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Moon and Companions

August 3, 2014

The first-quarter Moon snuggles close to two pinpoints of light this evening. A faint star with a great name — Zubenelgenubi — is close to the left of the Moon, with the brighter planet Saturn a little farther to the upper left.

Zubenelgenubi is the second-brightest star of Libra, the balance scales. Its name has nothing to do with the scales, though. Instead, it means “the southern claw.” In ancient times, the star represented one of the claws of Scorpius, which is to the east of Libra. But the Romans stripped away the scorpion’s claws to create the scales.

If you have good eyes and dark skies, you might just see Zubenelgenubi as two stars. They appear to be gravitationally bound to each other, so they travel through space as a pair. That means they most likely were born together, from the same giant cloud of gas and dust.

Such widely separated binaries are fairly unusual. Most binaries are much closer together — from millions to perhaps a few billion miles apart. In fact, each of the two members of Zubenelgenubi is itself a binary, with the two stars quite close together.

It’s difficult for wide-spread binaries to stick together. Their gravitational grip on each other is weak, and they’re constantly being tugged by the gravity of the rest of the galaxy. So such systems frequently split apart and go their own ways — a fate that may someday befall the two binaries of Zubenelgenubi.

More about the Moon and Saturn tomorrow.

Script by Damond Benningfield, Copyright 2014

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