Moon and Twins

In November, you can spot Castor and Pollux in the constellation Gemini, ascending in the eastern sky. They’re noticeable for being bright and close together on the sky’s dome. Most people know just two bright stars in the constellation Gemini.

Castor and Pollux represent the starry eyes of the two mythological Gemini Twins. Pollux is slightly brighter than Castor, but Castor is the more interesting star. If you peered at this star with a telescope, you’d find two stars of nearly equal brightness. These two stars revolve around each other every 340 years. Also through a telescope, you could see a third star that shares a common motion with the first two through space.

So Castor is at least three stars. But that’s not the end of the story. Spectroscopic analysis has revealed that each of these three stars is also a double star. In other words, the single point of light we see as Castor has been revealed by astronomers to be at least six stars — a complex star system — all bound by gravity.

So that’s what the eye sees as a single star, Castor in Gemini. If you look with just your eyes, you’ll find it near the star we mentioned earlier — Pollux in Gemini. These two bright stars are in the east by about 10 p.m. tonight, as seen from around the globe.

There are other bright stars — and a planet, Saturn — also in that part of the sky — tonight, not far from the moon. But you can recognize Castor and Pollux. They’re bright and noticeable for being close together on the sky’s dome.

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