A Wolf at the Door

My post on multiple star systems a while back somehow managed to miss one of the most interesting in the sky. Gamma Velorum (γ Vel), at a magnitude of +1.7, is amongst the brightest visible stars — it’s actually a huge star system containing at least 6 stars. Currently drifting as part of the Vela OB2 Associtaion, the main star in the group is a sizeable one.

Well, I say one. The main component, γ Vel A, is actually a spectroscopic binary. The main star is an intensely hot O-type blue supergiant, weighing in at 30 solar masses. Orbiting perilously close, a mere 1AU away, its binary companion is a carbon-rich Wolf-Rayet star.

Wolf-Rayet stars (sometimes WR stars, for short) are fascinating. Fascinating and a little bit scary. As massive stars evolve, they turn into Wolf-Rayets with surface temperatures of up to 50,000K. Such high temperatures cause enormous radiation pressure, which drives an intense stellar wind. This causes these stars to shed mass at a prodigous rate (about a solar mass every 10 thousand years or so). This image on the right is actually of the star WR 124, but it illustrates the point rather well. Those clouds around it are a Wolf-Rayet nebula — stellar material that’s been thrown off, condensing into gas and dust as it cools. At around 10 solar masses, the WR star in γ Vel A is actually the heaviest Wolf-Rayet star known, although in its glory days it would have been the larger of the two stars at over 40 solar masses. As with all WR stars, it’s ill-fated to eventually die as a supernova. Unless it sheds enough mass, it could even end up as a gamma ray burst.

As a result of the extreme temperature (and hence, ultraviolet output) of both components of γ Vel A, it has a very pretty optical spectrum. No dark absorptions. Instead, brilliant emission lines caused by ultraviolet excitation. The line in the blue betrays a lot of hot carbon in this star’s atmosphere. Poetically referred to as “the spectral gem of the southern skies”, 19th century astronomer Agnes Clerke had the following to say about it:

“An intensely bright line in the blue, and the gorgeous group of three bright lines in the yellow and orange, render the spectrum …incomparably the most brilliant and striking in the whole heavens. …a vivid continuous spectrum extends into the violet as far as the eye has power to follow it, and accounts for the brilliant whiteness of the star.”

(It’s also notable that the H-α line in the red is actually quite weak — WR stars tend to be slightly deficient in hydrogen as a result of them puffing it all away into space.)

Trailing the main binary pair is a small entourage of companion stars. They’re all distant enough to be easily resolvable with a pair of binoculars or a small telescope, making γ Vel a good target for any amateur stellar astronomers to observe. γ Vel B is a B-type subgiant, which lies 41.2 arcseconds away. γ Vel C, an A-star not unlike Sirius, sits 62.3 arcseconds away. γ Vel D and E are a second binary, more distant at 93.5 arcseconds away. D is another A star, while E is a practically unnoticeable magnitude +13 star just 1.8 arcseconds away from it.

It’s interesting to wonder about the full extent of a star system this size. Potentially there could be a number of other low mass companions in tow, too faint to be easily picked out. Red dwarfs or brown dwarfs. Planets, perhaps. While the hot central pair of stars would likely have blown away any planet forming material which was too close to them, this wouldn’t preclude the possibility of such objects forming further out around their less massive siblings. Given how resilient processes like star formation can be, I really wouldn’t be surprised.

Image Credits
WR 124 — ESO
Spectrum — Harry Roberts

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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