- M 46 (NGC 2437)
M 46 is a very rich open cluster lying within the Winter Milky Way in the constellation Puppis. It is one of the richest open clusters in Messier's list with 186 member stars brighter than magnitude 13 and probably an additional 500 fainter members. M 46 is located only about a degree away from another nice cluster, M 47, and the two make an attractive pair when viewed in a low-power telescope. The two cluster are not physically related, with M 46 lying at a distance of 4480 LY and M 47 lying much closer at 1600 LY. M 46 was discovered by Charles Messier in February of 1771.
One of the most interesting features of M 46 is the presence of a small, bright planetary nebula which is clearly visible in my image. This object, NGC 2438, was discovered by William Herschel in 1786. It exhibits the typical outer edge of red Hα emissions and intereior greenish-blue glow of OIII emissions, making it a miniature Helix Nebula. While the age of M 46 is estimated to be around 500 million years, NGC 2438 is much younger at only around 45,000 years. There is still debate as to whether this planetary nebula is physically a part of the cluster or just a foreground object. Studies indicate that the nebula and the cluster have similar proper motions, which supports the nebula being part of the cluster. If this is the case, NGC 2438 would have a physical diameter of around 1 LY.
NGC 2438 is not the only planetary nebula in my image. If you look about halfway between NGC 2438 and the left edge of my image, you will see a tiny red circular feature— this is the planetary nebula P231.4+4.3. The bright yellow star along the right edge of my image is 140 Puppis (HD 61772), a magnitude 4.98 class K giant lying at a distance of 694 LY.
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