Messier 46

Lore & Background

Messier 46 was discovered by Charles Messier in 1771. Dreyer described it as 'very bright, very rich, very large.' The cluster has a tidal radius of 37.8 ± 4.6 light-years and a core radius of 8.5 ± 1.3 light-years. It has a greater spatial extent in infrared than in visible light, suggesting mass segregation, with fainter redder stars migrating to a coma or tail region. Fainter stars extending to the south and west may form a tidal tail due to a past interaction.

The planetary nebula NGC 2438 appears to lie within the cluster near its northern edge, but it is most likely unrelated since it does not share the cluster's radial velocity. This makes for superimposed objects of interest, another instance perhaps being NGC 2818. On the other hand, the illuminating star of the bipolar Calabash Nebula shares the radial velocity and proper motion of Messier 46 and is at the same distance, so it is a bona fide member of the open cluster.

M46 is located close by to another open cluster, Messier 47, about a degree east of M47 in the sky, so the two fit well in a binocular or wide-angle telescope field.

Reader's Guide

Messier 46 is notable as a rich open cluster with an estimated 500 stars and a combined mass of 453 solar masses, located about 5,000 light-years away in Puppis. Its significance is enhanced by the superimposed planetary nebula NGC 2438, which appears near the cluster's northern edge but is likely unrelated due to differing radial velocities. This superposition creates a striking visual contrast between young cluster stars and an older nebula. Additionally, the illuminating star of the bipolar Calabash Nebula is a genuine member of M46, sharing its radial velocity, proper motion, and distance. The cluster's infrared extent suggests ongoing mass segregation, with fainter redder stars forming a tidal tail possibly from a past interaction. Its proximity to Messier 47, about a degree away, makes it a popular binocular or wide-field telescope target. The cluster's discovery by Charles Messier in 1771 and its description by Dreyer as 'very bright, very rich, very large' underscore its historical importance as a deep-sky object.

Did You Know?

• The planetary nebula NGC 2438 appears within M46 but is likely unrelated due to different radial velocities.
• The illuminating star of the Calabash Nebula is a bona fide member of M46.
• M46 is located about one degree east of the open cluster Messier 47.

A Place in the Grand Catalogue

Messier 46 occupies a numbered slot within one of the most celebrated inventories of the night sky ever assembled. The Messier catalogue, compiled by French astronomer Charles Messier alongside his assistant Pierre Méchain, ultimately encompasses 110 astronomical objects spanning a remarkable diversity of deep-sky phenomena. Originally born from a practical frustration—Messier was hunting comets and kept losing time to bright, non-comet objects that mimicked their appearance—the list grew from an unnumbered set of 45 entries in 1774 to a final 103-object publication in 1781, before later astronomers added the remaining entries through notes buried in Messier's and Méchain's manuscripts. M46, like its numbered neighbours, belongs to a collection that now serves as a shorthand reference system still in active use by astronomers worldwide. Rather than being defined by a modern discovery date, it carries the weight of an eighteenth-century designation that has outlasted the very instruments that first recorded it.

Observed from the Heart of Paris

The objects in the Messier catalogue, including M46, were identified using a modest refracting telescope of roughly 100-millimetre aperture, operated from the premises of the Hôtel de Cluny in Paris—today home to the Musée national du Moyen Âge. Messier's working sky was bounded by the north celestial pole down to a celestial latitude of approximately −35.7 degrees, a constraint that naturally excluded southern-hemisphere wonders such as the Magellanic Clouds. The fact that M46 and its catalogue companions could be resolved with such a small instrument from a city centre speaks to their intrinsic brightness and visual prominence. They represent, in their respective categories, some of the nearest and most spectacular examples of deep-sky objects visible from European latitudes. Today, professional-class telescopes can tease out fine structural details in these same targets that were invisible to an eighteenth-century observer peering through a four-inch glass, yet the fundamental appeal that drew Messier's eye remains unchanged.

A Target for the Backyard Astronomer

For the modern amateur astronomer, objects like M46 occupy a special place as accessible, rewarding targets that require nothing more than a modest telescope and a dark enough sky. The Messier catalogue was never designed as a recreational list; it was a practical tool to help a comet hunter avoid wasting hours on stationary objects. Yet the very qualities that made these objects frustrating for Messier—their brightness, their visual distinctiveness, their relative proximity—have made them the backbone of visual astronomy and astrophotography for nearly three centuries. Enthusiasts gather each early spring for Messier marathons, attempting to sweep through the entire catalogue in a single night, and M46 slots into that seasonal sequence. Larger-aperture equipment available to hobbyists today reveals far more detail than Messier's 100-millimetre refractor ever could, transforming what was once a faint smudge in a Parisian sky into a richly structured spectacle.

The Living Legacy of a Comet Hunter's List

What began as a working note to keep a comet search on track has evolved into one of the most enduring reference systems in all of astronomy. The Messier catalogue's numbering scheme—M1, M2, M3, and so on through M110—has become so deeply embedded in the language of the field that objects are routinely identified by their Messier designation rather than by their formal catalogue names. The list itself has a layered history: Messier's 1781 publication in the Connaissance des Temps contained 103 entries, but marginalia in his and Méchain's texts prompted later scholars to append additional objects. Flammarion added M104 in 1921, Helen Sawyer Hogg contributed M105 through M107 in 1947, Owen Gingerich added M108 and M109 in 1960, and Kenneth Glyn Jones included M110 in 1967. M46, sitting comfortably within the original numbered sequence, benefits from this stability. Its identity as a Messier object guarantees that it will continue to be referenced, studied, and admired long after the specific telescopic technology of any single era has passed.