R. W. Hilditch in "The Observatory" and S. Saad and M. Nouh in the Bull. of the Astr. Soc. of India.)
Written by Jim Kaler 7/20/12. Return to STARS.