ETA HER (Eta Herculis). An otherwise ordinary fourth magnitude (3.53, just across the line from third magnitude) class G (G8) giant some 112 light years away, Eta Herculis is perhaps best known for its placement, as it lies at the northwestern corner of Hercules' famed asterism, the "Keystone," which joins the star to Zeta Her (at the southwestern corner), Epsilon (southeastern), and Pi (northeastern). The star is also the gateway to the Great Cluster in Hercules, Messier 13, the most famed of northern globular clusters, which lies just to the south of it on the line to Zeta Her. Readily visible in binoculars, this grand cluster of a half a million stars, 25,000 light years away, is a marvelous sight through even a small telescope. Eta Her itself is a core-helium-fusing "clump star," so called because of so many others that lie near its position on a graph of luminosity and temperature. With a temperature of 4900 Kelvin, the star shines with the light of 50 Suns (after allowing for a significant amount of infrared radiation), which leads to a radius of 9.8 times that of the Sun and a mass of 2.3 solar. About a billion years old, the star started life as a class A0 dwarf rather similar to Vega (showing us what Vega will someday become). The interferometer reveals a disk 0.00256 seconds of arc across, which with the distance gives a physical radius of 9.5 times that of the Sun, very close to that derived from temperature and luminosity, showing that the various parameters are close to the mark. The only other significant characteristic is a depressed metal content, the iron abundance about 60 percent that of the Sun. Nearly two minutes of arc away is a purported 12.5 magnitude "companion," which if a real binary neighbor would have to be an M3 dwarf separated by at least 4000 AU with an orbital period of at least 155,000 years. However, measurements taken between 1873 and 1998 show that the separation is increasing at far too great a rate for an orbiting star, in turn showing that the alignment between the two is just an accident.
Written by Jim Kaler 10/10/08. Return to STARS.