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January 15, 2013

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 stars in the system really are related, because they were born from a single giant cloud of gas and dust, probably around 200 million years ago.

Two pairs 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, while the other star in each pair is smaller, fainter, and less massive than the Sun.

The stars in each pair 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 “fingerprints” of the stars as they go around each other, providing details on each star.

The third set of Castor twins is identical. Each star is smaller, cooler, and less massive than the Sun, and much fainter. And while the other two sets of twins are relatively close to each other, this set is so far away that it takes thousands of years to orbit the others — a set of twins that keeps its distances from its brighter siblings.

Castor is low in the eastern sky at nightfall, not far above Gemini’s other twin, Pollux. Castor is slightly fainter than Pollux, and shines pure white.

More about Pollux tomorrow.


Script by Damond Benningfield, Copyright 2012

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