Gemini

Gemini is a fine winter constellation, easily found to the upper left of Orion. The main stars form an extended rectangle, with the brightest two, Castor & Pollux at the far left (east) of the constellation. Castor, higher & slightly fainter than Pollux is the most interesting of the two. In fact it is not one star, but six! A moderate telescope will reveal a nice double, a nearby ninth magnitude star is also part of the system and each of the three stars is double again – although in each case they are so close that their double nature is only revealed spectroscopically. Castor was the first system beyond our own Solar System in which Newton’s universal law of gravitation was demonstrated to operate.

The ancient Greeks used their alphabet to list the stars of each constellation in order of brightness, starting with alpha as the brightest, beta as the second brightest, and so on. This system is still in common use today, but curiously in the case of Gemini it is Castor rather than the brighter Pollux which has the “alpha” designation. It may be that one or both stars has changed in brightness over the last 2,000 years.

As well as Castor, Gemini possesses a few other nice doubles, such as Delta, Kappa & Eta, but the deep-sky showpiece of the constellation is the open cluster M35. This is plainly visible in binoculars and a very attractive sight in small telescopes. Rather more challenging is the 12th magnitude cluster NGC 2158, which appears very close to M35, but which in reality is a much more distant background object. NGC 2158 is thought to be an intermediate type of cluster between the open or galactic clusters and the globular clusters which lie outside the plane of the galaxy. Finally in Gemini there is the planetary nebula NGC 2392, also known as the “Eskimo” or “Clown Face” Nebula, although it requires a fairly large telescope to discern the features that give rise to these names.

POWERPOINT SLIDE SHOW (209KB)