The above sky chart shows the wide waxing crescent moon on April 17, 2013, near the bright stars Castors and Pollux in the constellation Gemini – and also the star Procyon in Canis Minor the Lesser Dog. Castor and Pollux, the beacon lights of the constellation Gemini the Twins, are sometimes called ‘twin’ stars – allthough the kinship of these stars is more imaginary than real. Why twins then? These shimmering luminaries are named in honor of mythological twin brothers, Castor and Pollux.
As seen from Earth, the stars Castor and Pollux appear along the same line of sight by happenstance. These Gemini stars are not physically related or close together in space. Pollux resides some 34 light-years from Earth, whereas Castor lies at half again Pollux’s distance, at 52 light-years away.
Castor and Pollux’s contrasting colors reveal that they’re different sorts of stars. Can’t see their different colors? Try using binoculars. Castor is white, while Pollux radiates yellow-orange. Castor is in the prime of life, which means it shines by fusing hydrogen into helium at its core. But Pollux has depleted its hydrogen supply. It is now an aged helium-fusing star, and has changed into a yellow-orange giant. Pollux is the nearest giant star to Earth.
Castor is a magnificent multiple star. It is composed of three sets of binary stars – two stars revolving around a common center of gravity. That makes the single point of light we see as Castor six stars altogether!
And what about Procyon, the other star shown on the chart at the top of this post – near tonight’s moon? Since it’s not close to any other bright stars (in the way that Castor and Pollux are recognizable for being close to each other), it’ll be harder for you to identify. If you do think you’ve spotted it near tonight’s moon, know that it’s the brightest light in Canis Minor, the Lesser Dog. It’s sometimes called the Little Dog Star.
Bottom line: On the night of April 17, 2013, look for the waxing crescent moon close to the Gemini stars, Castor and Pollux, and the Little Dog star Procyon.