This is a CCD image taken with our ST-6 CCD camera thru Kopernik's 20 inch F/8.1 telescope. The exposure was 5.8 minutes, the image scale is about 5 x 7 arc minutes.
Planetary nebula, pretty bright, pretty small, very little extended, resolvable, but mottled
This Planetary Nebula is located well within the apparent borders of open star cluster M-46, some 7 arc minutes north of the center. It 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.
Controversy over the possibility of true membership in the cluster continues. Hipparcos satellite data shows M-46 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 M-46. 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.
Planetary Nebulae: To learn more about them, click here.
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George Normandin, KAS
March 13th, 2000