Gemini
The Heavenly Twins
Twins have been featured in stories throughout the ages. From the founders of Rome, Romulus and Remus, to Luke and Leia in the Star Wars saga, twins have always had some significance. Greek mythology is no exception, and it is here that we meet the subjects of our latest Constellation of the Month feature. The twin brothers, Castor and Polydeuces (Pollux to his mates), now stand high in our winter sky, closely following Orion in his silent hunt through the stars. Like our November constellation Aquarius, Gemini is a constellation of the zodiac meaning that planets and the sun all travel through it during their wanderings.
Gemini is easily seen in the winter sky, with its main stars forming two parallel lines, completing a large rectangle to the left of Orion. The stars at the eastern most end of the rectangle, the brightest in Gemini, are the heads of the twins.
The story of Castor and Pollux is a strange one, much of it confused by different versions, and no-one seemingly sure of the exact reason for their being in the sky. In the most well known version they aren’t actually twins, but half brothers. Castor, the mortal twin was brother of Helen (of Troy fame), whilst his brother Pollux was the product of a union between his mother Leda, and the god Zeus. He came down to Earth disguised as a swan and thus it was that Pollux hatched from an egg, and thanks to his Dad was immortal (presumably to make up for that undignified arrival on Earth!). When Castor died at the start of the Trojan War Pollux begged Zeus to let him share his immortality with his brother. Zeus granted this wish and put them in the skies so that they could spend half the year in the heavens and the other half in Hades, the Greek underworld.
Target Summary
| Target | Common Name | Type | Magnitude | Further Info |
|---|---|---|---|---|
| Alpha Gemini | Castor | Triple Star | 2 / 3 / 9 | Wikipedia |
| M35 | Open Cluster | 5.3 | SEDS Page | |
| NGC 2158 | Open Cluster | 8.6 | SEDS Page | |
| NGC 2392 | Eskimo Nebula | Planetary Nebula | 9.1 | SEDS Page |
Stellar Sights
The two brightest stars in Gemini are named for the twins themselves. Castor and Pollux (Alpha and Beta Gemini respectively, though in fact Pollux is the brighter) represent the heads of each twin. These stars however are definitely not identical twins. Pollux is a single cool, yellow-orange giant which has a planet 2.3 times the mass of Jupiter orbiting it with a period of just under 600 days. On the other hand Castor is a hot, white sextuple star system with three of these visible at high magnification through a telescope (they are 2nd, 3rd and 9th magnitudes).
Into The Deep
2800 light years from Earth, M35 is one of the richest open clusters in the galaxy. Through any sized telescope it is a rewarding sight, and at magnitude 5.1 on a really clear, transparent night should be visible to the naked eye from the Field.
In contrast to M35, close by is a much fainter open cluster, NGC 2158. This rarely shows any detail beyond a faint patch of light to moderate apertures, but can be used to compare it’s faint, ghostly glow with the stunning display offered by the nearby Messier target. It was once thought that 2158 was in fact a Globular Cluster, but astronomers have since proved it to be an Open Cluster of stars over 1 billion years old.
There are other, more spartan open clusters in the area, but my final target is the Eskimo Nebula, NGC 2392,a Planetary Nebula. It is strange that Charles Messier missed this object, which William Herschel described as “a very remarkable phenomenon”, from his catalogue. It is only very slightly dimmer than the famous Ring Nebula in Lyra (M57) but unlike that Planetary, the central star of the Eskimo is clearly visible, and on a clear night there is not one, but two rings on display around it. You might need averted vision to see this but it is detectable in moderate aperture scopes such as the Nexstar. The Eskimo gets its name from the stunning shots taken with the Hubble Space Telescope. The gases appear to have formed into the shape of a large hood, such as that which an Eskimo might wear, the face being buried deep inside its warm protection.
Conclusion
Gemini is a signature constellation of the late winter sky, and contains an interesting range of targets to be investigated whilst attempting to keep some feeling in your toes!
Clear skies!
Nick Bramhall
January 2007
M35 Image courtesy NOAO/AURA/NSF.