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Moon and Gemini

February 13, 2022

Pollux and Castor, the twins of Gemini, stay close to the gibbous Moon tonight. They line up to the upper left of the Moon as night falls, with Pollux closer to the Moon. The twins are to the right of the Moon as they set, in the wee hours of the morning.

Although they’re known as twins, Pollux and Castor aren’t related at all. Pollux is about 34 light-years from Earth, while Castor is half-again that far — 51 light-years. That means they’re more than 17 light-years apart.

There’s no indication that the stars are related by birth, either. Pollux is a good bit older than Castor. That means they probably didn’t come from the same original star cluster. So their only real connection is their chance alignment in our sky.

The stars also have different characters.

Pollux is a single star — a bloated giant near the end of its life. Castor, on the other hand, is a system of six stars — one of only a dozen or so known sextuple stars. Two of its stars are bigger, brighter, and heavier than the Sun. The other four are almost identical to each other. They’re red dwarfs — the most-common variety of star in the galaxy. They’re smaller, fainter, and less massive than the Sun.

Seen from Earth, Pollux is about half-again as bright as Castor. If you lined them up at the same distance, though, Castor would be the brighter star, and by almost exactly the same amount — making it the outstanding “twin” of Gemini.

Script by Damond Benningfield

Today's program was made possible by Dr. Cameron Mitchell, professor emeritus, supporting research and outreach at McDonald Observatory.

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