Several of the brightest stars in the night sky are systems of at least two stars. But only one of them consists of six stars — Castor, the fainter of the twins of Gemini. It’s about a third of the way up the eastern sky at nightfall, close to the upper left of Gemini’s brighter twin, Pollux. And tonight, the Moon is off to the upper right of the twins.

Seen across 50 light-years of space, the light from all six stars blurs together to form a single point. But binoculars reveal two points, known as Castor A and Castor B. Castor A is the brighter of the two.

And it turns out that A and B are both binaries — each of them consists of two stars. The stars in each pair are quite close together, so there’s no way to see them individually. Instead, their dual nature is revealed by sensitive scientific instruments.

Each pair consists of a star that’s bigger, hotter, and brighter than the Sun, plus a star that’s much smaller, cooler, and fainter than the Sun — a cosmic ember known as a red dwarf. Because of our viewing angle, the distance between the two pairs is increasing, so it’s easier to see them as separate points of light.

Castor’s third pair, Castor C, is a long way away from the other two. Both of its stars are red dwarfs, so the pair is quite faint — you need a telescope to pick it out. Even so, it helps make Castor the busiest bright star system in the night sky — three pairs of “twins” for the twins’ constellation.

Script by Damond Benningfield

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