Binocular tour

Explore a dog’s tail in Canis Major and a cluster even Aristotle was aware of

1 M50

M50 is an open cluster that lies 3,200 lightyears away and spans approximately 20 lightyears. The glow comes from just over 100 stars, of which you should expect to resolve only four or five in 10 50 binoculars. Find M50 by first locating mag. –1.5 Sirius (Alpha ( _ ) Canis Majoris). From here, navigate just over 5° northnortheast to mag. +4.0 Theta ( e ) Canis Majoris, then continue another 4° in the same direction. Here you will find an obvious circular glow measuring about half the apparent diameter of the Moon: this is M50. SEEN IT

2 M46 AND M47

These clusters lie in the same field of view as each other, 5° to the south of mag. +3.9 Alpha ( _ ) Monocerotis. They offer contrasting examples of how open clusters can appear in small binoculars: M47, 1,600 lightyears away, is large and loose: you should easily be able to resolve over half a dozen stars. M46 is over three times as distant at

5,500 lightyears, but is a similar apparent size. It is far more compact: although it contains many more stars than its neighbour, you may not be able to resolve any of them at all. SEEN IT

3 M93

If you place mag. +3.3 Xi ( j ) Puppis in the southeast of the field of a pair of 15x70s, the wedge-shaped M93 should appear approximately in the centre. This is a bright (mag. +6.2), rich and densely packed cluster in which 25-30 stars are visible in 15 70s, with more unresolvable stars giving a glowing backdrop. M93 is unusual in that the centre of the cluster, which is bounded by an arrowhead grouping of brighter stars, is relatively sparse; most open clusters are sparser at the periphery. SEEN IT

4 M41

M41 is 4° southeast of Sirius. This bright cluster is visible to the naked eye in a transparent sky, and was even noted by Aristotle in the 4th Century BC. It appears slightly larger than the previous four clusters and, in 10 50s from a reasonably dark sky site, you should be able to resolve up to 10 brighter stars against the background glow of fainter stars – you may need to use averted vision. If the sky is transparent enough, see that the stars differ in colour, with the brightest one, near the centre, being somewhat orange. SEEN IT

5 UW CANIS MAJORIS

UW Canis Majoris is a Beta Lyrae type variable, the distance of which is disputed. Find mag. +1.8 Wezen (Delta ( b ) Canis Majoris), then look 2.7º northeast to find mag. +4.4 Tau ( o ) Canis Majoris. UW Canis Majoris lies less than 0.5º north of Tau and is about 200,000 times more luminous than the Sun. It shines, usually at mag. +4.8, with a brilliant blue-white light, but every 4.39 days its brightness falls by half a magnitude as it is eclipsed by a fainter and smaller companion. SEEN IT

6 THE OMEGA CANIS MAJORIS GROUP

Mag. +4.0 Omega ( t ) Canis Majoris lies 1.5º east of Wezen. The first thing to note about it is that it is a brilliant white compared to the fainter (mag. +5.6) yellowish star that is just to the south of it. The two are part of a pretty C-shaped string of stars of varying colours, sometimes referred to as ‘the dog’s tail’, which extends from mag. +5.9 26 Canis Majoris in the north, through 27 and Omega Canis Majoris, to a mag. +5.4 yellow star that is just over 1º south of Wezen. SEEN IT