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.