GAMMA HER (Gamma Herculis). Gamma Herculis, which has no proper
name, is one of those stars that graphically illustrates that
Johannes Bayer did NOT always go by apparent brightness when in the
early seventeenth century he applied Greek
letters. Indeed such ranking is approximate at best in any of
the constellations. Hercules' top two
stars are Beta (Kornephoros) and Zeta, while Gamma, at number nine, is far
down the brightness list, even beat out by Pi Herculis, named after
the 16th letter in the Greek alphabet. The top three letters in
fact all fall into the southern part of the figure, showing that
position was as important as brightness. Gamma Herculis is on the
cool side of class A, at A9 right at the boundary with the class F
stars. Usually considered a giant, it has also been classed as an
F (F0) subgiant. From a distance of 195 light years, it shines at
us with the luminosity of 92 Suns (after
adjustment for five percent dimming by interstellar dust) from a
surface of 7050 Kelvin (which is more in keeping with the "F"
classification). The star may therefore be slightly metal-poor,
its spectrum fooling us into thinking it is hotter than actually
measured. From these figures we derive a radius 6 times that of
the Sun and a mass 2.6 times solar. The star, about half a billion
years old, has clearly stopped fusing hydrogen and helium in its
core and is now -- with a dead helium core -- in transition to
becoming a much brighter red giant. In less than 8 million years,
when it begins the fusion of its core helium into carbon, it will
be 3.5 times more luminous than today. It will then fade to roughly
its current luminosity while helium fusion takes
place, and then just before it finally dies with a dead carbon
core, it will become almost 1000 times more luminous than the Sun.
Gamma Her seems to have a faint companion under a minute of arc
away, but it is only a line of sight coincidence. It does,
however, have a real (though lesser) companion about which nothing
is known in a tight 11.9 day orbit, which implies an orbital radius
of only 0.15 Astronomical Units. Gamma Her may also be slightly
variable, changing its brightness by about 5 percent over a period
of a couple hundred days, but the observation has not been
confirmed.