Gemini, the Twins
This bright constellation dominates the eastern horizon in winter
What makes Gemini stand out is the presence of two bright and beautiful stars only 4.5° apart, mag. +1.9 Castor and mag. +1.1 Pollux. These stars form the heads of the legendary twins, and a stream of bright stars extends south from each. The best way to visualise Gemini is as a huge ‘U’ lying on its side.
M35 (mag. +5.2, 25’ across), located at the southern end of the constellation at the ‘foot’ of Castor, the western twin, is one of the finest star clusters in the sky. Easy to find even with a small telescope in heavy light pollution, M35 is a round, dense crowd of bright sparklers almost as large as the full Moon – its brightest stars are mag. +8.0.
NGC 2158 (mag. +8.6, 5’ across) is an open star cluster located only 27’ southwest of the centre of M35. Despite its closeness to the big cluster, the two are not actually associated. NGC 2158 is 13,000 lightyears out in space, compared to a distance of ‘only’ 2,800 lightyears for M35. Their placement alongside each other is just a line-of-sight effect. NGC 2158 is a cluster that cries out for aperture.
Planetary nebula NGC 2392 (mag. +3.5, 48” across) is located a little less than halfway down the constellation, 2°21’ east-southeast of the bright star Delta (δ) Geminorum (Wasat). Its small size means it can be mistaken for a star. On a good night, 250x magnification will resolve the mag. +10.5 central star, while going up to 300x reveals hints of an inner ring, what’s been described as looking like the ‘furry hood’ of a parka coat, a detail that is more visible in photographs.
Spiral galaxy NGC 2339 (mag. +12.3, 2.4’ across) is on the eastern side of the constellation, 4°13’ southwest of mag. +3.5 Wasat. Observing this galaxy requires patience, a good night in the suburbs and at least 10 inches of aperture.