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March 1, 2014

Sextuplets are as rare among stars as they are among people. Even so, a famous set of sextuplets arcs high across the sky on March evenings: Castor, the second-brightest star of Gemini. It’s high in the east at nightfall, close to Pollux, Gemini’s other bright twin.

All six of the stars of Castor appear to be moving through space as a group, bound to each other by their gravity. That suggests the stars all formed at the same time, from a single giant cloud of gas and dust.

Yet the stars aren’t clumped together. Instead, they’re separated into three pairs. The stars in each pair orbit each other, while the three pairs also orbit each other in a complex ballet.

Two of the pairs are fairly close to each other. Each pair consists of one big, bright, hot star plus one small, faint, cool star. The light of the two brighter stars merges to form the pinpoint of light that’s visible to the unaided eye.

Both of the stars of the final pair are small, cool, and faint. And they’re so far from the other two pairs that it takes light about a week to span the vast gulf between them.

There’s evidence that all of Castor’s stars once belonged to a much larger stellar family, which includes several of the night sky’s brightest stars. Over time, though, the star cluster was pulled apart. Most of the stars went their separate ways, but the stars of Castor stuck together — forming a widespread set of sextuplets.

We’ll talk about a companion for Pollux tomorrow.


Script by Damond Benningfield, Copyright 2014

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