The quite interesting planetary nebula NGC 2438 (PK231+04.2) is located, visually, within the lovely open cluster M46 (Messier 46). This is a very unusual combination. This purely RGB image does not show the remarkable outer shell of NGC2438. I will need to do a NB version to see this.
Wikipedia:
NGC 2438 is a planetary nebula in the southern constellation of Puppis. Parallax measurements by Gaia put the central star at a distance of roughly 1,370 light years. It was discovered by William Herschel on March 19, 1786. NGC 2438 appears to lie within the cluster M46, but it is most likely unrelated since it does not share the cluster's radial velocity. The case is yet another example of a superposed pair, joining the famed case of NGC 2818.
Messier 46 or M46, also known as NGC 2437, is an open cluster of stars in the slightly southern constellation of Puppis. It was discovered by Charles Messier in 1771. Dreyer described it as "very bright, very rich, very large." It is about 5,000 light-years away. There are an estimated 500 stars in the cluster with a combined mass of 453 M?, and it is thought to be a mid-range estimate of 251.2 million years old.
The cluster has a very broadest (tidal) radius of 37.8 ± 4.6 ly (11.6 ± 1.4 pc) and core radius of 8.5 ± 1.3 ly (2.6 ± 0.4 pc). It has a greater spatial extent in infrared than in visible light, suggesting it is undergoing some mass segregation with the fainter (redder) stars migrating to a coma (tail) region. The fainter stars that extend out to the south and west may form a tidal tail due to a past interaction.
The planetary nebula NGC 2438 appears to lie within the cluster near its northern edge (the faint almost rainbow array of colored smudge at the top-center of the image), but it is most likely unrelated since it does not share the cluster's radial velocity. This makes for superimposed objects of interest, another instance perhaps being NGC 2818. On the other hand, the illuminating star of the bipolar Calabash Nebula shares the radial velocity and proper motion of Messier 46, and is at the same distance, so is a bona fide member of the open cluster.
M46 is located close by to another open cluster, Messier 47. M46 is about a degree east of M47 in the sky, so the two fit well in a binocular or wide-angle telescope field.
This is a multi-shell planetary nebula with a bright inner nebula with a diameter of 60?, consisting of two somewhat detached shells. It is expanding with a velocity of 37 km/s. The structure is surrounded by a fainter, mostly circular halo that is more visible on the western half, and has a diameter of 130?. The mass of the main nebula is estimated at 0.45 M?, while the shell has 0.5–0.8 M?. The main nebula has a temperature of about 10–13,000 K, rising to 15–17,000 K at the inner edge.
The nebula consists of material ejected from the central star during the asymptotic giant branch stage, beginning some 8,500 years ago. The main nebula was formed at about half that age. The central star of this planetary nebula is a 17.7-magnitude white dwarf, with surface temperature of about 75,000 K (74,700 °C). It is one of the hottest stars known.
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