Sextuplets are rare — only a couple of hundred sets have been recorded. And they’re rare among stars as well — astronomers have recorded only about a dozen of them.

The most famous is Castor, one of the twins of Gemini. It’s one of the two dozen brightest star systems in the night sky, so it’s easy to pick out.

The system consists of three sets of twins. The stars in each set are so close together that we can’t see them as individual stars, even through the largest telescopes.

Astronomers know they’re twins, though, because they see the “fingerprints” of two stars in their spectra — the breakdown of their individual wavelengths of light. As the stars orbit each other, the imprints of the two stars shift back and forth a bit.

Two of the sets of twins are almost identical to each other. Each one consists of a star that’s bigger, heavier, and hotter than the Sun, plus a star that’s smaller, lighter, and cooler than the Sun. The two systems orbit each other once every 450 years or so.

The third set of twins consists of nearly identical stars. Both are about the same as the fainter stars in the other two sets. And this pair is so far from the others that it takes at least 14,000 years for them to make a single orbit.

Castor and Gemini’s other “twin” star, Pollux, line up to the upper left of the Moon at nightfall. Pollux is closer to the Moon, and is brighter than Castor — the best-known sextuplet in the night sky.

Script by Damond Benningfield

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