This Planetary Nebula is located well within the apparent borders of the open star cluster M46, some 7 arc minutes north of the center. Controversy over the possibility of true membership in the cluster continues. Hipparcos satellite data shows M46 to be about 7,250 light years and previous estimates of the distance to NGC 2438 are around 3,000 light years. However just recently an observing team announced results that place the nebula at the same distance as the cluster. Nevertheless, astronomers believe that planetary nebulae are relatively old objects compared to open clusters. It would be very unusual for it to be a true part of M46.
The nebula was first noticed by Sir William Herschel, and was described by John Herschel in 1827 as "exactly round, of a fairly equable light ....has a very minute star a little north of center .... it is not brighter in the middle or fading away, but a little velvety at the edges..." Lalande and Lord Rosse found it annular; the ring is about 65" in apparent size, with a faint central star.
The nebula's central star is 17.5 magnitude and is very bright in blue and ultraviolet light. The computed surface temperature is about 75,000° K, one of the hottest stars known. (Kopernik & Wikipedia)
This was my second image taken the same evening as the NGC2440 (Albino Butterfly Nebula) under less than ideal conditions.
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