Gemini: The Twins

Star chart. Diagram by Eric Fitzgerald

Learning the constellations isn’t hard if you do it bit by bit. The idea here in The Backyard Astronomer is to introduce you to the big, bright constellations first, building one on another. You can always refresh your memory of previous stars and their patterns explored in past columns by searching the M’Online website: 

Next up in the cosmic seasonal procession is Gemini, the twins. Gemini is the winter constellation that heralds all the spring stars rising in the east throughout the night.

It’s an easy constellation to spot because it is marked by, as you might guess, two fairly bright stars: Pollux and Castor. The Twins in Greek mythology are the children of Leda. Pollux was the offspring produced by that infamous meeting of Leda and the swan (Zeus in disguise). Castor was the son of Leda’s husband Tyndareus, king of Sparta. Because Pollux was immortal and Castor was not, Pollux hit up the old man to make his brother immortal too. That’s how they both ended up in the stars.

To find Gemini, locate Orion and the Winter Triangle (Sirius, Betelgeuse, and Procyon) in the southwest sky shortly after dark. Look above the top of the triangle and locate Pollux and Castor nearly overhead. It’s easy to remember which one is Pollux because he is on the Procyon side of the pair (Refer to Star Chart).

Fittingly, Castor is a spectacular double star and is easily resolvable in a small telescope at 100x. Castor A and Castor B, as the two components are named, revolve around one another. When I first started pointing my telescope at the sky in the 1960s, the double was quite hard to resolve at 1.8 arc seconds (an arc second is 1/60th of an arc minute, which is itself 1/60th of a degree. For comparison the full moon is about 30 minutes across). Currently the separation of the double is 4.8 arc seconds and increasing.

Castor A and Castor B are a “visual binary.” Rather than just appearing close together because they happen to be in the same line of sight here on Earth, they actually orbit one another in a multiple star system. There is a very faint third star, Castor C, which makes the system a triple visual binary. Staggeringly, all three stars are “spectroscopic” doubles, or stars that only reveal their very close companions in their spectra. This amazing star system is actually a triple-double system of six stars!

So, we’ve wandered down the Zodiac, or the path of the sun, moon, and planets through the sky from Taurus to Gemini. The next constellation of the Zodiac is a faint one, Cancer, the Crab. (See the star chart). Cancer is a small, faint constellation with a pretty open cluster, M44, or The Beehive, near his center. To the naked eye on a dark, moonless night, The Beehive might look like a small fuzzy spot. However, a small telescope or a pair of binoculars will reveal a glittering open cluster. It is among the first objects that Galileo pointed his telescope at. M44 is also sometimes called Praesepe (Latin for “The Manger” or “Cradle”)

As the weather gets warmer and the skies clear a bit, I hope you will venture out and explore The Twins and The Crab and add M44 to M45 and M41 on your deep sky list as we introduce the ancient stars and their patterns, one by one.

Urania’s Mirror. Illustration by Sydney Hall


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