Planetary nebulae offer many challenges for backyard observers with virtually any size telescope. For small telescope users, many bright planetaries show subtle structure and central stars that can be glimpsed under good seeing. For large telescope users, faint planetaries almost unheard of by amateurs twenty years ago dot the sky, offering the chance to spot the last wisp of light from a dead star.
Because the material forming planetaries is normally puffed off spherically, we see a typical planetary nebula as a disk. Some planetaries appear more like a ring – or even a face – than a disk. This is true especially for the Eskimo Nebula (NGC 2392) in Gemini the Twins.
NGC 2392 is easily located about midway between the stars Kappa and Lambda Geminorum, about 1° southeast of the wide double star 63 Geminorum. With low power it is an unimpressive sight, appearing to be little more than a star surrounded by dim haze. However, increased magnification and a little patience bring out some interesting details.
Most noticeable is the +10.5-magnitude central star, which shines through a dense 15″-wide shell of greenish blue gas. With careful scrutiny, the diffuse shell might appear mottled with dark patches. Photographs show these best; they also reveal another ring of gas some 40″ away. When seen together, the dark patches in the inner shell and the wispy ring of the outer shell suggest the face of an Eskimo, whose dark eyes are peering out of the hood of a fur-lined parka.
Like all planetary nebulae, the Eskimo Nebula is expanding, and has now reached a diameter of more than half a light-year. The gas shell probably first left the Eskimo’s central star some 1,500 years ago, making this planetary one of the youngest known.
Fortunately for city-bound stargazers, planetary nebulae can be observed in skies one might not normally consider satisfactory for deep sky observing. Of course, a dark sky is better than a relatively bright sky, but because they have reasonably high surface brightnesses, it is possible to observe most planetaries from moderately light-polluted locations.
Finder map – field width 15°, stars to magnitude +8.5.