NGC 188 in Cepheus
May 2025 - Nebula and Cluster of the Month
May is another month when I find myself struggling to find objects to write about. According to my own rules, the object in question should culminate at midnight some time in the current month, and should be visible from central Britain. Unfortunately, I’ve more or less run out of suitable subjects for May.
This month, then, I’m going to bend my own rules (possibly to breaking point, I’ll let you decide). I’m going to ask you to swing your telescopes away from the south and peer north for once.
Right up there, just 5° from the pole, is a very interesting open cluster. If you look at it during May, you will find it approaching its lower culmination, when it is actually at its lowest in the sky, but it will still be around 50° above the northern horizon. It is located 4° from Polaris, just east of the line that joins the pole star to γ Cephei.
It was missed by William Herschel and had to await discovery by his son, John. It entered his catalogue of observations, published in 1834, as no. 34. He described it as Cluster, very large, pretty rich 150 – 200 stars of magnitudes 10 – 18; more than fills the field
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The entry was carried forward into John’s General Catalogue (GC) as no. 92 and subsequently into the New General Catalogue (NGC) of 1888 as no. 188. The NGC description is the same as that originally penned by John Herschel, bar the comment about it more than filling his field.
NGC 188 is a large, rich cluster. It spans around 13’ in diameter. For once, I feel quite justified in using the term ‘diameter’ for an open cluster, because NGC 188 is pretty round. The overall magnitude is usually given as 8.1, but the brightest member stars of the cluster are of magnitude 12, not 10 as stated by John Herschel. There are stars in the field of 10th magnitude, but they are not members of the cluster.
NGC 188 was often touted as the ‘oldest open cluster in the Milky Way’. It is now not believed to be so, not because it isn’t very old, but because other, older clusters have now been observed. It is, however, probably the oldest open cluster that is readily available to users of modest telescopes.
Søren Meibom et al conducted an investigation1 into NGC 188 in 2008. They focused on a variable star known as V12, a member of the cluster. V12 is a spectroscopic eclipsing binary with a period of 2.9 days.
The mass and radius of each star in a spectroscopic binary can be determined by observation of the spectrum and light curve. A less direct method was used to determine the age of the stars and thus that of the cluster as a whole.
They concluded that the cluster was 6.2 billion years old (± 0.2 billion years) and that it lay at a distance of 1770 ± 75 parsecs, or 5770 ± 244 light-years.
The distance places it well north of the plane of the Milky Way and explains why it has survived to such a great age. It is well away from the gravitational influences that pull open clusters apart within the galactic plane. It is further from the galactic centre than we are.
Visual observers may find NGC 188 a little underwhelming at first sight, but it rewards careful study. I was fortunate to be in a very dark-sky site when I observed it in 2015. I noted that it was a large, fairly untidy cluster of scattered stars defined by two parallel lines of 11th and 12th-magnitude stars. It appeared fairly well detached and was rich. I estimated its diameter as 15’. Careful observation reveals seemingly hundreds of faint twinkles in the background.
The Trumpler type given in various catalogues for NGC 188 is usually I2r, indicating a well-detached cluster with strong central condensation, a moderate range of magnitudes and which contains over 100 stars. Archinal & Hynes quote a membership of 550 stars (including stars up to 10th magnitude) whereas the Deep Sky Field Guide to Uranometria suggests a more modest 120.
| Object | RA | Dec | Type | Magnitude |
|---|---|---|---|---|
| NGC 188 | 00h 47m 20s | +85° 15’ | Open cluster | 8.1 |
References:
- Søren Meibom et al 2009 Astronomical Journal 137 5086
If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Cluster of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.
