M 46

M46 was the first object Messier discovered after he had published the first edition of his list (M1-M45), on February 19, 1771, three days after presenting it to the academy.

The cluster is very rich, with 150 stars of mag 10-13 and probably a total population of over 500. The brightest of these stars are of spectral type A0, and each about 100 times more luminous than the Sun. This indicates an age of about 300 million years. The members are scattered over an angular diameter of about 27', corresponding to a linear extension of 30 light years, and are receding from us at 41.4 km/sec, according to Baade.

As a special and famous feature which is also obvious in our photograph, a planetary nebula (NGC 2438) appears within the apparent borders of M46. However, this nebula cannot be a true member but is superimposed, or perhaps a passing "guest", because of two reasons:

1. the radial velocity of NGC 2438 is about 77 km/sec recession, which is 43 km/sec different from the cluster's value and would not allow the cluster to hold it, even if it were at the same distance
2. planetary nebula are late states in stellar evolution which occur only at comparatively low mas stars of less then 3 solar masses. These, however, need over a billion years for their evolution until they eject the planetary, which is much longer than the age of M46 (more massive stars "go supernova").

Right ascension	07 : 39.6 (hours : minutes)
Declination	-14 : 42 (degrees : minutes)
Distance	5400.0 (light-years*10^3)
Visual magnitude	6.0