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.)