Canis Major, the Big Dog
Despite skimming the horizon, this constellation has plenty of sights worth seeking out
Although it never gets high enough in the sky to rival other open clusters, M41 (mag. +4.5, 39’ x 39’), the Little Beehive Cluster, is still a marvel – a large group of 60 stars spread across an area larger than the full Moon. Most of M41’s suns are bright, making it a naked-eye object from sufficiently dark sites. Luckily, M41 is located in the more northerly area of the constellation, 4° south of Sirius, and is somewhat away from the haze of the horizon. To improve the appearance of this group in the city, try using a mild light-pollution filter, a ‘Skyglow’ or ‘Deep Sky’-type filter. This has the effect of darkening the background a bit, but not dimming the bright cluster stars too much. One of M41’s more striking features is the strongly orange-red sun near the cluster’s centre.
The outstanding nebula, NGC 2359 (mag. +11.5, 10’ x 10’), Thor’s Helmet, is a faint cloud of ionised gas. Thrown off by a Wolf–Rayet star, this cloud of gas is just visible from urban sites using a 12-inch Dobsonian reflector and a UHC filter.
NGC 2362 (mag. +4.1, 8’ x 8’) requires a trip to the far south, to a declination of –24°57’. At best, this cluster only climbs 13° or less into the UK skies, so wait until it is on the local meridian, when it is as high as it gets, before attempting to observe it. Like most open clusters, NGC 2362 is not difficult to catch, and its 20 to 30 stars are obvious in the eyepiece even when it is at low altitude in the sky. In the midst of NGC 2362 is bright-yellow mag. +4.3 Tau (τ) Canis Majoris, the Jumping Spider Star. Due to contrast effects, when you jiggle the telescope, the halo of tiny amber suns surrounding Tau appear to move in one direction, while Tau seems to move in the opposite direction – the spider jumps!