Castor and Pollux

Gemini is still well overhead, north and east of the constellation Orion, in the evening hours in March.

Gemini is still well overhead, north and east of the constellation Orion, in the evening hours in March. Its brightest stars are Castor and Pollux. Created with SkyX Serious Astronomer edition by Software Bisque.

Along with Taurus, Gemini is one of the two most northerly constellations of the zodiac. It lies just east of Auriga and the bright star Capella, and it’s marked by the two bright stars Castor and Pollux which lie less than 5º apart (a little less than the width of your three middle fingers held at arm’s length). To find the constellation draw an imaginary line diagonally from Rigel past Betelgeuse in the constellation Orion about a distance equal to the separation of these two bright stars. This will land you smack in the middle of Gemini. In March and April, the constellation lies still well above the western horizon in the early evening hours.

According to Greek mythology, the stars Castor and Pollux take their names from two sons of Zeus who together were known as the Dioscuri. Pollux was a boxer, and Castor a horseman and fierce warrior. The brothers proved their courage by joining Jason and his Argonauts on his quest for the Golden Fleece, among other adventures. Upon returning, Castor and Pollux met their fate during more sordid matters, falling into a murderous grievance with another pair of twins, Idas and Lynceus over rustled cattle. Castor was killed by Lynceus, who was in turn slain by Pollux.

Pollux grieved for his fallen brother and asked Zeus if the two could share immortality. It pays to have a powerful father: Zeus placed both in the sky as the constellation Gemini.

Gemini constellation – Johannes Hevelius, Prodromus Astronomia, volume III: Firmamentum Uranographia, table DD: Gemini, 1690.

Gemini constellation in a star atlas by Johannes Hevelius, circa 1690.

The brightest star in Gemini, yellow-orange Pollux, makes a lovely color contrast with Castor. If you get these stars confused, as I did for years, simply remember that Castor is closest to Capella.

In a small telescope at 80x or more, Castor resolves into two bright stars.  The brighter “A” component lies some 4″ from the slightly fainter “B” component.  These stars revolve about their common center of mass every 400 years.  In 100 years, the stars will increase their apparent separation to about 6″.  At their closest, the two stars lie only 1.8″ apart. Castor A and B are blue-white stars only 45 light-years from the Sun.

A sketch of the star Castor (credit: Jeremy Perez)

A sketch of the star Castor (credit: Jeremy Perez)

There is a third star in the Castor system: about 72” to the south, you’ll see a much fainter red-orange companion in your field of view.  This 9th-magnitude third (or “C”) component is a small star, much smaller than our Sun, that revolves around the two primary stars every 10,000 years or so.

Each of the three stars in the Castor system is itself a double star… so there are six stars here in all.  Castor A and B each have reddish companion stars that hurtle around their brighter primary stars every 9 days.  And the fainter C-component has another red-orange companion less than 2 million miles away; the stars revolve around each other in less than 24 hours.  The close companions of Castor A, B, and C are far too close to resolve directly in any telescope, but they can be discerned by skilled astronomers through the motion of their spectra as they move around their brighter companions.

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