Degree of difficulty 3 (of 5)

Minimum aperture Naked eye

Designation NGC 2437

Type Open cluster

Class II2r

Distance 4480 ly (K2005) 4490 ly (proper motion, 2002)

Size 26 ly

Constellation Puppis

R.A. 7h 41.8min

Decl. –14° 49′

Magnitude 6.1

Surface brightness –

Apparent diameter 20′

Discoverer Messier, 1771

History M 46 was discovered by Charles Messier on the 19th of February 1771, as the first object after the completion of his initial catalog of 45 objects. He described this object as “cluster of very small stars, the stars are not visible except in a good refractor, the cluster contains a bit of nebulosity.” In 1786, William Herschel found the planetary nebula NGC 2438 in this cluster. His son John later described the view: “A very fine rich cluster which fills the field. Within the cluster at its northern edge is a fine planetary nebula.” He characterized the latter as “exactly round, of a faint equable light. Has a very minute star north of center. It is not brighter to the middle nor fading away, but a little velvety at the edges.”

Leo Brenner recommended M 46 to amateur observers, about 100 years ago, with the words: “Splendid object, even for smaller telescopes. 30′ diameter, very rich and bright. Stars of 10m and lower. The cluster contains a planetary or ring-shaped nebula of 3.75′ diameter and of considerable brightness.”

Curtis, based on the photographic images, gave a characterization of M 46 which does not quite agree with its visual appearance: “a very large, bright, sparse cluster about 25′ in diameter, in which is involved the planetary nebula 2438.”

Astrophysics M 46 and M 47 form a very appealing cluster pair in the winter sky. In real space, though, they are by no means close. Rather, M 46 is supposed to be two to three times farther away than M 47. M 46 has a position 400 light-years above the galactic plane and is among the richest Messier clusters.