EPS HER (Epsilon Herculis). The "rule" that Greek letter names in a constellation are in order of brightness is as often broken as it is followed. Hercules is a prime example, in which the brightest star is Beta Her (Kornephoros), the second brightest un- named Zeta. Instead, Bayer seems to have lettered the stars by giving Alpha to Hercules' head at the south; he then worked his way northward, which is why fourth magnitude (3.92) Epsilon at the southeastern corner of the "Keystone" is right next to much brighter (2.81) Zeta at the southwestern corner. Though seemingly another ordinary class A (A0) hydrogen-fusing dwarf, it's a curious and controversial double that has been rumored (probably falsely) to be triple. There is no question, though, about the star's status as a spectroscopic double (one known through the velocities of its components) with an orbital period of 4.0235 days. The pair has been classed as A0 plus A2. The temperature of the primary star, Eps Her A, 10,435 Kelvin, is normal for the class, but that deduced for the secondary (10,120) is too warm and suggests that it too is a slightly fainter A0 star. The combined luminosity then comes out to be 72 times that of the Sun. We next have two choices. The first comes from a measure that Eps Her A is four times brighter than Eps Her B, which gives luminosities for the pair of 58 and 14 Suns. However, according to the theory of stellar structure, Eps Her B is then impossibly faint for its temperature (either that or the temperature is wrong). A better approximation might be to assume that they are identical, each shining at 35 Suns. Radii then come in at 1.8 and 1.9 Suns, and the masses each at 2.5 Suns. From the masses and orbital period, the separation is just 0.085 Astronomical Units, 20 percent of Mercury's distance from the Sun. Not surprisingly, the orbital eccentricity is near zero, a mere two percent. A projected equatorial rotation speed of 69 kilometers per second leads to a rotation period under 1.3 day, notably less than the orbital period, which probably relates to Eps A (which in spite of the above approximation is no doubt the brighter). Various reports of one or more visually-observed components have not panned out. Epsilon Herculis has also been catalogued as a "Lambda Bootis star," one that is depleted in various metals as a result of accreting metal-poor gas, the metals bound up in interstellar dust. Others say no. The star clearly needs much more work.
Written by Jim Kaler 8/24/07. Return to STARS.