Object of the Week, February 24, 2019 - Messier 46, NGC 2438 and the Calabash Nebula
- OC: Messier 46
- PN: NGC 2438
- PPN: OH 231.8+4.2 ("Calabash Nebula", Rotten Egg Nebula")
Puppis
~RA: 07 42
~DEC: -14 48
"Deep, deeper, tough nut" could be the motto for the actual OOTW. Within a frame of around 20' we find three totally different objects for all kinds of telescopes and likes.
Lets start with the brightest one - Messier 46. Discovered by Messier itself in 1771 it is just visible with the naked eye under dark skies. With binoculars and small telescopes it forms a very nice trio with the two other OC Messier 47 and NGC 2423. The cluster itself counts around 500 stars and is a beauty in nearly all kinds of telescopes. The distance is around 4900ly.
Around 100 years later William Herschel discovered the PN NGC 2438 within the stars of Messier 46 in 1786. Personally I already pick out the object with a 4-inch bino without any problems. Most people name the bright 13mag star near the middle as the central star. But this false CS (which is a tight double with a 16mag companion) is shifted to the NW. The real CS stands 7" SE. With its 17.7mag the star is a hard target (the nut comes later) for bigger telescopes. Beyond the bright PN shell there is an extended double halo. I could not catch it yet with the 27-inch. Most sources speaks about a distance of around 2900ly. So the PN seems to lie in front of the OC.
The "tough nut", the Calabash nebula was already discussed here at February 23rd, 2012. It lies only 6.5' E of NGC 2438 so it is worth a comment. Physically the nebula belongs to the Protoplanetary nebula (PPN). It was first photographed in 1977 [1977ApJ...211..178C]. The name "Calabash Nebula" was first proposed in 1989 [1989A&A...211..409I]. The 27-inch brings out the first hints of the brightest knot. But even a 36-inch didn't show the longish and/or the bubble structure.
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