Messier 46, NGC 2437

Messier 46, also known as NGC 2437, is an open cluster in the constellation Puppis. The planetary nebula NGC 2438 appears to lie within the cluster, but is most likely unrelated.

M 46 was discovered by Charles Messier in 1771. Messier added it to his catalog three days after publishing the first edition of his list (containing M 1 - M 45). Dreyer described M 46 in the NGC as "very bright, very rich, very large."

M 46 appears in a rich region of the Milky Way in northern Puppis, about 1.5 degrees east of M 47. The two clusters fit well in a binocular or wide-angle telescope field. M 46 is very rich, with 150 stars of magnitude 10 - 13, and probably a total population over 500 stars. Its members are scattered over an angular diameter of about 27'; the brightest is of apparent magnitude 8.7. The cluster has a total visual magnitude of 6.0.

Properties and Planetary Nebula

M 46 lies at a distance of about 5,400 light years, implying a linear diameter some 30 light years across. Its brightest stars are of spectral type A0, and about 100 times more luminous than the Sun. This indicates an age of about 300 million years.

A planetary nebula, NGC 2438, appears within the cluster, near its northern fringes. However, this nebula is probably not a member, for several reasons. NGC 2438 is receding from us at about 77 km/sec, much faster than the cluster's radial velocity of 41.4 km/sec. The difference would not allow the cluster's gravity to hold the nebula, even if they were at the same distance. And with a derived distance of about 2,900 light years, the planetary a foreground object several thousand light years closer than the cluster.

Moreover, planetary nebulae are late states in stellar evolution, visible only for a few thousands of years before their material disperses into interstellar space. They occur only for stars of less then 3 solar masses (more massive stars go supernova). These stars, however, need more than a billion years to evolve until they eject their envelope to form the planetary. This is much longer than the age of M 46.