M35 and M38 are two beautiful open clusters that are well positioned for observing during the winter months. M35 is located in western Gemini, just a few degrees north and west of η Geminorum. It is a big cluster spread across an area nearly the size of the full moon. At magnitude 5.3 overall, and with nearly a hundred stars of 7th through 11th magnitude, M35 is an easy target even in heavily light polluted skies. Smaller and fainter at magnitude 7.5 overall, M38 is also a beautiful object. It consists of about a hundred stars, with the brightest ones forming a very distinctive letter “π” that is clearly visible even with small telescopes (you can see the “π” upside down in the photo to the right).
While these clusters are quite beautiful and easily observed from washed-out skies, there is something that sets them apart from the other many bright open clusters visible in the winter skies: both of these have fainter clusters within the same field of view. The combination of two clusters in one field of view makes them a particularly interesting sight. Because these fainter clusters are at the edge of visibility for urban observers, they make great challenge objects when observing in washed-out skies. I have more information for those of you wanting to take up this challenge....
Photos of M38 (upper, with NGC 1907 at the bottom edge) and M35 (lower, with NGC 2158 at the bottom-right edge) courtesy of Land of Oz Observatory [2].
M38 and NGC 1907
M38 is one of the three great Auriga clusters (M36, M7, and M38). Although not as spectacular as M37, M38 comes with a little extra feature: the open cluster NGC 1907 located about 30 arc minutes south and just slightly west of M38. NGC 1907’s overall magnitude of 8.2 and small size (7 arc minutes) would suggest that it shouldn’t be hard to see as a smudge of light even in urban skies. But the brightest stars of NGC 1907 are barely above magnitude 11 and the majority are 12th or 13th magnitude, making it harder to see than you might expect. In washed-out skies using my 10” (250mm) Dobsonian and low magnification, NGC 1907 is a soft smudge of light that is easily overlooked against the background of light pollution. Using moderate magnification (120x or more) brings out a pretty little small spray of faint stars. On the other hand, I haven’t been able to see NGC 1907 from urban areas with my 4.25” (110mm) Newtonian telescope—not even as a faint smudge. I think you’ll need at least a 6” (150mm) aperture to see NGC 1907 from urban and bright suburban locations. Give it a try and let me know what your experience is. (Photo courtesy of Land of Oz Observatory [2].)
M35 and NGC 2158
M35 is one of the most beautiful of the Messier open clusters, right up there with M11 and M37. M35 is easily visible in binoculars, but you’ll need a telescope to see open cluster NGC 2158 located about 15 arc minutes southwest of M35. NGC 2158 is very dense cluster that appears small (5 arc minutes across) only because it is over 5 times more distant than M35. It is so concentrated that in the 1950s astronomers debated whether it was an open cluster or a globular cluster. Since then its classification as an open cluster has been definitively established.
Various sources list an overall magnitude of 8.6 for NGC 2158, which in theory makes it just slightly fainter than NGC 1907. However, Burnham's Celestial Handbook lists NGC 1907 at magnitude 11, which is more consistent with my experience: NGC 2158 is much harder to see than NGC 1907. In washed-out skies using my 10” (250mm) Dob, I have to look very carefully using averted vision on clear, moonless nights to glimpse NGC 2158 as a faint smudge of light. Using high magnification to improve the contrast and pull out the individual stars doesn’t help that much because the member stars are very faint. Although a handful of the brightest stars in NGC 2158 are around magnitude 12.7, only a couple dozen stars out of the many hundreds in NGC 2158 are brighter than magnitude 14.0. However, on one night last winter of particularly good transparency, using 200x I did get a great view of NGC 2158 as a faint ball of light with a fine sprinkling of star dust across it—much like a globular cluster at the edge of resolution. It was a beautiful view. I have also had the opportunity to observe NGC 2158 from a dark location, and while it was much easier to see under dark skies, the view was not more spectacular than the great view I had last year from my light-polluted yard. (Photo courtesy of NASA DSS.)
Finally I want to highlight that NGC 2158 is located near the rim of the galaxy, about 13,000 light years away, making it one of the most remote open clusters you can observe. So on a clear, moonless night take the challenge and try to see NC 2158 from your urban/suburban location, and let me know if you succeed.