A Close Look at Gemini

By Bob Moler



The constellation Gemini with labels of the objects discussed. Created using Cartes du Ciel (Sky Charts).

This is a rewrite of an article first posted here and in the “Stellar Sentinel” in February 1999 as “Gemini the twins?”.

One of the constellations that surround the great central winter constellation Orion is Gemini, also known as the Twins. The brothers Castor and Pollux lie above left of the giant hunter. Though not as grand as Orion, Gemini has a rich mythology and great imagined importance as a member of the Zodiac.

According to Greek mythology their mother was Leda. Pollux was the result of the famous Leda and the Swan affair, when Zeus (the Roman Jupiter) disguised as a swan seduced Leda. Pollux then was immortal, like his father. Castor on the other hand was mortal. Though some authors say Zeus was his father too, others site Tyndareus, who had relations with Leda the same night. And rather than twins, Leda bore quadruplets. Pollux and Helen, the famous Helen of Troy were fathered by Zeus; while Castor and Clytemnestra who became the wife of Agamemnon the commander of the Greek forces of the Trojan war, were the mortal offspring of Tyndareus. Greek mythology tends to read like modern soap opera, only with even more sex and violence.

Castor was a horseman, and could tame the wildest horse. Pollux was a boxer. the brothers were very close and shared many adventures. In their last quest they shipped out with Jason and the rest of the Argonauts in search of the Golden Fleece. Castor was killed in battle with Idas, who was in turn slain by Zeus himself with a lightning bolt. Pollux was inconsolable by the death of Castor and petitioned his father to let him die too. Zeus, touched by his son's devotion placed them both in the heavens as the constellation of Gemini. The god Poseidon (the Roman Neptune) gave them power over the winds and waves, and so they were worshiped by sailors. In the Acts of the Apostles, Chapter 28 verse 11 states that St. Paul, after having been previously shipwrecked, boarded a ship whose sign or figurehead was Castor and Pollux.

Speaking of the two brothers, their namesake stars lie at their respective heads. Pollux is a tiny bit brighter than Castor. Pollux is magnitude 1.14, while Castor is 1.58, rounding. Pollux is a yellow-orange star of spectral type K0, much like the brighter Arcturus in Boötes. Pollux lies at a distance of 33.8 light years.

Pollux is a single star. Castor, however, actually contains six stars. They are arranged in three close pairs. Each pair is a spectroscopic binary, the individual components only discernible with a spectroscope by their Doppler shifts. The two brighter pairs are called Castor A (magnitude 1.9) and Castor B (magnitude 3.0) which are currently about 3.9 seconds of arc apart and slowly increasing. The Castor C pair lies nearly twice the apparent diameter of Jupiter (71”) to the southeast of the main pair and is of magnitude 9.

The main pair of Castor A and B is difficult to separate in small telescopes. The NMC Observatory's telescopes can do it. Smaller telescopes require good optics, high power, steady skies and patience to resolve them. The stars of Castor are at a distance of 51.5 light years

Another interesting star of Gemini is Zeta (). It is a Cephied variable star which varies from magnitude 3.68 to 4.16 in 10.148 days.

The brothers appear to be wading in the Milky Way, so Gemini contains objects which are found in and near the milky band: star clusters and nebulae. Here are a couple.

The best star cluster in Gemini, and only Messier object is M 35 a galactic star cluster, near Castor's foot. The cluster is huge, and has been traced out to the apparent diameter of the moon, though visually it appears about half that size The cluster contains several hundred stars and lies at a distance of about 3,870 light years. M 35 appears as a fuzzy spot in binoculars, but is easily resolved in any astronomical telescope.

Just half a degree southwest of M 35 is the faint and distant star cluster NGC 2158. This compact cluster, not quite resolvable in a 6 inch telescope, is about 11,000 light years away and appears rather old for a galactic or open cluster. The winter sky looks out to the outer regions of the Milky Way, so NGC 2158 must be nearly half again as far as the sun's 25,000 light years from the center of the Galaxy.

NGC 2392, the Clown Face Nebula, is a small planetary nebula that appears about the size of Jupiter in telescopes. It is magnitude 10.1 and appears blue-green in color. The color is due to the Hydrogen Beta and Oxygen III emission lines. In large telescopes the glow of the main elliptical mass is surrounded by a faint ring of light.

Gemini is also the constellation in which Pluto was discovered in 1930 by Clyde Tombaugh.


Questions? Send Email to me at bob@bjmoler.org

Updated: 02/01/21