Messier 46: Open Cluster in Puppis

Messier 46 is a rich open cluster in the northern part of Puppis, east of Sirius. It is a dense swarm of faint stars with a bonus: a small planetary nebula that appears inside the cluster's edge but is much closer to us.

What it is

M46 is an open cluster, a group of stars that formed together from the same cloud and are still loosely bound. It lies about 5,070 light-years away, according to Gaia measurements, and its 27′ apparent size works out to roughly 40 light-years across. Near its northern edge lies the planetary nebula NGC 2438, the glowing shell of gas from a dying star. It is not part of the cluster: it lies in the foreground and only happens to line up with it. Its neighbor M47, about 1.3° to the west, is a looser cluster with fewer but brighter stars, and it is much closer to us than M46.

When to see it

M46 is a winter object. From about 40°N it is highest in the south around midnight in mid-January and around 10 p.m. in mid-February, reaching about 35° above the horizon. It is visible in the evening from January to April. From the Southern Hemisphere it rides high in the summer sky and is easy to see.

How to find it

Start at Sirius, the brightest star in the night sky. M46 lies about 14° east of Sirius, slightly north. A more precise guide is Alpha Monocerotis, a 4th-magnitude star in the southern part of Monoceros: M46 lies about 5° due south of it. In binoculars, look for two hazy patches side by side: the brighter, more scattered one to the west is M47, and the fainter, smoother one about 1.3° to the east is M46.

What you will see

In binoculars M46 is a soft, round glow next to the more sparkling M47; from a dark site it looks grainy. A 70–100 mm telescope at low power resolves it into dozens of faint stars packed fairly evenly across the field. A 150–200 mm telescope shows a rich field of stars, and NGC 2438 appears near the northern edge as a small, round, grey disk; at higher power it shows as a faint ring with a darker center. An O-III or UHC filter makes the planetary nebula stand out clearly against the cluster stars, which dim through the filter. From the suburbs the nebula needs the filter.

History

Charles Messier discovered M46 in February 1771 and described it as a cluster of very small stars that needs a good telescope to see. He did not record the planetary nebula. William Herschel discovered NGC 2438 in 1786.