Messier 46 — Open Cluster in Puppis

M46 is a rich, impressive open star cluster in the constellation Puppis, located about 5,400 light-years from Earth. It is one of the richest open clusters in the Messier catalog, containing an estimated 500 or more stars packed into a sphere about 30 light-years across. At magnitude 6.1, it is just below naked-eye visibility but is an easy binocular target. The cluster is approximately 300 million years old. M46 is famous for the planetary nebula NGC 2438 that appears to lie within the cluster, though this nebula is actually a foreground object at roughly 2,900 light-years — a chance superposition.

Located about 5 degrees east of M47 and about 1 degree north of the 4th-magnitude star Sigma Puppis. Binoculars show a large, grainy glow. A 4-inch telescope at 80-100x resolves the cluster into a stunning carpet of faint, uniformly bright stars — it has a very even, granular texture unlike most open clusters. With an 8-inch telescope, look for the planetary nebula NGC 2438 near the northern edge of the cluster — it appears as a small, ghostly ring about 1 arcminute across. An OIII filter makes the planetary nebula pop out dramatically. Best observed from January through March.

Discovered by Charles Messier on February 19, 1771. Messier described it as 'a cluster of very small stars.' The planetary nebula NGC 2438 within its borders was discovered by William Herschel in 1786. For many years it was debated whether NGC 2438 was a true cluster member, but modern radial velocity and distance measurements confirm it is a foreground object, not physically associated with M46.