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Challenge #3 – Star Cluster NGC188

NGC188 Statistics

TargetTypeRight AscensionDeclinationConstellationMagnitudeSize
NGC188Open Cluster00h 49m 37s+85° 21′ 28″Cepheus+8.117.0 arcminutes

NGC 188 is a star cluster lying in a quiet part of the night sky just a few degrees away from Polaris in the constellation of Cepheus. It is visible throughout the year for the northern hemisphere because it is so close to the celestial pole, but it’s not an easy target to locate, which is why I’m confident you’ve not yet seen it.

There are believed to be over 500 stars in the cluster. However, your telescope won’t show that many because they range from 12th magnitude – which is in range of most scopes – to dimmer than 18th magnitude, which is too dim for your average backyard telescope.

It’s made even tougher to find because its brightness is nowhere near as great as the magnitude reading in the statistics box above would have you believe. The stars within the cluster are diffusely spread, there is no bright core and, consequently, you’ll need a dark sky with good seeing conditions to pick out this object.

We’ll be hunting in the area of the sky shown with the blue ring in the picture below, which represents a 4° finderscope view. Note Polaris is in the 4 o’clock position, with Errai outside the ring at 10 o’clock. The other star shown inside the ring at 10 o’clock is HR 285, which we’ll use shortly.

Place Polaris in your viewfinder at the edge of the field furthest from Errai in Cepheus.

The obvious place to begin locating our quarry is at Polaris. With the pole star in your finder scope, look for the star labelled HR285 in the diagram below (also known as SAO 181), this shines at a naked-eye-visible magnitude 4.2. The blue ring is a 4° field of view and the dimmest stars in the field are magnitude 7.0.

Get Polaris and HR285 in the same eyepiece view (click image for full screen)

The final step is to move HR 285 so that it is off towards the 3 o’clock position of your finder scope. You should now have centred in your eyepiece NGC 188. The diagram below retains the 4° ring of your finder, which is the outer ring, and adds the 1° field of view ring in the centre. This should be your approximate field of view through the eyepiece once you’ve found the cluster.

NGC is approximately 1° south of HR285, as shown in the diagram above (click to enlarge).

A 4-6 inch telescope at a magnification of 50x will show the faint circular glow of myriad tiny stars against a dark background. More aperture will be needed when observing under urban skies or in less than ideal conditions.

What you’re looking at in NGC 188 is an unusually ancient open cluster, estimated to be 5 billion years old!

All screen grabs courtesy of Sky Safari 6.