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January 7, 2017

The star known as Castor is like a miniature star cluster. It consists of at least six stars, which are divided into three pairs. Only a few systems are known to have more individual stars.

Castor represents the head of one of the twins of Gemini. Right now, the constellation is low in the eastern sky as night falls, with Castor above Pollux, Gemini’s other twin. They climb high overhead during the night.

A telescope turns this point of light into two points, known as Castor A and Castor B. And a spectrograph — a device that splits a star’s light into its individual wavelengths — reveals that each point actually consists of two stars.

One of the stars of each of these two pairs is bigger, brighter, and heavier than the Sun. In fact, the combined glow of these two stars is what we see as Castor with our eyes alone.

But each of these stars has a companion that’s much smaller and fainter than the Sun. They’re in tight orbits with their more impressive companions, so it’s impossible to see them directly.

The third set of Castor’s twins is known as Castor C. It’s billions of miles away from the other two. Each of its stars is about two-thirds as big and heavy as the Sun, but only about a tenth as bright. Their brightness changes, though, because they produce bright flares of energy and dark magnetic storms that are like sunspots — causing this pair of twins to flicker.

The Castor sextet orbits the galaxy as a family — a small “cluster” of stellar twins.


Script by Damond Benningfield

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