The unequal twins of Castor and Pollux
12/29/2013 4:06 PM
I'm a bit partial to Gemini. I don't believe in astrology at all, but my "sign" is Gemini, so I've known about the twins for a very long time, though less-so about the stars and astronomy of this region until much more recently in life. What fascinates me is how un-twin-like these two stars are: Pollux is this orange-looking, K-class star in the later phases of of it's life, orbited by a planet some 2.3 times the size of Jupiter. Castor, by contrast, is a six-star system composed of four A-class stars. The ones we see visually naked-eye on the sky is really 2 pairs of 2 spectroscopic binaries. We can split the "A" and "B" pairs with sufficient magnification, but not all four stars because Aa and Ab are too close together, as are Ba and Bb.
So how to observe these? Well, I like to start with Pollux. It really does look orange in the eyepiece. Now, star colors can be a bit more subtle than looking at primary color paint chips at a home improvement store, but I think Pollux has some real color going for it here. At least that was my impression the other night through both a 6" reflector and 10" reflector the following evening.
But the real fun aspect is then hopping up to Castor. The A-class stars glow with a white that, to my eyes, looks even slightly bluish-white in comparison. Now that could be because the nearby Rho Geminorum, a 4-th magnitude F-class star, appears more pure white in color by comparison. And at 200x to split the Castor "A" and "B" stars, I detected a hint of the blue around them - perhaps an artifact of my eyepieces (Orion Expanse 6mm and TMB Planetary II 5mm), but it certainly wasn't from the primary mirror. In any case, these are well worth checking out visually and telescopically during the winter months.
And if you'll be on your way towards Messier 35 at the foot of the twins, check out the color of Mu Gem and Eta Gem while you're at it. Though slightly dimmer, these are M-class red giant stars, and should have an even deeper orange-hue to them than Pollux does. They'll also help you appreciate the color of Castor as well. For more info on these stars and Gemini, check out this video.
Looking for more great objects to see in the night sky? How about gazing at Messier 35 in Gemini, or perhaps Orion's nebulae M42 & M43, the open cluster NGC1981, and the double and multiple stars of Orion's sword? For binocular stargazers, there's always Davis' Dog in Taurus.