by Shaphan Shank | Mar 6, 2024 | 0 comments

star clusters in Gemini, M35 and NGC 2158
M35 and NGC 2158. Photo © Scott Rosen.

Gemini, the twins, lies northeast of Orion along the band of the Milky Way. The most obvious part of Gemini is the pair of bright stars, named Pollux and Castor, which mark the northeastern end of the constellation. The rest of Gemini consists of several chains of stars which extend southwest toward Orion.

A number of double and multiple stars lie within the bounds of Gemini, but one of them outshines all the rest—literally. Castor, the second-brightest star in Gemini, is a fairly tight double consisting of two whitish stars separated by about 5 arcseconds. The component stars are similar in brightness, making the pair easy to split at moderate magnification. This star system is technically a sextuple system rather than a double—in addition to the bright pair, a third star which is much fainter lies about 70 arcseconds to the south. All three of the visible component stars have dwarf companion stars which orbit too closely to be visually separable.

The brightest and best-known deep sky object in Gemini is the open star cluster M35. This cluster lies in the western part of Gemini, about 2° northwest of the star Propus and 1.5° northeast of 1 Geminorum. M35 is bright enough that you may be able to see it as a faint spot of light with the unaided eye. However, it won’t be very noticeable without optical aid. With binoculars, you should be able to see the cluster as a fuzzy patch of light with just a few of the brightest stars resolved. Nearly any good-quality telescope at low magnification will turn M35 into a sparkling mass of dozens of stars.

As an added bonus, a much more distant cluster called NGC 2158 lies next to M35. Both clusters are easily visible in the same field of view at low magnification. Since NGC 2158 lies so much farther away than M35, its stars are much dimmer and harder to resolve. The cluster looks like a fuzzy or sparkly patch of light at low magnification, but you may be able to resolve some of its stars with high magnification.

Most of the nebulas in Gemini are quite faint, but there is one exception: NGC 2392, or the Eskimo Nebula. The Eskimo is a small planetary nebula named for its resemblance in photographs to a face surrounded by a fur hood. To find this nebula, start by finding the star Wasat southwest of Pollux. About 2° east of Wasat is a faint star called 63 Geminorum. NGC 2392 lies approximately 0.5° southeast of this star.

Use moderate magnification to find NGC 2392, as this will give you a reasonably large field of view while still magnifying the nebula enough to clearly show that it is not a star. Once you’ve found the nebula, experiment with higher power until you find the magnification that reveals the most detail. Backyard telescopes will not reveal the level of detail that photographs do, but a mid-sized telescope should show a brighter area in the middle of the nebula. Careful observation at high magnification may reveal additional ring-like structures and/or the nebula’s central star.

star map of Gemini

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