GAMMA COL (Gamma Columbae). Dropping south of Orion (or if in the southern hemisphere,
climbing up from) we run into the Hunter's prey, Lepus (the Hare) and then into the pretty
triangle that makes the most prominent part of the modern constellation, Columba (the Dove), composed of the three
brightest stars, Alpha, Beta, and Epsilon Columbae. Gamma Col,
shoveled off 1.5 or so degrees east-northeast of the third magnitude
(2.64) luminary Alpha, oddly ranks not third in the constellation, but
sixth. Indeed, at fourth magnitude (4.36) it just barely beats out
Kappa (4.37). But it's still the Gamma star, so we should take a look
at it. And as a quite-hot class B (B2.5) subgiant, it's certainly
worthy of attention. And it's received it, as the temperature of
17,100 Kelvin is quite well known. The hefty distance of 870 light
years (plus or minus 42) is not badly determined either. Rather well
to the west of the Milky Way, the star is subject to only 0.19 magnitudes
of dimming by interstellar dust.
Adding in a lot of ultraviolet
radiation, Gamma Col shines with a magnificent luminosity of 5010 times
that of the Sun, which leads to a radius of 8.1
times the solar value. The only statement of projected equatorial
rotation velocity is an uncertain 96 kilometers per second, which
loosely defines a rotation period of under 4.2 days. The theory of
stellar structure and evolution gives a mass of 7.0 Suns and shows that
at an age or 43 million years the star has, or will shortly, run out
of hydrogen fuel in its core. (As much as 7.2 Suns is possible depending
on the exact state of ageing.) It will soon expand and brighten to
become a giant star as it
prepares to ignite its internal helium (the ash of the original
hydrogen fusion) into a mix of carbon and oxygen, the star eventually
dying as a white dwarf of
about a solar mass, its outer layers having been ejected back into
interstellar space (in the process forming glowing planetary nebula). That might be the end of it. But
Gamma Col has a dim 13th magnitude (12.7) companion of considerable
interest. At an angular separation of 34 seconds of arc, over the past
century and a half little Gamma B has tracked Gamma A pretty well and
seems to be really connected. If so it is somewhat dimmer than the Sun,
a G7 or so dwarf with a luminosity two-thirds solar. But if it truly
belongs to the brighter star, then it must also be very young (42
million years old). As dwarfs age, they slowly brighten. When the Sun
was young its luminosity was about 70 percent what it is today (giving
us the "young Sun paradox": how could life have developed with a frozen
Earth?). Gamma Col B thus carries about a solar mass, a star so young
that it is considered a "post T-Tauri star," one that has recently (or
still in the act of) settling down onto the hydrogen-fusing "main sequence" of dwarfs. So we have a double in
which one massive star is starting to die while at the same time the
other is still frolicking away in exuberant youth. At Gamma Col's
distance, the pair must be separated by at least 9000 Astronomical
Units and from Kepler's Laws must take at least 300,000 years to orbit
each other. If the physical separation is correct, then from the
bright star, little Gamma B would shine with the light of four times
that of our Venus, while From B, A
would appear as bright as eight full
moons.
Written byJim Kaler 1/29/16. Return to STARS.