This time of the year is open cluster season for sure, with a whole host of prime examples of the species to choose from. Why not head in the direction of the under-observed southern constellation of Puppis to observe Messier 46 (NGC 2437), Messier 47 (NGC 2422) and Messier 93 (NGC 2447), a superb trio of clusters.
M46 and M47 lie under 1.5° apart in a sparkling, star-studded winter Milky Way field, and are great objects to view together through a pair of 10 x 50 binoculars. There’s an added bonus too: peering through a moderate- to large-aperture telescope will reveal tenth-magnitude NGC 2438, a tiny planetary nebula that’s embedded within M46’s swarming stars. M93 lies nearly 20° south and with a declination of 23° south, can prove a challenging target from mid-northern latitudes.
M46 and M47
Puppis’ northern region is reasonably well seen from UK shores; if you can see Sirius (alpha [α] Canis Majoris), then M46 and M47, located around 13° east of the brightest star in the sky, are accessible to you. Along with Carina and Vela, Puppis formed part of the huge constellation Argo Navis, specifically the stern, or poop of the ship of the Argonauts, until that unwieldy beast was split up by Nicolas Louis de Lacaille, in 1763.
Messier 47, the more westerly-lying cluster, shines with an integrated magnitude of +4.4, making it the brighter of the pair. A small telescope shows around 30 stars among half a dozen or so brighter members, with many more becoming visible through a 250mm (10-inch) aperture. M47 spans around a full-Moon-sized half a degree. M47 has a dozen or so bright members which give impact visually.
The rich seventh-magnitude open cluster NGC 2423 lies just over half a degree north-northeast of M47. Images show the cluster is less spherical overall than its Messier neighbours, with an extension to the north-east which rather blends into the rich Milky Way background.
Messier 46, which lies just 1.3° to the east-south-east of M47, its close neighbour on the sky, sports a similar apparent diameter but shines significantly fainter, having an integrated magnitude of +6.1. It lacks the immediate impact of M47’s brighter members, but makes up for that by being richer in fainter stars, giving it the appearance of a very loose globular cluster. A telescope of 100- to 150mm (four- to six-inches) aperture can reveal over 100 stars on a transparent and moonless night at a dark-sky location.
M46 and M47 are not close neighbours in space; at a distance of around 4,500 light years, M46 lies two to three time farther away than M47.
In late-February, Messier 46 and 47 cross the southern meridian from London at around 9pm GMT, culminating not far short of 25 degrees in altitude. In addition to Sirius, alpha Monocerotis is a handy, magnitude +3.9 guide star which lies 5.3 degrees due north of Messier 46.
Go low for M93
Messier 93 is a very rich cluster of stellar jewels, picturesquely set in Puppis’ crowded winter Milky Way star fields, with magnitude +3.3 xi (x) Puppis lying just 1.5 Messier 93 is a very rich cluster of stellar jewels, picturesquely set in Puppis’ crowded winter Milky Way star fields, with magnitude +3.3 xi (x) Puppis lying just 1.5° to the south-east. Sirius lies 15° to the north-west.
M93 has around 80 stars that are confirmed cluster members spread over around 10’, giving it an integrated magnitude of +6.2, within range of binoculars despite it horizon-hugging environment. M93 culminates at about 9pm an altitude of around 15° from the south of England.
