Before 1752, star maps of the southern sky looked a little different. The large constellation Argo Navis, the ship of the Argonauts, spanned over 70° in east-west direction and contained over 800 naked eye stars within its huge borders. Due to its vast extent, the French astronomer Nicolas Louis de Lacaille subdivided it into four component constellations: Carina (the hull of the ship), Puppis (the stern), Vela (the sails) and Pyxis (the compass, formerly the mast).
For observers in the Northern Hemisphere, Puppis is the most familiar part of the fragmented archaic constellation Argo Navis. It lies east of Canis Major, in an area notably devoid of any bright stars. Although it lacks naked eye luster, Puppis is home to many bright star clusters.
M47 is one of the most appealing open clusters in the constellation, ranking as the sky’s 14th-brightest object of this kind. Draw a line from Mirzam, Beta Canis Majoris, through Sirius and continue east for a little more than twice that distance, and you will land just north of M47.
Dark skies reveal the cluster as a very faint fuzzy patch to the naked eye, and although M47 is terrific through binoculars or a finder scope, it is somewhat disappointing through a telescope at magnifications above 50x. This is probably because the cluster’s stars spread out over an area equal to that covered by the Full Moon.
M47 contains at least 50 true member stars, which range in brightness from 6th to 12th magnitude. Near the center look for the easy double star Struve 1121, which consists of two 7.5-magnitude stars separated by 7.4 arcseconds. Two other nicely spaced doubles reside in the cluster, as do two orange stars of about 8th magnitude (one north of the brightest star in M47, the other to the south).
M47 lies at an estimated distance of 1,600 light years from Earth and its diameter is 18 light years. Several Be stars are known in this cluster, the brightest of which is HD 60856 at magnitude +8, readily visible in amateur telescopes. Be stars are very hot, with a B spectral type and presumed to be fast rotators, spinning at 150 miles (240 kilometers) per second or more.
Finder map – field width 15°, stars to magnitude +8.5.