Messier 46 is an open cluster in Puppis. All those bright blue stars apparently looked enough comet-like in Messier’s telescope that he thought he needed to put it in his list of “fuzzy things that aren’t comets”. What his telescope wasn’t good enough to see (especially if he thought the cluster was fuzzy!) was the small planetary nebula. That is NGC 2438.

Although the planetary nebula appears to be within the cluster, their radial velocities are different so they aren’t related. That is to say, the nebula didn’t come from a star within the cluster. The nebula is about 1,370 light years away and the cluster has an average distance of 5,000 light years sot he nebula is most likely passing in front of the cluster relative to us.

The cluster contains around 500 stars and is estimated to be about 250 million years old.

This is 12h of RGB data. For all the technical data, see astrobin.