IOM November 2010 – NGC 7789 in Cassiopeia, the open cluster that thinks it’s a globular

For November’s Imaging Object of the Month I have chosen one of my most favourite objects of all – the open cluster NGC 7789 in Cassiopeia.  Open Cluster?  This amazing collection of stars looks pretty much like a huge globular cluster measuring a massive 25 arc minutes across and blazing away at magnitude 6.6.  This open cluster was discovered by Caroline Herschel in 1783 and it lies just 3 degrees south west of Beta Cassiopeiae.  NGC 7789 also contains an amazing object as can be seen in the image below which is an animation of 2 frames taken around a year apart.  Is it an alien interstellar beacon, or perhaps a nova?  No, it’s WY Cassiopeiae an amazingly variable, variable star undergoing a magnitude change from around 7 at its brightest to around 14-15 at its dimmest – what a contrast!

As NGC 7789 is almost the diameter of a full Moon you can get away with slightly shorter focal lengths on this one, and I have managed to grab a nice image of this region using the Takahashi Sky 90 at f#4.5 and 405mm focal length.  Usual prescription for clusters, shortish subs (from 3 to 5 minutes maximum) and lots of them to get the depth and smoothness that this object rightly deserves.  It also makes sense to take two images of this object – one at WY Cass’ maximum and one at minimum, it really is a highly impressive variable.  Or why not image every clear night you can get throughout the whole cycle for a very unique recording of this star?

We are now well into the long dark evenings, astrophotographer’s heaven – let’s just hope the weather Gods look favourably upon us and provide us with crisp, crystal clear evenings for our imaging.

Until December’s IOM – clear skies – and happy imaging to you all 🙂

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