M46 (NGC 2437) Observing Guide — Open Cluster in Puppis

Overview

M46 is a rich open cluster in Puppis, catalogued as NGC 2437. At magnitude 6 and about 5,400 light-years away, it is one of the most richly populated Messier open clusters — and it carries a bonus: a planetary nebula (NGC 2438) visible against its face, creating an apparently embedded nebula that is actually an unrelated background object much closer to us.

Open clusters form from a single molecular cloud. M46 is relatively old for an open cluster, estimated at around 300 million years.

Basic Data

Physical Characteristics

M46 lies about 5,400 light-years away and shines at magnitude 6. It contains roughly 200 confirmed members in a relatively uniform, dense arrangement spanning about 27 arcminutes. The cluster's greater distance compared to M47 explains why its individual stars are fainter, despite the two clusters appearing in the same field of view. The planetary nebula NGC 2438 sits projected against the northern part of the cluster.

Observing Guide

Finding It

M46 sits in Puppis and transits around 8–10 PM in winter. It lies about 1.3 degrees east-southeast of M47 — the two clusters share a low-power Eyepiece field, making M46 trivial to locate once M47 is identified. The dense, more uniform star cloud of M46 immediately distinguishes it from the brighter, looser M47.

Observing Tips

At low magnification, M46 appears as a rich, mottled glow — more cluster-like than M47 but with fainter individual stars. The planetary nebula NGC 2438 is visible through 150mm or more as a small round ring projected near the cluster's northern edge. A UHC or OIII filter dramatically improves detection of the planetary.

Recommended Equipment

Binoculars show M46 as a bright, slightly granular patch. A 100mm telescope resolves individual cluster members and makes the planetary nebula NGC 2438 accessible. A narrowband nebula filter (OIII or UHC) is useful for clearly separating the planetary from the cluster background.

Astrophotography Tips

The M46/M47 pairing in a single wide-field frame is a winter imaging classic. Deep exposures also capture the faint but photogenic NGC 2438 planetary nebula projected against the cluster. A color image shows the warm yellow glow of M46's evolved stars alongside the blue-white point sources of its main sequence members.

Summary

M46 in Puppis is an intermediate winter cluster worth seeking for two reasons: its own richness as one of the most densely populated Messier clusters, and the bonus planetary nebula NGC 2438 visible against it. Together with neighboring M47, it forms one of winter's best demonstration pairs for showing how radically different two open clusters can appear when observed at different distances.