Deep-Sky Tour

From a pair of glow­ing eyes to a plan­e­tary ne­bula with a sur­prise at its cen­tre

Sky at Night Magazine - - CONTENTS -

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.


2 M103

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

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