From a pair of glowing eyes to a planetary nebula with a surprise at its centre
Tick the box when you’ve seen each one
1 NGC 457
At mag. +6.5, NGC 457 isn’t a tricky object to find. It’s located adjacent to mag. +4.9 Phi ( ) Cassiopeiae and this star, together with mag. +7.0 HIP 6229 located 2 arcminutes to the southeast, create the impression of two eyes staring back at you. The cluster stars form a ‘body’ with outstretched ‘arms’. Unsurprisingly it is known by various unofficial nicknames including the Owl, E.T. and Dragonfly (where the arms become wings) Cluster. NGC 457 is 7,900 lightyears distant, the two bright stars being foreground objects. There are around 150 stars down to mag. +15, around 45 of which can be seen through a 6-inch scope, a figure that more than doubles through an 8-inch instrument.
Our next target is another cluster located 1° west-northwest of mag. +2.7 Ruchbah (Delta (b) Cassiopeiae), forming an isosceles triangle with Ruchbah and mag. +4.7 Chi (r) Cassiopeiae. Messier 103 is a mag. +7.4 open cluster with a rather loose appearance through the eyepiece. In fact, for a long while this cluster appeared so loose that astronomers weren’t certain whether it was a bona fide cluster at all. It is now known that at least 40 of its members share the same proper motion, so they are indeed related. The non-associated double star Struve 131 slightly overpowers the cluster, appearing as a mag. +7.3 primary separated from a mag. +9.9 secondary by 14 arcseconds.
3 NGC 663
To find the next item on this month’s tour – the large, mag. +7.1 open cluster NGC 663 – imagine the mid-point between Ruchbah and mag. +3.3 Segin (Epsilon (¡) Cassiopeiae). Head southeast from this location for 1° and you’ll arrive at NGC 663. A 6-inch scope will show around 30 stars in an area roughly 0.25° across. There appear to be a number of slightly brighter stars ringfencing the fainter members of the cluster. Overall there also appears to be a slight concentration of stars towards the cluster’s core. A 10-inch scope will double the number of cluster members visible through the eyepiece to around 60 in total. SEEN IT
4 NGC 654
Our next stop is open cluster NGC 654. You’ll find this one 0.5° southeast of the line between Ruchbah and Segin, slightly displaced towards Segin. A bright star appears embedded within it, but unlike NGC 457, in this case the mag. +7.3, yellow supergiant HIP 8106 may well be part of the cluster. This is a small grouping of stars that looks like a fuzzy patch through smaller instruments. It appears around 3 arcminutes across with 20 members through a 10-inch scope. NGC 654 is young, estimated to be around 15 million years of age, although there is some uncertainty about the precision of this figure. It’s located 7,830 lightyears from the Sun.
5 NGC 559
Our penultimate target lies 1.7° northwest of the mid-point of the line between Ruchbah and Segin. NGC 559 is a richly populated open cluster containing an estimated 120 stars. As ever, telescopes of different sizes will reveal different levels of detail with a 10-inch scope revealing around 20 members contained in an area that’s approximately 3 arcminutes across. NGC 559 is quite irregular in shape with a number of brighter stars around its periphery and three notable stars forming a right-angled triangle within the core region. A 12-inch scope shows around one third of the total cluster population. Estimated to be around two billion years old, the cluster is listed at mag. +9.5 , lies 3,700 lightyears away and has a spatial diameter of seven lightyears.
6 IC 1747
Our final target for this month presents quite a challenge. Planetary nebula IC 1747 has a listed magnitude of +13.6 with a mag. +15.4 central star. It lies 0.5° southeast of Segin and with an apparent diameter of 26 arcseconds, so it is small and easy to mistake for a star. Start hunting using a low magnification and when you think you’ve found it, pile on the power. A 12-inch scope is recommended. At around 100x power the ‘star’ appearance gives way to a fuzzier look. However, it’s when you’re over 400x magnification that the odd appearance of IC 1747 becomes apparent. In its centre is a rather striking small dark circular hole. Although a ring shape is not an uncommon occurrence in planetary nebulas, it is the relatively small size of the dark hole at the centre of IC 1747 which makes it striking – almost as if it’s been created by a cosmicscale hole punch.
NGC 457 has many names – the Dragonfly Nebula, the Owl Nebula and others – but its two brightest stars always seem to represent eyes