To the eye alone, the star Castor looks like a single pinpoint of light. It’s one of the “twins” of Gemini. But Castor takes the “twins” business to an extreme. Instead of a single star, it appears to be a system of six stars, consisting of three binaries – three sets of twins.
The sets are known as Castor A, B, and C. The members of each pair are so close together that not even the biggest telescopes can see them as individual stars. Astronomers use special instruments to determine the details of each star.
Castor A and Castor B each consist of a star that’s bigger, heavier, and brighter than the Sun, plus a small, faint companion.
A and B orbit each other once every 450 years or so. Astronomers have been watching the two binaries for hundreds of years. That means they’ve seen enough of an orbit to know for sure that the two binaries are gravitationally bound to each other.
Both of the stars of Castor C are small and faint. The system appears to orbit the other two binaries about every 14,000 years. But astronomers haven’t been watching Castor C as long as the others. So they still can’t be completely certain that Castor C is tied to the others – making one of the “twins” of Gemini a family of three sets of twins.
Look for Gemini near the Moon at nightfall. Castor is to the left of the Moon, with brighter Pollux to the Moon’s lower left. Mars rises well below them, and we’ll talk about it tomorrow.
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