Articles: Gemini, The Twins

By Michael O'Connell, MAC

Gemini, the Twins, is one of the twelve constellations of the zodiac. The constellation forms two parallel lines with it’s brightest stars on the eastern side. From ancient mythology, these two stars Castor and Pollux represent the twins heads while the remainder of the stars represent their bodies. The idea of twins is quite fitting actually; both Castor and Pollux are of similar brightness, the lines representing their bodies are of similar length and have stars of similar magnitude. In ancient times Castor and Pollux were thought to represent the protectors of sailors during their long voyages across the seas. Their presence on a ship was believed to be associated with flames from the ship’s mast (a phenomena now understood to be caused by lighting and now called St. Elmo’s fire).

As mentioned earlier, the brightest stars in Gemini are Castor and Pollux. People often get confused as to which star is which. Personally, when I look at Gemini I think of an alphabet aligned vertically with A at the top and Z at the bottom. This way, Castor would appear “above” Pollux, just like it does in the sky. Castor, which is officially designated as a Gem is actually the second brightest star in the constellation. Normally, the brightest star is designated as a (alpha) ,the second brightest b (beta) and so on. However, when Johann Bayer was preparing his official Greek letter designation according to star magnitude, he made an error and accidentally “robbed” Pollux of it’s true status. This however was an easy mistake to make due to their similar magnitude.

To the naked eye, Castor appears as a bluey-white star of magnitude 1.6. However, Castor is a highly complex multiple star system with a total of SIX stars! Training a small scope at Castor will reveal two stars; one at mag. 1.9 and the other at mag. 3.0. These stars orbit each other once every 470 years. Each of these stars also have a star orbiting very close to them, however they cannot be resolved telescopically. Finally, a red dwarf star can also be seen nearby in a telescope. This star is a binary star system bringing to a total of six stars!

Pollux on the other hand appears as an orangey-white star, Although Pollux and Castor appear at similar magnitude, Pollux is 34 light years from Earth while Castor is 52 light years. This would indicate that Castor is a much bigger and brighter star than Pollux.

Wasat, d (delta) Gem, is a white star of magnitude 3.5. Wasat is located midway in the “body” of the lower “twin”. Wasat is a double star with a faint orange companion of mag. 8.2. These two stars are gravitationally bound together and orbit each other once every 1000 years.

Alzirr, x (zeta) Gem, is located just below the “foot” of the lower “twin” in the lower-right corner of Gemini. This star is not only a binary system but is also a variable star. The star appears slightly yellow and is actually a Cepheid variable with it’s brightness varying from 3.6 to 4.2 every 10 days. A small scope will show a nearby mag. 7.6 star which is not gravitationally linked. Alzirr is extremely distant in comparison to the remainder of naked eye stars in Gemini at a whopping 1,400 light years away!

The western side of Gemini just skims the edge of the Milky Way. As a result, we have a lovely open cluster, M35, to look at. M35 is located just above the “foot” of the upper “twin” in the upper-right corner of Gemini. M35 is an open cluster located within our own galaxy at a distance of 2,800 light years. From a dark sky this cluster appears as a small fuzzy patch to the naked eye. Small telescopes will reveal a dozen or so stars while a larger scope will show M35 in all it’s glory. When looking at M35 with a small scope, make sure to look out for a line of stars which appear to form a “chain”.

Finally we go to one of the most photographed objects in the entire night sky – NGC 2392 or more commonly known as the Eskimo Nebula. In a telescope, this nebula appears as a bluey-green oval disc. Don’t be afraid to try relatively high magnification as more detail will be seen due to the brightness of the nebula. Larger telescopes begin to show more detail although none can compare to amazing images captured by the Hubble Space telescope.


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