Discovered 1771 by Charles Messier
Planetary nebula NGC 2438 appears superimposed over the open star cluster M46, although it is certainly not a member of the cluster, but a foreground object that is approximately 3,000 lightyears distant. NGC 2438 is one of the discoveries of William Herschel, who cataloged it as H IV.39. The faint central star, which is now a white dwarf, can be seen close to a background star. This background star is probably a member of M46, as are many of the bright stars in the image. Surrounding the bright planetary is a ring, or halo, of hydrogen gas that was expelled during the earlier red gaint phase of the dying star. This faint, diffused, feature is composed of ionized hydrogen and is visble in long Ha (Hydrogen Alpha) filtered exposures.