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.
Written by Jim Kaler. Return to STARS.