95 HER (95 Herculis). Not just a double star, 95 Herculis (in far eastern Hercules and known best by its Flamsteed Number) is a glorious double that allows a two-for-one special. It's also quite unusual in that, rather like Capella, it consists of a pair of fifth magnitude (4.96 for 95 Herculis A, 5.18 for 95 Her B) giants separated by 6.3 seconds of arc that combine into what appears as a single fourth magnitude (4.31) star at a healthy distance of 470 light years. Though both are evolving stars that have given up core hydrogen fusion, they are nevertheless in different stages of life, rendering them slightly different colors, 95 Her A being a relatively uncommon white class A (A5) giant, 95 Her B a class G (G8) yellow-white giant. Their proximity and subtle brightness difference, however, greatly magnifies the color contrast to the eye, which resulted in vivid early descriptions, Admiral Smythe calling them "apple-green and cherry-red" (one of the reasons people love looking at double stars). Some astronomers even thought the colors changed with time. They do of course, but not in any human lifetime. With a temperature (estimated from spectral class) of 8000 Kelvin, the warmer and brighter star (95 Her A) shines with a total luminosity 167 times that of the Sun. While 95 Her B is somewhat dimmer to the eye, when its infrared radiation (the result of its cooler 4900 Kelvin temperature) is taken into account, it actually comes out as the more luminous star (194 solar). It is also the larger, 19.4 solar diameters as opposed to 6.8 for 95 Her A. As would be expected from its spectral class and smaller size, 95 Her A is by far the faster rotator, spinning at its equator with a speed of at least 233 kilometers per second, while 95 Her B ponderously spins at (minimum) 5.7 km/s, giving them respective highly contrasting rotation periods of less than 1.5 and 170 days (the latter quite expected for a large class G giant). Masses are similar, 2.8 solar for 95 Her A and 3.2 solar for 95 Her B, as expected, since higher mass stars evolve earlier and 95 Her B is the more advanced (the pair about half a billion years old). While 95 Her A has recently given up hydrogen fusion and is in a transition state in which it is rapidly (on an astronomical time scale) expanding and cooling, 95 Her B has either just completed its transition or is happily now fusing its core helium into carbon and oxygen. The metal content of 95 B is a bit low, 60 percent that of the Sun, while that of 95 A is not known. Together, they move about twice as fast relative to the Sun than average. While no orbital motion has been detected, the separation between the two has stayed the same for the past 180 or so years, showing that they are travelling through space together. The consistency of their computed evolutionary states also shows them to be at the same distance and that they are clearly a real binary pair. With a separation of at least 900 Astronomical Units, they would (given their masses) take at least 11,000 years to make a full orbit around each other, so it is no surprise that we have not seen relative orbital movement. If you can, find a telescope and admire them. Do you see apples and cherries too? (Thanks to Jenny Hall, who suggested this star.)
Written by Jim Kaler. Return to STARS.