NGC 2437

Messier 46 (NGC 2437) is a rich open star cluster in the constellation Puppis, approximately 5,400 light-years away and spanning about 30 light-years. Charles Messier discovered it on February 19, 1771. NGC 2437 contains over 150 conspicuous stars with a total population perhaps exceeding 500, all roughly 300 million years old — one of the richer old open clusters in the Messier catalog. What makes M46 particularly remarkable, however, is what appears to be a planetary nebula visible on its northern edge: the round, ghostly disk of NGC 2438, the glowing shell of gas expelled by a dying star.

Despite appearances, NGC 2438 is almost certainly a foreground object unrelated to M46. Its radial velocity is significantly different from the cluster's, and distance estimates place it at roughly half the distance to NGC 2437 — the two simply happen to overlap along our line of sight. A second unusual object also lurks in this field: a strikingly red smudge to the east known as OH231.8+4.2, a bipolar protoplanetary nebula — material being violently ejected by a star partway through its death throes. Even closer to us than NGC 2438, it shows twin expanding lobes of emission and reflection nebulosity ejected from a central star on the asymptotic giant branch. The coincidence of three independent objects — an old open cluster, a foreground planetary nebula, and a protoplanetary nebula — in the same small patch of sky is remarkable.

In a telescope M46 appears as a rich, uniform spray of stars of similar brightness; NGC 2438 appears as a faint, round haze floating within the cluster field, clearly non-stellar. This approximately true-color image was assembled from eleven BVR exposures taken in September 1997 at the Burrell Schmidt telescope of Case Western Reserve University's Warner and Swasey Observatory on Kitt Peak.