The star Castor represents one of the twins of Gemini. But it takes the business of “twinhood” to extremes. Although we see it as a single point of light, Castor actually consists of three sets of stellar twins — a total of six stars in all.

All six of them really are related. They were born from a single giant cloud of gas and dust, probably a few hundred million years ago.

Two sets of Castor’s twins are more fraternal than identical. One star in each pair is a good bit bigger, brighter, and heavier than the Sun. The other is smaller, fainter, and less massive than the Sun.

The stars in these pairs are so close together that they orbit each other in just a few days. At such close range, there’s no way for telescopes to see them as individual stars. Instead, special instruments separate the “barcodes” of the stars as they go around each other, providing details on each star.

The stars in the third pair of Castor’s twins are identical. Each star is smaller, cooler, and less massive than the Sun, and much fainter. And while the other two sets of twins are fairly close to each other, this pair is a long way away. It takes thousands of years for it to orbit the others — a set of twins that keeps its distance from its brighter siblings.

The twins of Gemini are in the east at nightfall, near the Moon. The brighter twin, Pollux, is above the Moon, with Castor about the same distance to the upper left of Pollux.

Script by Damond Benningfield

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